L;

of the
FLORIDA STATE MUSEUM

Biological Sciences
Volume 19 1974 Number 1

ECOLOGY OF THE AQUATIC BOX TURTLE,
TEARAPENE COAHUILA (CHELONIA, EMYDIDAE)

IN NORTHERN MEXICO

WILLIAM S. BROWN

-,6.-7-r.-i--.-s=~,F,Jiri-3*Fir-SI]55-ITi{*-'-W,r,-'~,-11~2/

-m- 44'' _'> "12•_LA _~IC*-.

UNIVERSITY OF FLORIDA GAINESVILLE



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Publication date: October 1, 1974 Price: $2.30



ECOLOGY OF THE AQUATIC BOX TURTLE, TERRAPENE
COAHUILA ( CHELONIA, EMYDIDAE ), IN

NORTHERN MEXICO

WILLIAM S. BROWNl

SYNOPSIS: An ecological study of the Coahuilan Box Turtle, Terrapene coahuila, was
undertaken between December 1964 and November 1967 in its natural habitat on
the northern Mexican Plateau. The species is endemic to an intermontane basin of
the Chihuahuan Desert near Cuatro Ci6negas, Coahuila. Its geographic range con-
sists of disjunct populations in an area that does not exceed 800 kmz.

Preferred habitats of T. coahuila in the area studied are small, north-south
trending, spring-fed marshes, characterized by mud bottoms, shallow water, and
dense aquatic vegetation, principally Chara spp., Eleocharis rostellata, and Scimus
olneyi Marshes are distinct aquatic communities surrounded by desert grassland
and shrub communities.

Ecological adaptations of this species more closely resemble those of other North
American aquatic turtles than of terrestrial members of its own genus. Population
densities, foraging behavior, food habits, and thermal relationships best exemplify the
aquatic mode of life of T. coahuila

T. coahuila remains active throughout the year except for short periods of en -
vironmental extremes. Mating occurs from September to June and appears concen-
trated in spring. Copulating pairs of box turtles were found in October, November,
December, and April, frequently in shallow water. The ovarian cycle appears to be
intermediate between the lengthy cycle of tropical emydid species and the compressed
cycle of northern species caused by cold weather. Follicle enlargement occurs be-
tween late August and early April when ovulation begins. Egg laying begins in May
and continues to September. Complements of 2 or 3 eggs are produced most fre-
quently. An estimated half of the females can produce second clutches, and about
one-third may deposit three sets annually. These females produce a mean of 6.8
eggs/female per season, a higher reproductive potential than in certain northern
populations of Terrapene.

Coahuilan box turtles forage in shallow water, with the carapace usually above
the surface and dry, and the head extended underwater. T. coahuila is opportunistic

i The author is an assistant professor in the Department of Biology, Skidmore
College, Saratoga Springs, New York 12866. Research for this paper was done in
partial fulfillment of requirements for a Master of Science degree at Arizona State
University, Tempe, Arizona. Accepted for publication 21 August 1971.

Brown, William S. 1974, Ecology of the Aquatic Box Turtle, Terrapene coahuila
( Chelonia, Emydidae), in Northern Mexico. Bull. Florida State Mus., Biol. Sci.,
Vol. 19 No. 1, pp. 1-67.



2 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

and omnivorous, feeding extensively on aquatic plants ( Eleocharis) and insects
( stratiomyid fly larvae, beetles, hemipterans, dragonfly nymphs). Foraging behavior
and food habits of T. coahuila are comparable to other aquatic or semiaquatic emy-
dids ( Chrusemys picta, Clemmys muhlenbergi).

Cloacal temperatures of T. coahuila active in marshes closely approximate water
temperatures at all seasons, as is generally true for most other aquatic turtles while
in water. There is wide seasonal and daily variation in the mean cloacal temperature
of active turtles, so no single optimum temperature within the activity range is re-
ported.

In summer, when water temperatures can exceed tolerable levels, activity occurs
mainly in early morning, late afternoon, and at night. Marsh bottoms provide a cool
refuge into which a turtle can burrow to avoid potentially harmful midday surface
temperatures. Most T. coahuila presumably undergo temporary states of winter
inactivity, although some remain active in water despite low air temperatures. In
December, when several individuals on land had cloacal temperatures elevated well
above air temperatures, basking was indicated.

Most T. coahuila remain within a given marsh for relatively long periods, but
about 20% of recaptured turtles had moved longer distances, possibly overland, from
one marsh to another. Within marshes, movements between successive points of
capture averaged about 13 m. Box turtles move in a sinuous fashion over mats of
Chara and around sedge tussocks. Individuals seemed socially tolerant of others in
nature, and were occasionally close together; no aggressive encounters were observed.

Three mark-recapture census techniques were used to estimate the population
size in the study area. Population densities ranged from 54 to 63 adult turtles per
marsh acre ( 133-156/ha). T. coahuila occurs in relatively higher numbers and is
restricted to smaller areas of activity than its terrestrial congeners, T. carolina and
T. ornata. Its population density is more comparable to certain aquatic species, such
as Chrusemi/s picta and Psetidemus scripta.

Although populations of the Coahuilan Box Turtle are relatively dense in many
marsh communities in the Cuatro Ci6negas basin, T. coahuila can be considered a
rare species by virtue of its restricted aquatic habitat. Destruction of marshes by
draining and excessive collecting of specimens clearly represents threats to the turtle's
existence. Terfapene coahuita should be obtained, therefore, only by those seriously
investigating its biology.



1974 BROWN: Terrapene coahuila ECOLOGY 3

TABLE OF CONTENTS
INTRODUCTION 3
ACKNOWLEDGEMENTS 4
METHODS 4
GEOCRAPHICAL SETTING AND HABITAT 5

Study Area 6
REPRODUCTION 11

Mating 11
Sexual Maturity and Seasonal Changes in Males 14
Sexual Maturity and Seasonal Changes in Females 16
Reproductive Potential 20
Egg-laying Season 24
Eggs 26

GROWTH 26
BIOTIC ASSOCIATES 28
INJURIES AND PRI£DATION 29
THERMAL RELATIONSHIPS 31

Activity Temperature 31
Daily and Seasonal Activity 37

FOOD AND FEEDING 39
Foraging Behavior 39
Diet 41

MOVEMENTS 48
POPULATION 51

Composition 51
Density 

59
54

Mortality and Replacement
SOCIAL RELATIONSHIPS 60
SURVIVAL STATUS 61
LITERATURE CITED 2

INTRODUCTION

Turtles of the family Emydidae are the most nearly cosmopolitan
and generalized of living chelonians. Most emydids are aquatic or semi-
aquatic, but some genera have evolved terrestrial adaptations. One such
North American group is the terrestrial genus Terrapene, the widely dis-
tributed and familiar box turtles, of which 4 species and 11 subspecies are
recognized ( Milstead and Tinkle 1967). One species, Terrapene coa-
huila, is exceptional among living members of the genus in that it alone
is aquatic.

Terrapene coahuila is restricted to a semi-isolated intermontane
desert basin of about 800 km2 lying generally south of the town of Cuatro
Ci6negas, Coahuila, in northern Maxico. The Coahuilan box turtle
occurs principally in marshes of the Cuatro Ci6negas basin, though
other aquatic habitats occur there ( Minckley 1969).

Since the description of Terrapene coahuila by Schmidt and Owens
( 1944), habitats, habits, and distribution of the species were surveyed



4 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

by Webb, Minckley, and Craddock ( 1963). Data concerning food, re-
production, and populations were lacking.

As the turtle is ecologically and evolutionarily unique in the genus
( Milstead 1969; Brown 1971), it seemed pertinent to investigate its
ecology and compare this information with that of aquatic emydids and
with the terrestrial Terrapene. This paper reports the results of a field
study conducted on 87 days between December 1964 and November
1967. Heaviest concentration of work in Mdxico was in July and August
1965.

ACKNOWLEDGMENTS

This study could not have been undertaken without the advice and encourage-
ment of W. L. Minckley, to whom I am particularly indebted. National Science
Foundation Grants GB-2461 and GB-6477X to Minckley helped defray some field
expenses. I thank Jorge Echaniz R., Lie., Director General de Pesca, Direcci6n
General de Pesca e Industrias Conexas, Secretaria de Industria y Comercio, M6xico,
D. F., for granting me a scientific collecting permit. The advice of Ismael Ferrusquia
V., Instituto de Geologia, Universidad Nacional Autonoma de M6xico, is most ap-
preciated.

Thanks are due to the following persons who helped in various phases of labor-
atory identifications: Frank F. Hasbrouck, insects; Donald J. Pinkava and Elinor
Lehto, plant material; Arland T. Hotchkiss, algae; Gerald A. Cole, crustaceans; and
Dwight W. Taylor, snails. Lauren E. Brown, W. L. Minckley, M. J. Fouquette, Jr.,
David I. Rasmussen, and Gerald A. Cole read all or portions of the manuscript.
Certain work was conducted in the laboratories of H. Gray Merriam ( University of
Texas, Austin) and John M. Legler ( University of Utah ). For field assistance I am
grateful to William S. Parker, Walter Kingsley Taylor, Josd Luis Lugo Diaz de Leon,
and Armando Moncada Diaz de Leon. Patricia J. Brown and the late Lynette Hansen
assisted with preparation of the manuscript.

I especially want to thank the people of Cuatro Ci6negas de Carranza, whose
friendship and warm hospitality made my stay most pleasant. I express my sincere
gratitude to Jos6 Lugo Guajardo and Catalina Diaz de Leon de Lugo for their en-
thusiastic aid and encouragement. I am indebted to former Presidente Municipal of
Cuatro Ci6negas, Francisco Manrique Davila, and to his successor in that office,
Victor Castillo Soto, for official clearance and introductions. Generous thanks are
also given to Modesto B. de la Garza Palos, Gines Nilo de la Garza Palos, Dr. Ro-
dolfo Castro Villarreal, Manuel Gonzklez Verduzco, Jos6 Ibarra Gonzdlez, Armando
Moncada Rivas, and many others for numerous courtesies.

METHODS

T. coahuila were hand-collected and marked by notching the marginal scutes of
the carapace. Movements were studied by capture and release. Points of capture
were plotted on maps of the study area. Maps of individual marshes were pre-
pared from field measurements obtained with a surveying transit. A total of 169
T. coahuita was marked; 36% of these were recaptured a total of 271 times. The
following data were recorded for each individual captured: general weather condi-
tions, cloacal and environmental temperatures, body measurements, presence of ecto-
parasites, injuries, and markings.

I examined 59 preserved specimens. Reproductive systems and digestive tracts
of 48 specimens ( 14 males and 34 females) showed food habits and gonadal cycles
in spring and summer. Reproductive systems only of seven females in autumn were
inspected, for a total of 55 subadult or adult size individuals surveyed. Of these,



1974 BROWN : Terrapene coahuila ECOLOGY 5

29 were collected in July and August 1965, 19 in April 1966, 6 in October 1967, and
1 in November 1967. They are deposited in the following collections: Arizona State
University ( ASU) 5853-5900, and University of Utah ( UU) 12551-12557. Four
additional preserved juvenile specimens examined for growth rings are from the
following collections: United States National Museum ( USNM) 159578; UU 3646;
and ASU 8000-8001.

AIl mean values in this paper are followed by + one standard error ( SE) of
the mean.

GEOGRAPHICAL SETTING AND HABITAT

The Cuatro Ci6negas basin is in central Coahuila ( Fig. 1) at the
eastern edge of the Mesa del Norte, or Ridge and Basin Province, of
northern M6xico. The basin is an expansive plain nearly enclosed by
mountain ranges of Cretaceous limestone ( Minckley 1969), and lies
within the Chihuahuan Desert faciation of the Hot Desert Biome ( Muller
1947; Shelford 1963). Climate of Coahuila is influenced primarily by its
continental position and its mountain ranges ( Muller 1947). Most of the
state is arid, with an annual rainfall of 0 to 200 mm ( Shreve 1944; Viv6
Escoto 1964).

Elevation of the basin floor is about 740 m above sea level; surround-
ing mountains are generally between 1500 and 3000 m high ( Minckley
1969). Three passes into the basin sequentially break the eastern-
bounding Sierra del Carmen-Sierra Madre Oriental axis, or Coahuila
Folded Belt ( West 1964), and connect the lowland Gulf Coastal Plain
with the Mexican Plateau. External draining of some of the basin's
waters through these gaps is by canals to the Rio Salado de los Nadadores
in the drainage system of the Rio Grande.

IO,9 103' 102' 101' IOO'

-29'

300--. COAHUILA -28"

21 - 1, -27'
CUATRO -26'Cl<NEGAS

-25'

8,0 160
1

1100 100° 90° KILOMETERS

FIGURE 1.-Geographical location of Cuatro Ci6negas, CoahuiIa, M6xico.



6 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

This small ( ca. 30 x 40 km) intermontane bolsdn lies just inside the
Chihuahua-Zacatecas Biotic Province near its zone of contact with the
Tamaulipan Biotic Province to the east ( Goldman and Moore 1945;
Stuart 1964). Innumerable springs, marshes, rivers, and ponds, many of
which are parts of distinct internal drainages, contribute to the biological
diversity of the Cuatro Cidnegas basin.

High endemism exists among the snails, crustaceans, and fishes
( Minckley 1969). Three of the four taxa of turtles known from the basin
are endemic. In addition to T. coahuila, basin endemics are Pseudemys
scripta taylori ( Legler 196Oa) and Trionyx ater ( Webb and Legler 1960).
Trionyx spiniferus emoryi, the fourth taxon, occurs widely in south-
western U. S. and northern Maxico, including the Cuatro Ci6negas basin
( Webb 1962; Winokur 1968).

STUDY AREA.-A site approximately 9.5 km almost due southwest of
the town of Cuatro Cidnegas was chosen for intensive field work ( Fig.
2). It is a gently-sloping grass zone immediately below the rock pedi-
ment ( bajada) of the northeast tip of Sierra de San Marcos, extending
finger-like into the basin from the south.

To the west and east around Sierra de San Marcos, many springs
and marshes lie immediately downslope from the bajada of the mountain.
Terrapene coahuila is perhaps more plentiful in this region of extensive
marshes than in other more widely-scattered areas of suitable habitat
within its limited range ( Fig. 2).

Transition from the rocky bajada to the grassy Hats of the basin
floor ( barrial) is abrupt. Here, the soil is bleached and Ene-textured,
often becoming soft and spongy after a rain. Halophytic grasses, par-
ticularly Distichlis spp., are prominent in the study area. Patches of
iodine bush, Allenrolfea occidentalis, are abundant. Stands of marsh
grass, Spartina spartinae, and alkali sacaton, Sporobolus airodies, are less
extensive. Bare regions, sparsely covered with scattered clumps of
mesquite ( Prosopis iuli#ora), catclaw C Acacia greggii), inkweed ( Suaeda
fruticosa), and Allenrolfea, are occasionally encountered within the
farther-ranging Distichlis Hats.

Near moist, spring-fed drainages, slight local subsidence of the basin
floor is evident. All marshes are characterized by spike-rush, Eleocharis
rostellata, approximately 50-70 cm tall. This sedge provides most ( ca.
70%) of the vegetative cover, but is supplemented in many marshes by
thick, submersed mats of stonewort, Chara spp. Bullrushes, Scirpus
olneyi, are prominent sedges in several marshes ( Fig. 3A). In others,
Scirpus is dispersed without noticeable zonation among the more abun-
dant Eleocharis.



1974 BROWN: Terrapene coahuita ECOLOGY 7

o SIERRA DE LA MADERA (·ANTEOt'-\~~,»2ZIZEL»f-_

CUATRO
dC I fNEGAS

E
V

F

-=Lj -.. -

H

T
study

area

SIERRA DE .* .-L-~3.1:~~
SAN MARCOS -1<~u

6 R-\ SIERRA

OE LA PURIS/A
I TOWNS, RANCHES

C/5 LAKES. SPRINGS. SIKHOLES-/A,· -

L -6 WIERS, CREEKS

6,-· INTEAMITTENT CHANNELS

----'  ABANDONED CANALS SCALE IN KILOMETERS'~

- ACTIVELY-USED CANALS 3 0 3~

11 12 13 14 IS 16 17 18 19 20 21 22 23

FIGURE 2.-Distribution of T. coahuita in the Cuatro Ci6negas basin as shown by
known sites of collection or sight records, 1958 to 1968 ( W. L. Minckley, unpub-
lished). Map ( provided by W. L. Minckley) is arranged with an approximately
2.5 km grid, letters designating N-S sections, numbers designating E-W sections.
Precise grid locations are indicated by subdivision of the square sections ( e.g., study
area is in NE 1/4, H-12). For place names of localities see gazeteer in Minckley
(1969).

Among other plant species that occur frequently in and around the
marshes or grow near them, seep-willow, Baccharis glutinosa, commonly
fringes a marsh, usually along the northern border. Of the 11 marshes
in the area of intensive study, 7 had conspicuous clumps of Baccharis
along them ( Fig. 3B). The sedge, Fimbristylis thernmlis, sawgrass, Cla-
dium californicum, and cattails, Typha spp., are less common plants of
the marshes. Distichlis stricta is usually found near moist spots close to
a drainage channel or a marsh, and occasionally occurs on raised patches
within a marsh.

The distinctness of the marshes from surrounding halophytic grass
communities is readily apparent. Marsh borders end abruptly as they
meet the dry, or seasonally water-logged, saline soil of the barrial.



8 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

A

I f

4

B

#.4

FIGURE 3.-Marsh habitat of T. coahuita, 28 July 1965, views northeast. Grasses
surrounding the marshes are largely Distichlis spp. A. Marsh 2-A; vegetation is
mainly Scirpus olneyi and Eleocharis rostellata, with scattered Spartina spartinae
and Cladium californicum at north end of marsh. B. Marsh 5; vegetation is mainly
Eleocharis rostellata with Baccharis glutinosa prominent around edges. ( Stakes
around marsh perimeter were for mapping purposes.)



1974 BROWN: Terrapene coahuila ECOLOGY 9

9,1 3900 D
6 8 8-A 2 10 5 9

6

N

6 25 so
METERS

{f f2-A 3 11
FIGURE 4.-Outline maps of 10 marshes in the study area. Marshes are arranged
from smallest ( left) to largest ( right).

Eleven marshes in the study area were visited regularly, and several
others infrequently. The last marshes were approximately 0.8 km east
of the study area. Ten marshes were mapped ( Fig. 4) and their areas
were determined by means of a planimeter ( Table 1). Lengths ranged
from 13 to 130 m, and widths from 7 to 100 m. Excluding the largest
marsh ( no. 11, which was more than 150 times larger in area than the
smallest), maximum dimensions averaged 30 x 12 m.

All marshes studied receive water directly or indirectly from a
number of springs and narrow stream channels entering from the south.
All are oriented with their long axes in a general south-north direction.
Flow is north and northeast toward El Mojarral, a series of several large
ponds (lagunas) about a kilometer north of the study area. One stream
approximately a meter wide carries the greatest volume of water of those
in the study area.



10 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 1. SIZES OF 11 MARSHES IN THE STUDY AREA.1

Area
Marsh No. mz Acres

11 8,745 2.16
3 751 0.18

2-A 394 0.10
9 234 0.06
5 186 0.05

10 172 0.04
2 157 0.04

8-A 118 0.03
8 104 0.03
1 102 0.02
6 56 0.01

Total Area 11,019 2.72

1 Areas of all except Marsh 1 ( area estimated) were
determined with a planimeter.

The channels tend to widen imperceptibly near a seep, or marsh,
then become braided. Within a marsh flow is reduced in many shallow
( 2-15 cm) rivulets. Substrate of the marshes is usually a dark mud.
Less frequently a lighter, more calcareous, flocculent material is present.

Most marshes have no visible water outlets, and evaporation and
seepage apparently balance inflow. During July and August 1965 the
surface water in several marshes evaporated during the day, exposing
patches of wet mud, but water was always replenished overnight. No
marsh in the study area dried completely.

Two marshes ( 8A and 3) had small, underground exit tubes along
their northeast borders, where water flowed out in small streams about 10
cm wide. Subterranean channels were further evidenced by several
small ( 50 cm diameter) sinkholes in the grassy terrain between marshes.
No surface channels connected one marsh with another, with one ex-
ception: marshes 8 and 8A. Marshes 9, 3, and 11 received their water by
surface streams directly from nearly circular, spring-fed pits ( posos), ap-
proximately 4 to 6 m in diameter ( Fig. 5).

Much of the study area, especially the intervening grass zones out-
side the marshes, is periodically burned by man. Burned areas were
locally extensive, but did not cover more than a few hundred or perhaps
a few thousand mi Charred grass tusssocks and young second-growth
plants evidenced old and recent fires over much of the area. Fires did
not appear to kill larger shrubs, such as mesquite or acacia, and did not
seem to cause long-term damage to marsh vegetation. During winter
when vegetation is dry, some large marshes of the basin are occasionally
destroyed by fires.



1974 BROWN: Terrapene coahuila ECOLOGY 11

*#r

FIGURE 5.-Poso, or small spring-fed pool, in study area, 31 July 1965. Water
temperature at all seasons averaged 33.5 'C in this sinkhole, typical of those in which
T. coahuila occasionally forage.

Herds of 5 to 15 horses and mules grazed in and around the marshes
almost daily during July and August 1965. They seemed to relish Eleo-
charis, and thus often diminished the cover this sedge normally pro-
vided the box turtles. Horses had severely grazed 7 of the 10 smaller
marshes. An occasional transient herd of goats entered the study tract,
but these usually did not remain longer than an hour and caused no
observable damage.

REPRODUCTION

MATING.-Approximately 30 Coahuilan box turtles have been main-
tained in an artificial outdoor pond at Arizona State University, Tempe,
and several of these attempted to mate. The pond is roughly circular,
about 8 x 10 m. About half of it is shallow ( ca. 1-20 cm) and half deep
( ca. 1 m). In April 1966 I saw a male mounted on a female in rela-
tively deep ( ca. 25 cm) water in this pond. The male's rear claws
gripped the female's posterior, apparently on the skin of the gluteal
region, and he snapped at the female's head. The female was com-
pletely submerged and made vigorous attempts to climb out of the
water, which she could not do because of the slippery inclined bottom.



12 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

After about 15 minutes, she succeeded and the pair separated. Whether
intromission had occurred is not known.

W. L. Minckley saw copulations between captive T. coahuila in
the artificial pond on 10 dates between 16 September 1965 and 11
June 1966. No matings were recorded in October, January, and Feb-
ruary. Minckley saw two different copulating pairs on 4 March 1966
at 7:40 AM, and three pairs on the mornings of 17 March 1966. Most
pairs were in shallow water and in all cases males were lying on their
backs.

On 1 November 1965 a male in shallow water "butted the female's
shell twice" ( presumably with his shell) before mounting the submerged
female. On 23 March 1966 a male followed a female with his head
extended, pushing the back of the female's carapace. The male then re-
tracted his head and "bumped the female with his carapace. Two hours
later these individuals were found in copulation. Two copulations were
timed at approximately 2 hours, and 2 hours and 20 minutes ( W. L.
Minckley, unpublished).

The first of three phases of mating in T. carolina observed by Evans
( 1953, 1968) consisted of the male pushing, cirding, and biting at the
female's carapace and striking it with the anterior portion of his plastron.
The entire courtship of T. carolina lasted up to 6 hours with up to 2 hours
in copulation ( Evans 1953). Brumwell ( 1940) recorded a 30-minute
copulation time in T. ornata and saw a male of that species striking the
carapace of a female with his plastron and biting at her carapace before
mounting.

Copulating pairs of T. coahuila were found several times under
natural conditions. On 31 December 1964 at midday, a pair was dis-
covered at the edge of a dense growth of Baccharis along the edge of a
marsh in the study area. The substrate was soft and muddy, with shal-
low water nearby but not directly under the turtles. The weather was
slightly overcast, air temperature 26.7°C. Temperature of the mud di-
rectly beneath the turtles was 17.2°C. The female was partially hidden
by overhanging vegetation and the male lay on his carapace. When
disturbed, cloacal contact was broken and both individuals withdrew
into their shells, remaining in their original location. Two other turtles,
a male and a female, were found 1 m and 3 m, respectively, from the
copulating pair.

Copulation was recorded on 8 April 1966 at 4:30 PM in marsh 5.
The central and eastern parts of the basin had received light rain 2 hours
earlier, but the study area received only a trace. Weather was clear and
humid, air temperature 29.0°C. Water temperature near the marsh inlet
was 22.6°C. Both turtles were in shallow ( 2-5 cm) water. The male



1974 BROWN: Terrapene coahuila ECOLOGY 13

was lying on his carapace and was being dragged slowly by the female.
When disturbed they separated and began to burrow rapidly into the
mud. Three other turtles were foraging within 5 m of the copulating
pair. Two of these turtles, a male and a female, were examined; the
third escaped.

On 20 October 1967 at 11:30 AM in marsh 8, a pair of marked T.
coahuila was copulating in water ca. 10 cm deep. The male made rapid
movements above the submerged female. Water temperature was
29.2°C, and air approximately 23.0°C. A month later, on 21 November,
another pair of marked individuals was copulating in shallow water of a
localized spring overflow among Distichlis grass near marsh 5 at 2:30
PM. Air temperature was approximately 29°C. The animals were dis-
covered after I heard their shells striking sharply together. The male
was on his back immediately behind the female.

Cahn and Conder ( 1932) and Evans ( 1953, 1968) described copu-
lation in captive T. carolina in which the hind legs of the male were in-
serted between the plastron and carapace of the female and were held
tightly in place. Secured by the hind legs of the female, the male leans
back and effects coitus. Legler ( 196Ob) states that coitus of T. carolina
differs from that of T. ornata in the position of the male's legs. It
seems necessary for male Terrapene to tilt backward to achieve effective
copulation, perhaps because of their relatively short tails and high-domed
shells.

Legler, in Webb et al. ( 1963 ), noted that a female T. coahuila was
drowned by a male during mating in an aquarium. Females probably
rarely drown while mating in the shallow water of marshes, but may
do so in the deeper water of sinkholes or pools. Few turtles were seen
in such habitats in the study area, and no mating turtles were noted in
pools along the Rio Mesquites ( Fig. 6).

Mating in T. carolina and T. ornata takes place in the spring after
emergence from hibernation and less frequently in the fall prior to
hibernation ( Ewing 1933, 1935; Allard 1935; Rosenberger 1936). It has
been reported to occur sporadically throughout the season of activity,
approximately from April to October in Washington, D.C. for T. c.
carolina ( Allard 1935, 1949), and from mid-April to late October for
T. o. ornata in Kansas ( Legler 196Ob). Penn and Pottharst ( 1940)
noted that T. carolina major in New Orleans, La. mated most often
after a rain or when temperatures were between 21.1° and 26.7°C.
Twice mating occurred in water.

The records of November and December matings by T. coahuila in
nature suggest extended, perhaps nearly year-round, sexual activity of
at least part of the population. Studies of captive and wild T. coahuila



14 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

FIGURE 6.-Two Terrapene coahuita C indicated by arrows) foraging in pools along
Rio Mesquites, ca. 7.5 km SW of Cuatro Ci6negas, 30 July 1965. Water tempera-
ture 33°C. Note dense sedge cover.

showed mating is frequent from September to June and most common
in March and April. No matings were recorded during July and
August 1965, the period of most intensive field work.

SEXUAL MATURITY AND SEASONAL CHANGES IN MALES.-Measure-
ments and weights of testes were determined with vernier calipers and
triple-beam balance. Testes volumes were determined by water dis-
placement. Epididymal smears from each male and several smears of
gonadaI tissue were examined microscopically for the presence of sperm.

Of 14 males dissected 10 had sperm in the epididymides and were
considered to be sexually mature ( mean carapace length, 109.5 *2.2
mm). Based on this sample, 95% confidence limits indicate that carapace
lengths of mature males from the study area would be expected to fall
between 104.6 and 114.4 mm. The smallest mature male was 93.1 mm.
The smallest mature male of T. o. ornata reported by Legler ( 196Ob)
had a plastral length of 99 mm; 76% of the males were mature at plastral
lengths of 100 to 109 mm, and all were mature between 110 and 119 mm.
In T. o. ornata plastral lengths are shorter than carapace lengths in most
young specimens, but are somewhat greater than carapace lengths in
mature animals.

A few sperm were present in the epididymides of four T. coahuila



1974 BROWN: Terrapene coahufla ECOLOGY 15

with small testes in late July and August. In two males with greatly
enlarged testes in late August, sperm were much more numerous. Sperm
were most abundant in epididymal smears of four males in April. The
epididymides of these turtles were slightly distended and contained a
milky fluid, presumably semen, whereas the epididymides of turtles in
July and August lacked any noticeable fluid.

Of the four males lacking sperm in the epididymides, two were
clearly subadult and probably immature ( carapace lengths 85.1 and
89.2 mm), whereas two collected in the last week of July appeared to be
adults on the basis of size ( 117.2 and 131.5 mm) and external appear-
ance. Testes of the last two contained no sperm and were very small,
with combined weights of trace and 0.08 g and volume displacements
of 0.01 and 0.12 ml, respectively. Two additional adult-sized males
( testes 0.07 and 0.15 g; 0.08 and 0.18 ml) caught at about the same time
( 26 July) had sperm in their epididymides. These individual differences
may be explained by prior expenditures of sperm toward the end of the
spring-early summer mating period at a time when the ensuing sperma-
togenic cycle may well be in its early stages before new sperm have ma-
tured. Mature spermatozoa were found in the epididymides of T. c.
carolina throughout the year ( Altland 1951) and in T. o. ornata through-
out the activity season ( Legler 196Ob). Tinkle ( 1958b) discovered no
sperm in the testes of 20 large male Sternotherus carinatus and con-
cluded that they were "out of season" when collected, commenting:
"The complete absence of spermatozoa was unusual, as a few generally
may be found even in out of season males of other forms." Moll and
Legler ( 1971) noted the presence of some epididymal sperm in Pseu-
demys scripta from Panamd in all months, with fewest numbers occuring
in June and July.

Seasonal changes in testis size in turtles is generally coincident with
the stage of spermatogenesis, the testes reaching maximum size at the
height of the cycle before the spermatozoa enter the epididymides
( Risley 1938; Altland 1951; Gibbons 1968c). Fluctuation in testis size
is evident in T. coahuila ( Table 2). The testes were small in specimens
from April and July, but had increased dramatically by late August in
two of four specimens. Available data on testis size and relative abun-
dance of sperm indicate a spermatogenic cycle not greatly different from
the north temperate pattern for turtles in which spermatogenesis takes
place in summer and mature sperm overwinter in the sex accessories
( Miller 1959; Moll and Legler 1971).

On the basis of observed matings of T. coahuila in nature and under
semi-natural conditions, and because of its habitat in a southern, thermal-
spring environment which permits a more extended period of sexual



16 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 2. TEsns SIZES OF 10 MATURE MALE T. coahuda.

Mean Weight Mean Volume
Month Mean Testis of Both of Both

Diameter ( mm) Testes (g) Testes ( ml)

April 6.4 0.13 0.17
(n=4) (5.0-9.0)1 ( 0.05-0.31) (0.09-0.30)
July 6.2 0.11 0.13
(n=2) ( 6.0-6.3) (0.07-0.15) (0.08-0.18)
August 10.4 1.13 1.28
(n=4) (5.0-17.1) ( 0.09-2.94) (0.10-3.40)

1 Ranges in parentheses.

activity than in more northern turtles, spermatogenesis may be extended
longer into the winter, as in Panamanian Pseudemys scripta ( Moll and
Legler 1971). If this occurs, T. coahuila has a spermatogenic cycle
differing in extent from the known cycle of Sternotherus odoratus ( Risley
1938), Chrysemys picta ( Gibbons 1968c; Ernst 197la), and the two U. S.
species of Terrapene ( Altland 1951; Legler 196Ob). Comparative data
are lacking for T. carolina populations of southeastern M6xico and T.
nelsoni of western M6xico.

SEXUAL MATURITY AND SEASONAL CHANGES IN F~MALES.-OVarieS and

oviducts of preserved female T. coahuila were removed and ovarian
follicles, corpora lutea, and oviducal eggs were counted. Follicles
greater than 1 mm in diameter were measured with vernier calipers.
Ovaries and eggs were weighed after being trimmed of connective
tissue and blotted with an absorbent paper towel. The condition and
relative size of oviducts were noted.

Female T. coahuita with one or more ovarian follicles larger than
5 mm in diameter were considered mature, but size and color of the
oviduets were also used to indicate sexual maturity, especially in postre-
productive females that lacked enlarged ovarian follicles. In 28 of 30
mature females, the uterine portion of each oviduct was black. All
oviducts in mature females were noticeably thickened, and had larger,
more expanded ostia than those of immature females.

Carapace lengths of 30 mature females ranged from 90.7 to 147.5
mm, mean 101.6 + 2.1 mm. Based on this sample, 95% confidence limits
indicate that carapace lengths of mature females in samples from the
study area would be expected to fall between 97.3 and 105.9 mm.

The smallest mature female T. o. ornata found by Legler ( 196Ob)
had a plastron length of 107 mm; 47% of his sample were mature at a
plastron length of 100 to 109 mm, most maturing when they had attained
a plastron length between 120 and 129 mm. Males became sexually ma-
ture at a smaller size than females. In T. coahuila this situation is re-



1974 BROWN: Terrapene coahuila ECOLOGY 17

TABLE 3. OVARIAN WEIGHTS AND NUMBERS OF FOLLICLES IN
OVARIES OF 30 MATURE F~MALE T. coahuila.

Mean Weight of Mean Number of
Month Both Ovaries (g) Follicles > 1 mm

April 2.16+0.57 8.150.8
(n=13) ( 0.36 - 6.24) (3-14)
July &
August 1.21 + 0.27 11.8 +1.0

(n=17) (0.32-4.17) (5-21)

versed: sexually mature females were significantly ( P<0.05) smaller
than males.

The spring and summer samples examined contained 15 female T.
coahuila taken in the first week of April 1966, 10 in July 1965, and 9 in
August 1965. July and August individuals did not differ in condition of
their reproductive tracts and were combined. Seasonal comparisons were
made between the April ( spring) and the July-August ( summer) group.
Two individuals from each sample were considered subadult or im-
mature, reducing the numbers to 13 mature females in April and 17 in
July and August. Although carapace lengths of mature females in the
summer sample averaged slightly larger ( 103.4 mm) than those in the
spring sample ( 99.2 mm), there was no significant difference in size
between the two groups ( P>0.30). Follicles were arbitrarily grouped
into size classes as follows: 1-4 mm, 5-9 mm, 10-14 mm, and >15 mm.

In spring all 13 mature females were prereproductive and had not
yet ovulated. These had heavier ovaries than females in summer ( Table
3), but the difference between them was not statistically significant
( P>0.10). Weights of preserved ovaries of T. coahuila were consider-
ably less than those of T. c. carolina ( Altland 1951) and T. o. ornata
( Legler 1960b). Ovarian weights of T. coahuila early in July tended to
be greater than those in late July and August.

There was no correlation between an adult turtle's size and the total
number of follicles greater than 2 mm in the ovaries in both the spring
sample ( P >0.05) and the summer sample ( P>0.05). Females in sum-
mer had a significantly greater (P= 0.01 ) mean number of follicles per
female than did the spring females ( Table 3). Most of these were
small follicles, indicating that most of the turtles in the July-August
sample were postreproductive, in a period between a previous ovulation
and the beginning of a new ovarian cycle. Legler ( 196Ob) reported
the formation of many small follicles in the ovaries of female T. ornata
in July or August.

Figure 7 shows the distribution of follicle sizes in all mature females
from both samples; 44 percent more females in spring than in summer



18 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

20 -

15 -

APRIL (13) JULY - AUGUST (17)

N
U

M
BE

R
 

O
F 

IN
DI

VI
DU

AL
S

10 -

85% 54% 8% 41% 35%
F~-3 F-'-3

5-

-

5 10 15 20 5 10 15

FOLLICLE DIAMETER (mm)

FIGURE 7.-Follicle size distribution in ovaries of 30 mature female T. coahuila.
Percentages of total number of individuals that contained follicles in arbitrary 5 mm
size groups are shown for each sample.

contained follicles in the 5-9 mm range, and 19% more had follicles be-
tween 10 and 14 mm. Follicles in the latter size range were preovula-
tory. Nearly twice as many females in spring contained at least two
follicles greater than 5 mm in diameter, compared to summer females.
Conversely, postreproductive July-August females had 26% more small
follicles ( 1-4 mm) than did prereproductive April females. Enlarged
follicles ( > 10 mm) present in April females probably would have ma-
tured and been ovulated later that month.

One female taken on 4 April 1966 with two ovarian follicles 15 and
17 mm in diameter was apparently on the verge of ovulation. These
were the largest follicles found in any female. Yolks of two oviducal
eggs examined had average diameters of 16 and 17 mm, indicating the
probable size attained by ova just prior to ovulation. Judging from this
individual, and from the large follicles in the ovaries of the other females
in the spring sample, ovulation occurs as early as the Brst week in April
but may be concentrated in the last half of April.

Several female T. coahuila with enlarged follicles were collected in
late August. One, taken 23 August 1965, had follicles 5 and 7 mm in
diameter, and another taken 30 August 1965 had three follicles measuring
5, 5, and 6 mm. On 24 August 1965 a female contained three enlarged
follicles, two of which ( 11 and 12 mm) were approaching ovulatory
size. Ovulation of the two largest ova in the Iast female might have oc-
curred in mid-September and the smaller follicles in the first two females



1974 BROWN : Terrap€ne coahuila ECOLOGY 19

TABLE 4. SIZE RANGES OF OVARIAN FOLLICLES IN SEVEN FEMALE
T. coahuita ( UU 12551-12557) IN AuruMN 1967.1

Date of
Collection Enlarged Follicles SmalI Follicles
2 October 5.2-5.6 (2) 39-4.2 (3)

7.2 (1) 3.6 (1)
" 4.6 (1) 3.3-4.1 (6)

18 October 7.1-9.1 (5) (0)
(0) 2.5-4.0 (8)

20 October (0) 3.0-4.0 (4)
12 November 5.7-7.2 (2) 3.2 (1)
i Measurements in mm; number of follicles in parentheses.

could have been ovulated later that season, or they may have been held
over until the following spring.

In autumn seven mature females had a total of 34 follicles >2 mm.
Mean size of the follicles was 4.6 mm ( range 2.5-9.1). Of these, 11
( 32%) were in the 5-9 mm size range and occurred in five of the seven
females ( Table 4). Enlarged follicles in this small autumn sample
averaged 6.9 mm. No corpora lutea were discernible macroscopically.

T. coahuita remains active throughout the year except for short
periods of environmental extremes. Sexual activity occurs mainly from
September to June. Follicular enlargement occurs between late August,
when ovarian weights are low, and early April when ovaries are heavy
and when nearly all mature females have one or more enlarged follicles
( Tables 3 and 4, Fig. 7). Follicles of females that deposit clutches in
late summer may undergo enIargement in late winter and the following
spring.

Follicular atresia in T. coahuila is not great. Only two slightly en-
larged, discolored follides were considered atretic. Both were in the
same ovary of a female containing oviducal eggs on 9 July 1965. Altland
( 1951) observed follicular atresia most frequently in August in T. c.
carolina, but he indicated that atresia did not account for a complete loss
of enlarged follicles over the winter in that species. Legler ( 196Ob)
observed brown, orange, or purplish atretic follicles in ovaries of many
female T. o. ornata.

Ovaries of T. c. carolina were heaviest in May when they contained
2 to 8 enlarged follicles ( Altland 1951). Ovulation usually occurred in
June and July with a corresponding decrease in ovarian weight, but ovu-
lation could occur as late as August 15. Follicles began to enlarge in
July and August, following ovulation. Altland suggested that some of
the enlarged follicles formed prior to hibernation were held over to the
next reproductive season. Likewise, follicles of Chrysemys picta in
Pennsylvania remained in an enlarged, quiescent state through winter



20 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

after a period of postovulatory summer growth ( Ernst 197la). The
ovarian cycle of T. ornata ( Legler 19605 ) is similar to that of T. carolina .
Ovaries weighed most in March and April prior to initial ovulation dur-
ing May and June; an estimated 33% of females were capable of a second
ovulation in July. The cycle began in late summer, ovarian weights in-
creasing in October before hibernation.

Essentially the same timing occurs in ovarian cycles of northern
temperate aquatic turtles that have been studied ( Chrysemys picta,
Powell 1967 ; Gibbons 1968c ; Ernst 19718 ; Sternotherus odoratus, Risley
1933). Moll and Legler ( 1971) studied the ovarian cycle of a tropical
aquatic species, Pseudemys scripta, in PanamA and found that ovulation
occurred in the first half of the calendar year ( December to May), and
oviposition was completed by August. Follicles began to enlarge again
in the last half of the year following a quiescent period in July and
August.

Except for a several-month period of interruption imposed by cold
weather and resulting hibernation in northern species, similar patterns
of timing occur in temperate as well as certain tropical emydid turtle
species known to date. In the Cuatro Cidnegas basin, low temperatures
during an estimated 3-month period from December through February
( see "Seasonal Activity") may inactivate box turtles. Although the T.
coahuila population apparently does not undergo any sustained period
of hibernation, cool weather probably delays completion of follicular
enlargement and ovulation until around April. Ovulation can seemingly
continue into August and oviposition into early September ( see below).
The ovarian cycle of T. coahuila appears to be intermediate between the
lengthy cycle of tropical species and the shorter cycle of northern
species compressed by cool weather.

REPRODUCTIVE POTENTIAL.-Studies of 16 female T. coahuila ( 6 with
large preovulatory follicles, 7 with corpora lutea or enlarged follicles or
both, and 3 with oviducal eggs or enlarged follicles or both) represent-
ing 23 potential clutches indicate that complements of 2 or 3 eggs are
produced most frequently ( Fig. 8), with an over-all mean clutch size of
2.3 ( range 1 to 4).

Mean clutch size for T. c. carolina near Washington, D.C., has been
reported as 4.2,3.0, and 3.6 eggs ( Ewing 1933, 1935; Allard 1935).
Altland ( 1951) recorded 2 to 5 eggs in T. c. carolina from Pennsylvania
and Maryland. Legler ( 196Ob) found 2 to 8 eggs ( mean 4.7) in 23
clutches of T. o. ornata in Kansas . These data indicate that T. c. CaTo-

lina and T. o. ornata living at more northern latitudes have higher aver-
age clutch sizes than does the southern T. coahuila. Tinkle ( 1961)
gave a mean of 2.2 eggs in southern and 4.6 eggs in northern Sterno-



1974 BROWN: Terrapene coahuda ECOLOGY 21

10-

W

6
3
0
IL 5-
0
Cr

NUMBER OF EGGS
FIGURE 8.-Number of eggs in 23 clutches of T. coahuila. Shaded portions repre-
sent clutches determined from counts of oviducal eggs and corpora lutea; unshaded
portions represent potential clutches determined from counts of enlarged ovarian
follicles.

therus odoratus, and showed that clutch size in S. odoratus decreases
progressively as one moves south to lower latitudes. Selective factors
involved in this variation are discussed by Tinkle ( 1961) and Moll and
Legler ( 1971). Chrysemys picta produce clutches averaging 8.5 eggs
in Nova Scotia ( Powell 1967), 6.6 in Michigan ( Gibbons 1968c), 6.3 in
Illinois, and 4.3 in Tennessee ( Cagle 1954). Small dutch sizes ( mean
2.7 eggs) of Terrapene n. nelsoni from Nayarit, M6xico, reported by
Milstead and Tinkle ( 1967), together with that reported here for T.
coahuila, concur with Tinkle's ( 1961) data on S. odoratus and those
available for C. picta, and suggest that latitudinal variation in clutch size
also occurs in Terrapene. Whether this conclusion remains valid will
ultimately depend on detailed work within a single geographic region
to determine the extent of reproductive variability between local popula-
tions. Particularly, Gibbons and Tinkle ( 1969) have shown significant
differences in clutch size between three closely situated populations of
Chrysemys picta in Michigan.

Of 10 July and August female T. coahuila, 7 had corpora lutea in the
ovaries but no oviducal eggs, indicating recent oviposition. In the three
females with oviducal eggs (mean 3.0), corpora lutea were cuplike
structures approximately 6 to 7 mm in diameter and appeared similar
to the corpus luteum of T. c. carolina ( Altland 1951). In each case
there was agreement between the number of corpora lutea in the ovary



22 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

and the number of eggs in the oviduct on the corresponding side, so
there was no indication of extra-uterine migration of ova to the contra-
lateral oviduct as recorded for other species by Legler ( 1958) and Tinkle
( 1959a), and no indication that any eggs had been laid before the turtle
was preserved. Altland ( 1951) and Moll and Legler ( 1971) noted de-
generation of corpora lutea in some female T. c. carolina and Pseudemys
scripta, respectively, while eggs were still in the oviducts. Atresia of
corpora lutea in T. c. carolina was completed by August, shortly after
the egg-laying period. Corpora lutea of T. o. ornata underwent rapid
involution and were barely discernible 2 to 3 weeks after oviposition
( Legler 196Ob).

A large female T. coahuita containing four oviducal eggs had both
oviducts, each containing two eggs, displaced to opposite sides of the
body cavity. This individual was the largest turtle of either sex taken
( 147.5 mm carapace length). Legler ( 1958) noted a similar phenom-
enon in T. o. ornata when oviducts contained large complements of eggs.

Six April females approaching their first ovulation of the season had
2 to 4 preovulatory follicles ( mean 2.7 ), These follicles are thought
to represent the first clutch. One female on 4 April 1966 contained
follicles of two distinct size groups, which possibly represented two
future clutches. Ovulation seemed imminent for two follicles. with the
other group of three representing a second clutch. Several other April
females also gave indications of multiple-clutch capability, containing
five and six follicles greater than 5 mm in diameter that presumably
represented two developing clutches.

Ovulation first occurs for most of the population in April with egg
laying in May. An additional set of ova could be ovulated and de-
posited by June or early July. Ten females taken between 3 July and
24 August 1965 provide evidence for second and third clutches in T.
coahuila. Of seven females with corpora lutea, four had 1 or 2 preovu-
latory follicles, and of three carrying oviducal eggs, two had 1 and 3
preovulatory follicles. Thus, six females were at a stage between a sec-
ond ovulation, as indicated by corpora lutea, and a third ovulation, as in-
dicated by preovulatory follicles. Three individuals had corIora lutea
only. Based on these nine females ( 53% of July-August sample), plus
the female in April with its second set of enlarged follicles, mean size
of the second clutch is 2.4, range 1 to 4. ( One female with two oviducal
eggs on 26 August 1965 is not included because it contained no enlarged
follicles deBnitely indicating a second dutch.) All females in the popu-
lation would probably not lay second clutches, as some individuals in the
July-August sample had neither corpora lutea nor enlarged follicles.
Rapid disappearance of corpora lutea, however, and collection of females



1974 BROWN: Terrapene coahuila ECOLOGY 23

in an intermediate period may, as mentioned earlier, influence this con-
clusion. The six females ( 35% of July-August sample) believed capable
of depositing third clutches contained from 1 to 3 preovulatory follicles
(mean 1.7).

Multiple clutches in turtles are generally known ( Legler 196Ob).
Chfusemys picta apparently produce two similar-sized clutches annually
in Michigan ( Gibbons 1968c), but only one in Pennsylvania ( Ernst
197la). Tropical Pseudem!~s scripta produce up to six clutches per sea-
son ( Moll and Legler 1971). Approximately 33% of female T. o. ornata
in Kansas produced two clutches of eggs in the same season; first clutches
averaged 4.7 eggs, second clutches 3.5 ( Legler 196Ob). Clutch sizes in
T. coahuila decrease from a mean of 2.7 eggs in the first dutch to 2.4 in
the second and 1.7 in the third.

Clutch sizes in turtles are correlated with the size of the female
( Cagle 1944b, 1950; Einem 1956; Legler 196Ob; Tinkle 1961; Moll and
Legler 1971). Although variable, in 12 T. coahuila with 13 potential
clutches determined by counts of enlarged preovulatory follicles, 5 of 8
females between 90 and 100 mm in carapace length would have laid
two eggs; 2 of 3 females 100 to 110 mm, three eggs; and 1 of 2 females
over 110 mm, four eggs. There was also a direct correlation between
carapace length and clutch size in the three females containing oviducal
eggs ( see below).

Reproductive potential can be estimated by counting the number of
enlarged follicles that could be ovulated in one season and adding to
this the number of oviducal eggs or corpora lutea, or both ( Tinkle 1961).
Tinkle ( 1961) noted the difficulty in calculating the reproductive poten-
tial in turtles in which a new ovarian cycle may begin late in the season,
resulting in enlarged follicles that may not be ovulated until the follow-
ing season, and in which more than one clutch per yearrnay be pro-
duced. These phenomena are known to occur in T. carolina and T.
ornata ( Altland 1951; Legler 1960b ), and in T. coahuila. Tinkle ( 1961)
states that "counts of follicles, lutea, and eggs will give an estimate of the
maximum egg production... but the actual production may be much
lower." The maximum annual reproductive capacity of T. coahuila can
amount to 11 eggs ( maximum of 4 eggs in the first two clutches and 3
eggs ( see below).

Several difficulties in using this method for T. coahuila were ap-
parent: (1) that all the females collected in April had developed a full
complement of potential ovulatory follicles could not be determined ac-
curately; (2) the probable rapid disappearance of corpora lutea made
it impossible to determine whether some postreproductive females had
already ovulated, and, if so, how many eggs they had laid; and (3) some



24 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

postreproductive females lacked enlarged follicles, possibly because they
were preserved before a new ovarian cycle had become advanced.

In April the prereproductive potential, as estimated by counts of
follicles greater than 5 mm in diameter in nine mature females, was 3.9
eggs/female. As T. coahuita can produce more than one clutch of eggs
per season, forming new follicles in each of three possible reproductive
periods, the above may be an inaccurate estimate for a single year. The
difference of 1.2 eggs between the prereproductive potential ( 3.9) and
the average first clutch size ( 2.7) further indicates early follicular en-
largement for second ovulations. Of the July-August sample, 53% would
be expected to produce two clutches. In the estimated 35% of females
producing three clutches per season, the expected mean annual repro-
ductive potential, determined by adding the mean number of eggs in
each clutch, is 6.8.

As is true for clutch size, reproductive potential of Sternotherus
odoratus varies geographically ( Tinkle 1961). After comparing Terrapene
n. n€lsoni from western M6xico with T. o. ornata and T. c, carolina on
the basis of average single ( or first) clutch sizes produced in these
populations, Milstead and Tinkle ( 1967) proposed that reproductive
potentials may be lower in southern than in northern Terrapene. From
the sample examined, they concluded that T. n. nelsoni produces one
clutch annually, but state: "it must be admitted that the southern turtles
may produce more than one clutch per year. The mean reproductive
potential ( 6.8 eggs/female per season) realized by an estimated one-
third of T. coahuila females is below the potential of 8.2 (mean first and
second clutch sizes added) realized by a similar portion of the T. o.
ornata population in Kansas ( Legler 196Ob ), but is higher than the mean
single clutch size ( = mean reproductive potential) of 4.2 eggs/female
per season in northern T. c. carolina ( Allard 1935).

EGG-LAYING SEASON.-The earliest date of laying indicated by pre-

served specimens of T. coahuila was approximately 3 July ( corpora
lutea), the latest date approximately 26 August ( oviducal eggs), Legler
( 196Ob) and Gibbons ( 1968c) noted that T. o. ornata and Chrysem!/s
picta, respectively, normally retained eggs in the oviducts for 2 to 3
weeks before laying. Although length of egg retention in T. coahuila is
unknown, if 3 weeks is added to the approximate earliest date of ovula-
tion ( early April), oviposition could begin in late April or the first week
in May. The egg laying period continues to the first week in September,
if one week is added to the latest date when a female was found with
eggs. One female had two preovulatory follicles on 24 August, extend-
ing oviposition to the latter portion of September if ovulation were to
have occurred in early September.



1974 BROWN: Tefrapene coahuita ECOLOGY 25

Incubation periods of turtle eggs are subject to wide variation de-
pending largely upon environmental temperatures. Allard ( 1949) found
that eggs of T. carolina hatched in 52 days at summer laboratory tem-
peratures, but incubation periods varied between 69 and 135 days in
nests; 87 to 89 days was the "average" incubation period under natural
conditions. Incubation periods of T. c. carolina varied from 69 to 103
days at Washington, D.C. ( Ewing 1933). Conant ( 1951) reported an
incubation period of a single clutch of T. c, carolina in Ohio to be 105
days. Ewing ( 1933) and Allard ( 1948) reported hatching of T. carolina
in September and early October. A typical incubation period for T. o.
ornata eggs in eastern Kansas under natural conditions was about 65
days; eggs laid in mid-June usually hatch in mid-August, but may be
delayed until October in years when summer temperatures are cooler
than normal ( Legler 196Ob). Legler ( 196Ob) noted wide fluctuations
in laboratory incubation periods of T. o. ornata eggs. At an average
daily temperature of 32.8°C, the mean incubation period was 59 days;
at 27.8°C, 70 daysi and at 23.9°C, 125 days. Moll and Legler ( 1971) de-
termined the mean incubation period of Pseudemys scripta eggs in Pan-
ama at seasonal environmental temperatures ( 21° to 32°C) to be 78
days.

Climatological data indicate that mean monthly air temperatures
in the 7-month period May through November range from a low of
19.0°C in November to 29.4°C in August at Cuatro Cidnegas ( Con-
treras Arias 1942). Mean temperatures during this period in 1965
ranged from 18.5° in November to 29.5° in July ( Modesto de la Garza P.,
pers. comm.). If the incubation periods given by Legler ( 196Ob), Moll
and Legler ( 1971), and others, are assumed to approximate those of T.
coahuila in the period from May through November when environmental
temperatures would generally correspond to temperatures given for an
incubation period of approximately 70 days, projected dates of hatching
of the eggs contained in the three gravid female T. coahuila would be
approximately mid-September, early October, and late October or early
November.

Hatchlings appearing in October or early November from eggs laid
in August should not experience thermal difficulty in emergence. Aver-
age maximum air temperatures in November ( 26.3°C) and December
( 20.7°C) ( Contreras Arias 1942) would provide suitable conditions for
activity; if, as seems probable, nests are placed in moist, soft soils of
sedge tussocks, conditions may differ widely from those indicated by
air temperatures alone. The relatively warm water of the marshes would
mitigate environmental extremes.

A 1- to 3-month old hatchling was discovered in the study area on



26 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 5. SIZE OF EGcs IN THREE CLUTCHES OBTAINED FROM OVWUCTS OF FEMALE
T. coahuila.

Carapace
Date Length of Length ( mm) Width (mm) Weight (g)

Preserved Female ( mm)

9 July 116.0 31.2 15.8 4.44
30.9 16.6 4.71
30.5 16.8 4.87

1 August 147.5 34.5 17.6 6.21
34.6 18.2 6.76
36.3 17.9 6.81
34.8 17.2 6.28

26 August 93.4 33.5 16.1 5.51
32.8 16.2 5.35

Mean + 1 SE 33,2 2 0.67 16.950.28 5.66 * 0.30

15 October 1966. The juvenile was sunning on a Chara mat in a marsh;
nearby water temperature was 23.8°C, air 20.0°C. Extended periods of
incubation ( 230 days) as reported by Driver ( 1946) for T. c. carolina,
or overwintering in the egg by hatchling turtles ( Myers 1952; Sexton
1957) would not be expected to occur in T. coahuila, which inhabit a
warmer climate.

Eccs.-Nine eggs from oviducts of three preserved female T. coahu-
ila are ellipsoidal and white. The shell is smooth to the touch, but
Bnely granulated when viewed under a dissecting microscope. For
the eggs' dimensions and weights see Table 5.

Lengths of T. coahuila eggs approximate those reported by Allard
( 1948) and Cahn ( 1937) for T. c. carolina, but their widths are slightly
less. Eggs of T. c. bauri, T. c. maior, T. 0. ornata, and T. n. nelsoni are
all larger than the eggs of T. coahuila, while those of T. c. triunguis
seem to be of approximately equal length ( Carr 1952; Crooks and Smith
1958; Legler 196Ob; Milstead and Tinkle 1967). Mean weight of the
nine preserved eggs of T. coahuila ( 5.66 g ) is less than mean weights of
T. c. carolina eggs ( 8.4 g, Allard 1949; 9.24 g, Cunningham and Huene
1938) and T. o. ornata eggs ( 10.09 g, Legler 196Ob).

GROWTH

Growth in turtles was reviewed by Cagle ( 1946) and Legler
( 196Ob). They also extensively analyzed growth in Pseudemys scripta
and Terrapene ornata respectively. I have followed Legler's terminology.

The usefulness of major growth-rings as indicators of growth and
age depends upon four assumptions ( Sexton 1959a ): (1) a discernible
increase in growth occurs each year, (2) one major growth-ring is added



1974 BROWN: Terrapene coahuila ECOLOGY 27

TABLE 6. SIZE AND AGE OF JUVENILE T. Coahuita As DETERMINED FROM GROWTH-
RINGS.

Plastron
Estimated Plastron length at PIastron length

Specimen plastron length length at second up to time of Estimated
No. at hatchingl Erst winter winter capture age

ASU 8000 26.1 - - 30.5 1-3
(July-Sept. 1966)2 ( 15 Oct. 1966) months

ASU 8001 29.0 36.3 43.6 49.0 > 1 year (?)
( shell found)

Field no. 66 ? 36.4 54.2 57.1 1 year,
(Sept. 1963) ( 15 July 1965) 10 months

USNM 159578 ? ? - 49.8 1 year
( Sept. 1963) ( 20 Aug. 1965)

UU 3646 28.7 36.9 - 47.2 1 year
( July-Sept. 1959) ( 18 Aug. 1960)

iMeasurements are in millimeters.
2Probable dates of hatching and dates of collecton are indicated in parentheses.

per year, (3) no major growth-rings are lost, and (4) a major growth-
ring of any selected scute does not change in length after its formation.

It was possible to estimate growth and age only in a few small T.
coahuila. Growth-rings are presumably obscured by wear. No turtles
were found to be shedding their epidermal scutes. Nearly all subadult
and adult individuals had a completely smooth carapace and pastron,
or, at best, exhibited only traces of recently-formed growth-rings on the
abdominal or pectoral scutes of the plastron. No other methods of aging
were attempted.

Four juveniles collected in or near the study tract and one ( shell
only) from an unknown locality in the basin provide some data on ap-
proximate size at hatching and early growth, as growth-rings are still
evident. Growth-rings were measured to the nearest tenth of a milli-
meter on the medial side of the right abdominal scute in the manner
described by Legler ( 196Ob). Sergeev's ( 1937) proportion was used
to calculate previous plastron lengths.

Plastron lengths at hatching are estimated at 26.1, 28.7, and 29.0
mm in three juveniles ( Table 6). One specimen ( ASU 8000) may be
1 to 3 months old as estimated from its growth increment ( 17% of the
plastron length at hatching) and from the probable season of hatching
in the population ( mid-July to December). September, holding a
roughly median position in the hatching season, is a likely month of ap-
pearance for two other juveniles.

Legler ( 196Ob) recorded only a 17.5% increment in plastron length
of T. ornata in the year of hatching, a 68.1% increase in the first full
year of growth, and decreasing each year thereafter ( 28.6% in the second



28 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

season, 18.1% in the third, etc.). Ernst ( 197lb) recorded a mean plastral
increase of 111% in Chrysemys picta following hatching, but that figure
includes growth of hatchlings that overwintered in the nest and whose
growth began early the following year ( called the "first season" by
Ernst). As hatchling T. coahuila may emerge earlier from the nest and
remain active longer than more northern species of Terrapene, growth
in the season of hatching could be considerably greater. Two juveniles
( ASU 8001, UU 3646) increased an estimated 25% and 28% of their
original plastron lengths in the season of hatching. , Three juveniles
( ASU 8001, field 66, UU 3646) made calculated increases of 20%, 28%,
and 49% of previous estimated plastron lengths ( attained by the end of
the hatching year) in their Grst full year of growth.

BIOTIC ASSOCIATES

Some T. coahuila had compact deposits of algal majl on the cara-
pace, most commonly on the anterior or posterior edges, or both. Two
individuals collected in December 1965 had algal encrustation on the
five posterior marginal scutes of each side, the posterior portion of the
third laterals, and all of the fourth laterals on both sides of the carapace;
one had coatings on the first two marginals anteriorly. Color of these
deposits on T. coahuila ranged from a pinkish hue to green.

In December 1965 and January 1966 samples were scraped from
carapaces of five individuals from marshes in or near the study area. Six
genera of blue-green algae ( Cyanophyta) were identifed from these
samples: Anacustis, Gloeothece, Lyngbya, Oscillatoria, Pleurocapsa, and
Spirulina. Diatoms ( genera unknown) occurred on two of the turtles
together with blue-green genera. No green algae ( Chlorophyta) were
in any of the samples fom turtle carapaces, but a sample of algae col-
lected from a marsh in January 1966 contained the green algae Spiroggra
and Mougeotia. A blue-green alga ( Gloeothece) and diatoms ( Synedra
and others) were also present in the marsh sample. A. T. Hotchkiss
(pers. comm.) believed that the blue-green algae were on T. coahuila
shells largely by chance, and that they might well have occurred on any
other solid substratum. Except for one unidentified blue-green alga,
none of the forms was an attached alga to the extent of having a hold-
fast.

Many species of aquatic turtles support flores of epizoic green algae,
mainly the genus Basieladia ( Edgren et al. 1953; Proctor 1958; Gibbons
1968a; Moll and Legler 1971). Basicladia, a genus restricted mostly to
turtles, was not found on any of the T. coahuila sampled, but a filamen-
tous algal growth that was not identified ( but which may have been



1974 BROWN: Terrapene coahuilti ECOLOGY 29

Basicladia) was on the carapace of an individual in the preserved series
from near the study area. In 1956 John M. Legler first noted algal
growth on T. coahuila. Basieladia chelonum and several blue-green
algae, including Pleurocapsa sp., were identified from a specimen in the
type series E. G. Marsh, Jr. collected in 1939.

One individual from the study area had six small round pits about
1 mm deep and a larger pit about 7 mm in diameter and 2 mm deep on
the second right lateral scute. Pitting and eventual erosion of the shell
in aquatic turtles could be caused by certain algae or fungi penetrating
under the epidermal laminae ( Hunt 1957, 1958). Potter ( 1887) de-
scribed the penetation of wedge-shaped masses of the green alga Derma-
tophyton radicans into the carapace of Clemmys ( - Mauremys) caspica
of Europe. Jackson ( 1964, 1969) noted carapace erosion in Sternotherus
m. minor from Florida, and suggested injuries from intraspeciftc aggres-
sion as a possible cause. Carpenter ( 1956) recorded carapace pits in
T. carolina triunguis in Oklahoma and speculated that parasitic fungi,
among other factors, might have caused the shell erosion.

Seven of 169 ( 4%) T. coahuila in the Beld harbored 1 to 4 small
unidentified leeches ( Hirudinea) attached to the skin at the base of the
tail or to the posterior ventral margin of the carapace, Leeches did not
exceed a length of approximately 1 cm when quiescent, and did not ap-
pear to discomfort the turtles; they were easily detached.

Of 48 dissected T. coahuila, 46% contained from 1 to 5 small, un-
identified nematode worms in the stomach, some of which were im-
bedded in the lining. A total of 68 nematodes was in 22 stomachs, and
averaged 2.4% of the volume of material in stomachs possessing them.
Nematodes were in 42% of the intestines examined; one individual con-
tained 104 and another 41. Nematodes in the latter turtle were matted
together in two compact aggregations. Esch and Gibbons ( 1967)
studied nematode parasitism in Chrysemys picta, reporting infection
rates of 31 to 78% in mature individuals. Sex and age of the host, water
temperature, and season of the year influenced the rate of infection.

INJURIES AND PREDATION

Injuries were noted in 24 of 218 ( 11%) T. coahuila examined in the
field and in the laboratory. Of these 7 ( 3%) were burn scars, 6 ( 3%)
were limb amputations, and 6 ( 3%) were scars on the shell.

Grass burning is practiced in the basin of Cuatro Ci6negas. M. A.
Nickerson ( pers. comm.) reported considerable burning in the basin in
late March and early April 1969. On 28 March three small 100-400 m2
areas that were charred black by recent burning were noted, one of
which was at the T. coahuila study area ( see «Mortality and Replace-



30 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

ment"); on 2 April a small fire was burning near the study area, and
Nickerson saw a large fire, estimated to have burned "many thousands
of square meters, in the east-central region of the basin.

Burn scars on the carapace of T. coahuila usually covered between
one-fourth and one-half of the surface and consisted of rough-textured,
regenerated epidermis, recently-exposed underlying bone, or raised
patches of dead bone sloughing from an old wound. Legler ( 196Ob)
described worn patches of enamel-like, shiny bone during shell regenera-
tion in burned T. ornata. Similar areas of exposed bone were on five of
seven burned specimens of T. coahuila. The worst burn injury recorded
was in a subadult male ( ASU 5854) collected in July 1965 near the
study area. All epidermal scutes of the carapace had been burned away,
and the exposed bone was smooth, polished, and lacking noticeable
sutures. The epidermis of most of the marginal scutes was loose and
peeling away. Despite its injury the turtle appeared healthy, and its
stomach contained food.

Amputations and some carapace scars probably result from attacks
by predators. Four of five adults had one hind limb missing, and the
other lacked its right foreleg. A post-hatchling ( ASU 8000) lacked most
of its right hind foot and the right posterior portion of its carapace was
gouged away. Four individuals had long shallow gashes through the
epidermis of the shell that may have been inflicted by some large preda-
tor. One male had a 4-cm gash on the second left lateral scute, and sev-
eral small pock-like scars, possibly tooth marks, on the carapace and plas-
tion. In addition, the right hind leg was missing. The coyote, Canis la-
trans, could inflict wounds of this nature and possibly succeed in preying
on some turtles. They are not common diurnally in the region. Only one
was seen crossing the study area during the summer of 1965. Minckley
( 1966) described a coyote catching a large Pseudemys scripta taylori
in a shallow lake ( Laguna Grande) in the Cuatro Ci6negas basin, and
found a live T. coahuila that was thought to have been attacked and
chewed at the same locality.

Coahuilan box turtles are extremely alert while foraging and, in
addition to protective coloration, seem to rely considerably on rapid
movement and escape for survival. Disturbing a foraging turtle usually
made it stop and raise its head, and it then remained motionless for
several minutes. Another movement by the intruder usually made the
turtle withdraw its head and limbs into the shell and remain motionless.
Not infrequently a turtle moved away rapidly and burrowed into the
mud. Some escaping individuals thrust themselves so vigorously into
the mud that the rear of the shell and hind legs tilted upward at an
angle.



1974 BROWN: Terrapene coahuila ECOLOGY 31

When T. coahuila were handled in the field, they all pulled the
lobes of the plastron tightly against the carapace and remained closed
until left undisturbed for several minutes. There was no variation in
this reaction. Nichols ( 1939b) and Legler ( 196Ob) have noted that
some T. c. carolina and T. o. ornata struggle to escape while handled,
whereas others close their shells and remain passive.

THERMAL RELATIONSHIPS

Cloacal temperatures of T. coahuila captured in the field were re-
corded with a Schultheis quick-recording mercury thermometer gradu-
ated in 0.2°C divisions. Even after several minutes of handling, cloacal
temperatures did not change noticeably and so were not apparently
affected by conduction of heat from my hand to the animal's body.
Fitch (1956) noted this in recording body temperatures of small am-
phibians and reptiles.

Environmental temperatures recorded were: (1) water tempera-
ture at a depth of 1 to 2 cm at the site of capture ( measured immedi-
ately after obtaining the cloacal temperature ); (2) air temperature
measured with a dry thermometer approximately 50 cm above the sub-
strate near the site of capture, with the thermometer bulb shaded from
the sun; and, when applicable, (3) mud substrate temperature beneath
the water at the capture location.

Terminology follows Cowles and Bogert ( 1944) and Brattstrom
( 1965) for the voluntary minimum and maximum, normal activity range,
and preferred or optimum temperature. The optimum body tempera-
ture is, in practice, the mean body temperature within the normal activ-
ity range; the voluntary minimum and maximum are, in practice, the
lowest and highest body temperatures, respectively, recorded for free,
active animals ( Brattstrom 1965).

In the field 254 cloacal temperatures were secured. Almost 90%
were accompanied by a simultaneous reading from the water in which a
turtle was found. The remaining temperatures were from animals in
terrestrial situations, such as on dirt roads or dry ground at the edge of
marshes. During July and August 1965 air temperatures were recorded
only for turtles captured on land, and were seldom obtained with turtles
found in water. Air temperatures were recorded regularly in December
1965, January 1966, and April 1966.

Acc[vrry TEMPERATURE.-TWO hundred cloacal temperatures and

corresponding water temperatures were recorded. A highly significant
regression ( P<0.01) exists between a turtle's cloacal temperature and



32 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

that of the water in which it was active. Brattstrom ( 1965) and Edgren
and Edgren ( 1955) reported cloacal temperatures of Sternotherus odor-
atus closely approximating the surrounding water, and Boyer ( 1965)
noted body temperatures of aquatic turtles in water were nearly identical
to water temperatures.

Of 121 T. coahuila caught in marshes in July and August 1965, cloa-
cal temperatures of 114 were slightly different from water temperatures.
Approximately half the temperatures varied from 0.1 to 1.7°C greater
than water, and about half had an identical range below the water
temperature. A trend for cloacal temperatures to be slightly lower than
the surrounding water in the morning was apparent, but during the after-
noon most individuals were warmer than the medium. Only 21% of 45
turtles between 6:00 and 8:00 AM had cloacal temperatures higher than
water, whereas between 4:00 and 7:00 PM, 64% of 69 turtles had tem-
peratures above that of the water. This may result from the more in-
tense afternoon sunlight and an increasing heating effect of light waves
as the angle of incidence becomes greater. Boyer ( 1965) found angle
of incident light to be a factor in increasing heat gains of turtle models,
and noted that turtles of the genus Pseudem!/s orient while basking to
receive maximum heat absorption through a more direct angle of in-
cidence. Moll and Legler ( 1971) reported that a basking Pseudemys
scripta changed its antero-posterior orientation as much as 360° in one
hour. In marshes almost all active T. coahuila were in shallow water
with the carapace dry and exposed to sunlight.

During December 1965 and April 1965 and 1966, approximately
two-thirds of the cloacal temperatures were higher than water tempera-
ture. Differences ranged from 0.2 to 1.3°C in December and from 0.1
to 3.4°C in April, but the mean cloacal temperature in December does
not reflect this trend, being slightly less than the mean water tempera-
ture ( Fig. 9). In several instances turtles that apparently had recently
emerged from deep in the mud had body temperatures as much as 3.3°C
lower than surface water temperatures, thereby lowering the mean. The
same situation obtained in January 1966, when temperatures in 9 of 10
turtles varied from 0.1 to 3.0°C below that of the surrounding water.
Three emerging T. coahuila had cloacal temperatures 2.7°, 2.9°, and
3.0°C less than that of the surface water.

Although cool, all days during December 1965 were clear and
sunny; air temperatures averaged 17.6°C. In contrast, 3 of the 4 days
on which turtles were captured in January 1966 were overcast, and the
air averaged 11.1°C. Some differences are to be expected, therefore,
between temperature data from these two winter months. Active turtles,
with carapaces exposed to air, probably are affected by low air tempera-



1974 BROWN: Terrapene coahuita ECOLOGY 33

1 1 1 A (10)

JANUARY FmphpmA C (to)

1 1 1 W (10)

/-7 A (28)

APRIL mmamm C ( 40)
W (40)

C (78)
JULY 07 W (78)

C (60)
AUGUST rt-1 W (60)

A (12)

DECEMBER C (12)

r-1-1 w (123
It 1 1 1
5 10 15 20 25 ' 30 35 40

TEMPERATURE (°C.)

m=j= C (22)
6-7 PM I I 1 W (22)

C ( 36)
5 - 6 PM F-7 W (36)

C (16)
4-5 PM li w (B)

C (31)
7- 8 AM r-»7 W (31)

C (16)
6-7 AM I 1 1 w (16)

lili 'lilli
20 25 30 35

TEMPERATURE (°C.)
FIGURE 9.-Temperature relationships of T. coahuila active in marshes. Vertical
and horizontal lines represent mean and range of observed variation, respectively.
Blocks represent 95% confidence limits ( approximately + 2 standard errors);
number of records in parentheses. Top: Seasonal variation in cloacal (C) tempera-
tures during five months of year, showing relationship with water (W) and air (A)
temperatures recorded at times of capture. Bottom. Daily Huctuation in water (W)
and cloacal (C) temperatures during July and August 1965.



34 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 7, WATER AND T. coahuila CLOACAL TEMPERATURES.

Water Cloaca
Month Mean + 1 SE Extrennes Mean zE 1 SE Extremes

July ( 78) 1 28.08 + 0.30 20.7-- 32.6 28.14+0.30 20.1 - 32.7
August ( 60) 27.84 =t 0.38 21.2-- 32.5 27.94* 0.38 20.9 - 32.6
December ( 12 ) 22.08 =t 0.79 17.0- 26.6 21.93 t 0.79 16.3-26.6
January ( 10) 21.75* 1.12 15.0 - 26.9 26.67 + 1.12 14.8 - 26.7
April ( 40) 25.74 IE 0.65 19.5 - 34.4 25.80+0.65 18.8 - 33.5

Hour2
6-7 AM ( 16) 25.47 + 0.56 20.7-30.1 25.12* 0.49 20.8 -- 29.8
7-8 AM ( 31) 25.93 + 0.40 20.8 - 30.5 25.68 * 0.36 20.1 - 30.4
4-5 PM ( 16) 30.51 t 0.56 28.2-32.6 30.88 E 0.49 28.5- 32.7
5-6 PM ( 36) 29.33 IE 0.37 26.2 - 32.1 29.53:!: 0.33 26.9 - 31.8
6-7 PM ( 22) 28.12+0.47 24.8 - 31.0 28.39 + 0.42 25.4 - 30.7

1 Number of records in parentheses.
2In July and August 1965.

tures. Clouds can effectively reduce insolation and rates of heat gain
( Boyer 1965).

Seasonal variation in the normal activity range of T. coahuila in
marshes is evident ( Table 7, Fig. 9). Cloacal temperatures in Decem-
ber and January are significantly lower than those in April, while the
April temperatures are significantly lower than records in July and
August. A voluntary minimum temperature of 14.8°C ( January) and a
voluntary maximum of 33.5 °C ( April) were the extreme cloacal tem-
peratures recorded at all seasons within marshes. These give the approxi-
mate limits of the normal activity range in shallow water. Cloacal tem-
perature variation closely follows the seasonal changes in marsh water
temperatures brought about by over-all climatic changes of the area.
Mean monthly air temperatures for Cuatro Cic#negas ( Contreras Arias
1942) show December and January as the coldest months, with July and
August as the hottest, and with April intermediate between the two.
With data presently available showing such wide seasonal fluctuations,
it is doubtful that calculation of an optimum or preferred cloacal tem-
perature would be meaningful for T. coahuila. Moll and Legler ( 1971)
reached essentially the same conclusion regarding Pseudemys scripta in
the tropics.

During July and August 1965, when most data were taken on T.
coahuila, records were divided into 1-hour periods to test variation in
cloacal temperatures at different times of day ( Table 7, Fig. 9). The
difference in mean cloacal temperatures between the time periods is
highly significant (F= 33.2, P<0.01 ). Cloacal and water temperatures
were lowest between 6:00 and 7:00 AM, the period shortly after sun-
rise, before insolation raised water or cloacal temperatures. Maxima



1974 BROWN: Te.apene coahuila ECOLOGY 35

for the time periods indicated were attained between 4:00 and 5:00 PM
( Table 7). Cloacal temperatures of Pseudemys scripta in Panamd
similarly varied with time and changing water temperatures throughout
the day ( Moll and Legler 1971). The few records available for midday
are highest of all cloacal temperatures recorded from T. coah#ila within
marshes. The paucity of records during these intervals seems to reflect
a lowered level of box turtle activity.

Mean cloacal temperatures slightly lower than surrounding water
temperatures in morning hours, and higher than water in afternoon, may
indicate a lag of cloacal temperatures in comparison to progressively
increasing or decreasing temperatures of the water through the day.
Baldwin ( 1925) recorded a lag in cloacal temperatures of 1.5 to 3.0°C
when aquatic turtles of several species were exposed to changing water
temperatures.

In T. coahuila, a greater range of variation in both cloacal and water
temperatures in morning than in the afternoon ( Fig. 9) suggests more
variable microdimatic conditions in marshes in the former period, and
is a partial result of turtles at widely-varied stages of warming.

Ten cloacal temperatures were taken from turtles collected from
pools of the Rio Mesquites ( Fig. 6) in July and August 1965. All were
recorded in late morning or early afternoon, and temperatures of both
water and cloacae were high. On 30 July, under overcast conditions,
water temperatures were near 33.0°C; cloacal temperatures of five turtles
ranged from 31.4 to 33.6°C.

Sinkholes ( Fig. 5) are generally fed by thermal springs and have
the highest water temperatures of all aquatic habitats sampled. Two in
the study area ranged from 30.9°C in January 1966 to 34.7°C in October
1966 ( mean of six records, 32.1°C), and from 32.8°C in January 1966
to 34.2°C in July 1966 ( mean of six records, 33.5°C). Temperatures in
these habitats are probably always above 30.0°C throughout the year.
On 31 July 1965 a female registered 34.1°C, the highest voluntary cloacal
temperature of any T. coahuita recorded while in water ( 34.0°C). Two
other cloacal temperatures of turtles in posos ( August 1965) were 33.4°C
and 32.1°C, each 0.1° above the water temperature. Another individual
seen 14 October 1966 in a sinkhole ( water 34.0°C) was not caught.

T. coahuila on sedge tussocks within marshes, at the edge of a
marsh, or in the grassy terrain away from water, provided an opportunity
to note the effects of a thermal environment uncommon to T. coahuila.
A female on 6 July 1965 at 6: 15 AM on a dry sedge clump, had a cloacal
temperature of 20.3°C (air 21.4°C); her lower temperature presumably
reflected the cool soil beneath her. Another female at the edge of a
marsh the previous day at 6:30 AM had a cloacal temperature of 21.8°C



36 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

( air 23.5°C ). A female found resting in a horse path 4 August 1965 at
6:25 AM had a cloacal temperature of 22.4°C (air 21.1°C). The
morning was cloudy, cool, and damp, with a brisk easterly wind. On
the afternoon of another overcast day, 12 July 1965, a turtle in grass
near a marsh, shaded and on a cool substrate, had a cloacal temperature
of 28.8°C ( air 31.6°C). The highest cloacal temperature recorded
during this study was 34.5°C in a male on 23 August 1965 at 5:25 PM
in grass about 50 m from the nearest marsh. The weather was clear,
air temperature 32.9°C. This body temperature was approaching the
upper critical voluntary thermal level for the species ( see below).

Although Heath ( 1964) has stressed that thermoregulation cannot
be definitely ascribed to an animal whose activities prior to measurement
are not fully known, field evidence suggests that T. coahuila does exhibit
basking behavior. The most convincing data for elevation of body
temperatures by basking were obtained on 21 December 1965. Air
temperatures at midday varied between 17.0 and 19.1°C under a clear,
sunny sky with a moderate southerly breeze. Three turtles caught on
land had cloacal temperatures considerably higher than air temperatures
(mean difference 10.8°C; range 6.5-14.O°C). The mean cloacal tem-
perature for these three turtles was 29.2°C, while three other individuals
in water at the same time had cloacal temperatures 5.0 to 9.00£ lower,
corresponding to the water temperatures.

In April 1965 and 1966, mean air temperature at times of turtle
captures was 27.2° C (slightly, but not significantly greater than marsh
water temperatures ) ( Fig. 9). All but 2 of 6 days during April 1966
were sunny and clear, probably contributing to the mean cloacal tem-
perature being slightly above that of the water. On the morning of 7
April 1966, three of five turtles captured had cloacal temperatures from
0.2 to 1.7°C higher than the water, despite a completely overcast sky
and light precipitation.

Data obtained in summer, like those from April, are less strongly
indicative of basking, but appear pertinent. Cloacal temperatures of
T. coahuila on a dry substrate in summer were raised at most only 2.4°C
above air temperatures. On 31 July 1965 at 8:05 AM, cloacal tempera-
ture of a female on dry ground at a marsh edge was 25.3°C, 2.1° above
the air temperature ( 23.2°C). The morning was hazy, with a slight
easterly breeze. Cloacal temperatures of two turtles also found on land
during the morning 2 days later were near the high prevailing air tem-
peratures. One had a cloacal temperature of 31.9°C (air 34.3°C), and
the other registered 33.3°C (air 32.9°C). These observations further
indicate that T. coahuila are capable of achieving elevated body temper-
atures, even under overcast skies. Changes in body temperature of T.



1974 BROWN: Terrapene coahuila ECOLOGY 37

coahuila, and levels of heating attained while basking, appear similar to
those reported for other aquatic species ( Cagle 1946; Boyer 1965; Bratt-
strom 1965; Sexton 1959b; Moll and Legler 1971).

DAILY AND SEASONAL ACTIvITY.-In summer, T. coahuila were active
from shortly after sunrise ( about 6:00 AM) until several hours after
dark ( 10:00 PM). Air temperatures were recorded in the study area
between 6 July and 9 August 1965 with two maximum-minimum ther-
mometers placed about 50 cm above the water surface in three marshes.
Between 6:00 and 7:00 AM, air temperatures averaged 21.7°C ( range
16.7-25.6°C, 34 records). Maxima ranged from 31.7 to 40.0°C (mean
of 47 records, 36.0°C). Minimum air temperatures came in the early
morning hours before daylight, ranging from 15.6 to 22.8°C ( mean of
49 records, 19.3°C). Environmental temperatures ( air and water) in-
creased to a maximum at midday or early afternoon. Few box turtles
were active in this period.

Legler ( 196Ob) noted that most T. ornata in Kansas remained in-
active at midday on hot summer days, and Penn and Pottharst ( 1940)
observed midday inactivity in summer for T. carolina mmor in Louisiana.
Two T. coa/mila on 11 July 1965 at 12: 15 PM were found under the
thick cover of a composite shrub and sedges, at the edge of a marsh.
Air temperature was 34.4°C, and cloacal temperatures of the turtles
were 29.1 and 29.5°C; shallow water beneath the plant cover was 28.6 ° C.
This retreat appeared to be in regular use, as it had a well-defined de-
pression in the mud. Two days later a third individual in this place at
5:00 PM had a cloacal temperature of 29.2°C, water 28.9°C.

A crude experiment was performed in the 8eld using an adult fe-
male T. coahuila to determine the approximate upper limit of tempera-
ture tolerance. The turtle was tethered on a string under direct sun-
light. After 16 minutes at a cloacal temperature of 36.6°C, the plastral
muscles contracted weakly and the turtle did not completely close its
shell when disturbed; after 22 minutes at 37.5°C it made almost no at-
tempt to close the shell. Gasping and frothing at the mouth appeared
early in the experiment. Distress was evident after 7 minutes, when
the body had reached approximately 35.0°C. This may approximate the
maximum temperature T. coahuila tolerates in nature. A similar upper
limit was reported for T. o. ornata; 37.4° C (mean of 17 records) repre-
sented the cloacal temperature when salivation ( frothing) began ( Rie-
desel et al. 1971). Although I conducted no experiments to determine
the critical thermal maximum ( CTM) of T. coahuila, body temperatures
near 40.0°C in all likelihood approach the lethal level. Hutchison, Vine-
gar, and Kosh ( 1966) report CTMs ranging from 42.5 to 43.0°C in T.



38 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

carolina, and give a mean CTM of 41.6°C for several species of semi-
aquatic emydids.

Water temperatures during summer at midday may often be above
tolerable levels for T. coahuila. High water temperatures in the study
area were recorded on 25 July 1966 between 12:30 and 1:45 PM. The
day was clear and hot, despite a brisk easterly wind. Air temperature
was 35.0°C. Water temperatures were 37.8°C in marsh 6 and 35.3°C
in marsh 2-A. On 14 October 1966 at 1: 15 PM open water in marsh 8
reached a temperature of 38.8°C. Such maxima are probably not tol-
erated by T. coahuila, and no box turtles were found in the marshes
where they were recorded. Just 5 or 6 cm below the water surface,
temperatures of mud bottoms were approximately 5.0°C cooler than
water. Mud temperatures drop steadily with depth, providing a cool
refuge into which a turtle can readily retreat by burrowing to avoid
potentially harmful temperatures of the shallow waters above.

In addition to avoidance of high temperatures on sunny days, T.
coahuila seem to alternate periods of activity with longer periods of rest.
One male remained in uninterrupted quiescense for nearly 4 days. I re-
corded the turtle at 7:45 AM on 20 July 1965 and watched it dig into
the base of a sedge clump. The tussock was checked each morning and
evening of the following days to verify the turtle's presence. It remained
there, well covered by vegetation and in shallow water, until the night
of 23 July or the early morning of 24 July. Resting for several days and
then resuming activity was a conspicuous feature of T. c. carolina be-
havior ( Stickel 1950). Legler ( 196Ob) noted that some T. 0. ornata I
were steadily quiet for several consecutive days in summer.

Unlike other box turtles, T. coahuila is to some extent nocturnal,
although Webb et al. ( 1963) observed that T. coahuila in captivity were
inactive after dark. Marshes in the study area were visited on three
nights in the summer of 1965, and on two occasions ( 5 and 18 July) four
active box turtles were found between 8:50 and 10:00 PM. Their activ-
ity was seemingly identical to that during the day; cloacal temperatures
ranged from 23.5 to 27.4°C. No turtles were sighted between 11:30 PM
and midnight on 3 August 1965. Breder ( 1927), Allard ( 1935), Cahn
( 1937), and Stickel ( 1950) noted no nocturnal activity in T. c. carolina,
and Legler ( 196Ob) showed by the use of thread trails that the activity
of T. o. ornata ended at dusk.

Winter activities of T. coahuila may be curtailed by low water tem-
peratures, except in thermal waters. On 28 December 1964 at Posos de
la Becerra, then a series of large marsh pools 13 km SW of Cuatro
Cidnegas, seven T. coahuila were found in shallow ( 20 cm) water.
Air Temperatures ranged from 10.0 to 15.0°C and water from 7.2 to



1974 BROWN: Terrapene coahuila ECOLOGY 39

16.5°C from about 8:00 to 11:00 AM. The box turtles appeared lethar-
gic when captured, most being partly buried in the flocculent bottom
material. Body temperatures ranged from 7.2 to 12.4°C, mean 10.0°C.
During December 1965 and January 1966 I caught no T. coahuila with
a body temperature lower than 14.8°C.

A rudimentary laboratory experiment using adult T. coahuila of
both sexes was performed to determine the approximate lower tempera-
ture limit that can be tolerated before body movements are impaired.
Turtles were placed in a water bath of crushed ice at 0°C. When
completely torpid, they were removed and placed on a dry surface at
room temperature and allowed to warm slowly. At the first successful
forward movement, cloacal temperatures ranged from 6.0 to 16.4°C in
12 trials. All temperatures except the extremes were in the range 10.2
to 14.4° ( mode 12.0°C, mean 12.2°C). These data indicate the possible
minimum effective temperature for movement in T. coahuila as around
12.0°C. The minimum temperature tolerated voluntarily may be 14.0 to
15.0°C. Cagle ( 1946) estimated the minimum effective temperature for
activity in Pseudemys scripta to be about 10°C. Ernst ( 1967) found
several pairs of Clemmys guttata copulating in 8.5°C water in Pennsyl-
vania; cloacal temperatures of the turtles ranged from 8.0 to 10.1°C.

Diurnal activity follows ambient water temperatures more closely
than air temperatures. Many nights in December, January, and Feb-
ruary are cold (air 0°C or below). Although I recorded no evening
water temperatures from the marshes in winter, Contreras Arias ( 1942)
gives average minimum air temperatures of 9.0,7.6, and 9.9°C and ex-
treme minima of -2.0, -0.3, and - 1.0°C, respectively, for these
months. Such low air temperatures could conceivably drive surface
water temperatures below that voluntarily tolerated by T. coahuila,
forcing them into the mud bottom of marshes where temperatures would
most likely be higher. Many turtles may undergo temporary states of
inactivity at night as a result of the cold, especially from December
through February.

FOOD AND FEEDING

FORAGING BEHAVIOR.-Practically all turtles captured were active.
Inactive turtles were found infrequently, usually concealed under a
sedge tussock, beneath mats of stonewort, or in the soft mud marsh bot-
tom.

Two marshes provided suitable places for studying foraging turtles.
The south and west edges of marsh 11 were relatively open, having areas
of shallow water with scattered Eleocharis closely cropped by horses. A



40 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

f· · - -1-lt=*3»tr'i-'-" r

V"7911%4113'Lors:Er-
ES; =.Lp

nal,A /
NNa@lk V :, - ir,

----- -1.m-01
8/2.819&"I'lae/g

4 -r  '„„B-~/~TWV *Cat~~

FIGURE 10.-Female T. coahuila foraging in a marsh near the study area, 24 August
1965. Nine mosquitolish, Gambusia marshi, are swimming near the turtle.

second marsh was essentially open and shallow with beds of Chara and
grazed Eleochans around the perimeter, offering an unusually good
situation where turtles could be watched easily with binoculars. Feed-
ing turtles could sometimes be approached to within a meter.

Water depth generally varied between 2 and 6 cm so that much of
the turtle's carapace remained above water level and was dry. The
animal nearly always held its head below the water surface as it moved
forward, apparently scanning underwater. Forelimbs were frequently
used to move Chara and basal portions of Eleocharis to the side while
the head entered the cleared area or the turtle nipped at portions of up-
rooted plant material or exposed aquatic animals. Typically the turtle
occasionally raised its head out of the water, neck extended, as if sur-
veying its surroundings ( Fig. 10). These pauses usually lasted only a
few seconds, but might be longer if an intruder was suspected.

The Bog Turtle, Clemmus muhlenbergi, in northeastern U.S., in-
habits Sphagnum moss-Carex sedge marshes similar in life form to the
Chara-Eleocharis marsh habitat of T. coahuila. Intersecting rivulets in
the moss provide runways for C. muhlenbergi ( Barton and Price 1955).



1974 BROWN: Terrapene coalluita ECOLOGY 41

Sexton ( 1959b) described foraging of Chrysemys picta on pond surface
vegetation mats in which their carapaces remained above the water and
their heads extended forward beneath the water. Patterns of foraging
of these two species are notably like those of T. coahuila, which appears,
on this basis, to be as well-adapted as do these, and possibly other,
aquatic emydids.

On 22 December 1965 at 2:35 PM, a female T. coahuila was feed-
ing at the basal portion of an Eleocharis clump that was raised above
a small pool of shallow water. This turtle pawed and bit at the roots
and mud of the clump apparently eating plant material. Cloacal tem-
perature was 23.1°C, water was 22.7°, and air 21.4°C.

Between 21 and 29 July 1965 I made 10 observations of foraging
turtles; 4 were between 6:30 and 7:30 AM and 6 between 4:45 and 7:25
PM on different days. Morning and evening foraging patterns did not
seem to differ. Cloacal temperatures varied between 26.3° and 32.3°C.

A female moved in shallow water, pushing with its forefeet at the
edges of clumps of vegetation and biting at the base of Eleocharis and
mati-bf Chara. Several times she climbed partly out of water, pulling
apart plant material and muddy debris with her forefeet, biting at sedge
stalks thus exposed. For approximately 20 minutes this individual
foraged in an area of only 25-30 cm on a side. Another individual
moved approximately 3 meters in 45 minutes while foraging among
clumps of Eleocharis. A male climbed partly out of water onto a raised
patch of Eleocharis and stalked and suddenly lunged at an unseen
object in the vegetation. This individual foraged with its head extended
underwater for periods up to a minute, largely motionless. One indi-
vidual snapped and tugged on plant material with such force that the
body jerked with each effort to pull the material free. Frequent turns
while following narrow channels though the vegetation, use of the fore-
legs to expose places for feeding, and occasional pauses to survey the
surroundings typifed all observations of foraging T. coahuila.

DIET.-Literature references to the food of T. coahuila are few.
Williams ( 1960) noted that captives ate dead or live sunfish and roaches.
Webb et al. ( 1963) stated that T. coahuila are omnivorous and scav-
engers on the basis of a wide variety of foods consumed by captives.

Food items were identified to order and family, and, where possible,
to genus and species. Plant material was lumped into a single category
for volume determination. Because items were often partially digested
and fragmentary, no attempt was made to count individuals or measure
volumes of organisms from the intestines. Methods of presentation of
data follow Larimore ( 1957 ): (1) percentage of stomachs in which
each kind of food occured ( frequency of occurrence ); (2) mean of the



42 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

DIPTERA

19.6 %

PLANT COLEOPTERA
456% 11.7%

ODONATA
11.3%

OTHER 1.2%7/1 1 1 f HEMIPTERA 3.2%

ARACHNIDA 1.2% ~ ~ EPHEMEROPTERA 2.4%

CRUSTACEA 1.3 % MISCELLANEOUS INSECT 2.5%
FIGURE 11.-Composition of diet by percentage of total volume of food in stomachs
of 45 T. coahuila.

percentage of volume comprised by each of the kinds of food in stomachs
containing the food ( individual volume ); and (3) percentage of the
total volume represented by each kind of food.

Three stomachs were empty and calculations presented in Table 8
and shown graphically in Fig. 11 are based on 45 stomachs containing
food. The total volume of all food items in the stomachs ranged from
0.19 to 2.54 ml, mean 0.92+0.09 ml. Volumes of intestinal contents
were not measured, but most appeared considerably greater than stom-
achs.

Based on percentage of total volume, insects ( 50.7%) and plant
material ( 45.6%) are by far the most prominent components of the diet
( Fig. 11). Crustaceans ( 1.3% ), spiders ( 1.2%), and Bshes ( 0.5%) are
relatively much less frequent. Plant material was found in 84.4% of
the stomachs and consisted of the following kinds with the frequency
of occurrence of each: Eleocharis rostellata 64.4%, mushroom 15.6%,
Chara spp. 11.1%, and large unidentified seeds 6.7%.

Several groups of insects are prominent in the diet. Of the 29 insect
families identified ( Table 8) 19 have one or more aquatic stages in



1974 BROWN: Terrapene coahuila ECOLOGY 43

TABLE 8. QUALITATIVE AND QUANTITATIVE ANALYSIS OF FOOD UTILIZED BY T.
coahuila.

Stomach (n=45) Intestine (n=48)
Frequency Mean % %

of Individual Total Number in
Taxon Occurrence Volume Volume which found

Plant material 84.4 - 45.61 48
Ephemeroptera

Baetidae (n. ) 1 20.0 9.0 ( 1-24) 2.31 0
unidentified (n. & a.) 4.4 2.2 ( 2-3) 0.04 2

Odonata
Aeshnidae ( n.) 2.2 13.1 0.27 0
Agrionidae ( n. ) 24.4 8.1. (2-23) 1.84 14
Comphidae (n.) 4,4 27.3 (20-35) 1.09 4
Libellulidae (n.) 26.7 30.1 (tr.-96) 7.14 10
unidentified (n.) 2.2 24.5 0.94 2

Orthoptera
Acrididae (n. & a.) 4.4 13.3 ( 12-14) 1.07 1

Hemiptera
B3lostomatidae ( a. ) 2.2 23.9 0.56 3
Nabidae ( a.) 2.2 4.5 0.05 0
Naucoridae ( n. & a.) 24.4 10.5 (1-41) 2.03 30
Nepidae (a.) 0 - - 2
Notonectidae (a.) 0 - - 1
Pentatomidae ( a.) 2.2 31.7 0.48 1
Podopidae (a.) 0 - - 2
Veliidae ( a.) 4.4 0.9 0.05 2
unidentified (n. & a.) 8.9 0.7 0.04 5

Coleoptera
Curculionidae (a.) 24.4 25.2 ( tr.-98) 8.98 22
Chrysomelidae ( 1. ) 0 - - 1
Dytiscidae (1. ) 4.4 23.9 ( 4-44) 0.44 5
Dytiscidae ( a. ) 6.7 1.9 ( tr.-5) 0.04 1
Hydrophilidae ( a. ) 11. 1 6.9 ( 1-25) 1.57 8
Limnebiidae ( a.) 4.4 2.9 0.06 0
Oedemeridae ( a.) 4.4 65 (4-9) 0.17 0
Staphylinidae ( a. ) 2.2 0.5 0.02 0
Tenebrionidae ( a.) 2.2 3.3 0.12 0
unidentified ( a. ) 2.2 9.1 0.27 8

Trichoptera
Hydroptilidae ( 1.) 4.4 0.9 0.07 0

Lepidoptera
Microlepidoptera ( a.) 2.2 0.9 - 0
unidentified (1. ) 2.2 14.3 - 1

Diptera
Chironomidae (1.) 6.6 0.7 0.04 0
Culicidae

Chaoborinae (1.) 15.6 0.9 ( tr.-2) 0.14 0
Culicinae (1.) 2.2 0.3 tr. 0

Culicidae (p.) 6.7 0.9 0.06 0
Ephydridae (p.) 0 - - 1
Stratiomyidae (1.) 64.4 29.9 ( 3-99) 18.59 39
Stratiomyidae ( p.) 2.2 6.3 0.24 0
Tipulidae ( 1.) 6.7 8.8 (2-18) 6.46 0
unidentified C all stages) 6.7 0.4 0.04 2



44 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 8.-Continued

Stomach (n=45) Intestine (n=48)

Frequency Mean % %
of Individual Total Number in

Taxon Occurrence Volume Volume which found

Hymenoptera
Apoidea ( a.) 2.2 1.2 0.07 1

Formicidae ( a.) 8.9 7.6 ( 1-28) 0.53 13
unidentified Insecta

( all stages) 11.1 3.6 ( tr.-11 ) 0.50 9
Arachnida

Araneae-Argiopoidea 8.9 14.9 ( 1-44) 1.21 0
Crustacea

Amphipoda
Talitridae

Hyaletta
azteca 17.8 5.5 ( tr.-36) 0.45 3

Isopoda
Armadillidiidae

Armadimdium
vulgare 4.4 16.9 ( 14-20) 0.46 4

Cirolanidae
Sphaerolana

interstitialis 2.2 35.7 0.24 1
Ostracoda

Cypridae 22.5 0.9 ( tr.-2) 0.14 13
Mollusca

Gastropoda
Hydrobiidae

Durangonella sp
& 15.6 0.4 ( tr.-2 ) 0.04 9

Paludiscala sp.
Osteichthyes

Cyprinodontiformes
Cyprinodontidae

Cuprinodon
atrorus 2.2 47.4 0.22 1

Poeciliidae
Gambuda

marshi 2.2 5.5 0.31 0
Serpentes

Colubridae
N atrix

erythrogaster 0 - - 1

1 Insect stages abbreviated: n., nymph; 1., larva; p., pupa; a., adult.

their life cycles. Stratiomyid fly larvae were the most frequent animal
food in the diet, occurring in nearly two-thirds of all stomachs contain-
ing food, and were important on a volume basis ( nearly one-ffth of
total volume). Many stratiomyid species live in shallow stagnant pools
or in mud. Adult curculionid beetles were in almost one-fourth of stom-
achs and were an important component by volume. Although the adults



1974 BROWN: Terrapene coahuila ECOLOGY 45

are not truly aquatic, the larvae of some of these beetles live in the stems
or roots of aquatic plants. Libellulid dragonfly and agrionid damselfly
nymphs were present in more than one-fourth of the stomachs. Odonata
adults are seen frequently in and around marshes in the study area.
Naucorid nymphs and adults made up the bulk of the hemipterans re-
corded. These bugs may be easier for box turtles to catch, as they move
more slowly through submerged vegetation than other groups of aquatic
hemipterans occasionally eaten. Baetid mayRy nymphs occurred in one-
fifth of the stomachs.

Most of the insects T. coahuila ate were presumably obtained from
the water, with the exception of curculionid beetles and other terrestrial
forms encountered rarely during overland movements or possibly after
having fallen into the water from overhanging vegetation.

A total of 167 amphipods was found in 8 turtles, giving a mean of
20.9 amphipods for those stomachs in which they occurred. The mean is
strikingly high because of a single individual foraging in a low bed of
Chara on the bottom of a poso ( Fig. 5) in water about 25 cm deep;
its stomach contained several small fragments of Chara along with 141
amphipods, 1 cirolanid isopod ( Sphaerolana insterstitialis [Cole and
Minckley 1970] ), and remains of ostracods and small snails. Tiny
amphipods, Hyalella azteca, may represent a primary food item the
turtle was hunting in the Chara beds, but amphipods made up less than
0.5% of the total volume for all food items. On 4 April 1966 amphipods
occurred in 6 stomachs of 17 T. coa/mila collected from marshes, but
the number in any one stomach did not exceed 14, mean 4.2. Amphi-
pods were plentiful during April in the marshes and clung to the skin
of box turtles when they were removed from the water.

Of five box turtles collected on dirt roads adjacent to marshes, three
had food in their stomachs. In one individual the food did not differ
markedly from turtles taken directly from marshes. The stomach of
another contained 21 large curculionid beetles making up over 98% of
the individual volume, and the only recorded rove beetle ( Staphylini-
dae). The third contained 39 small ants, 19 tiny limnebiid beetles, a
grasshopper nymph, and a terrestrial isopod, Armadillidium oulgare.
The turtles probably ate these items while traveling overland between
marshes.

T. coahuila can apparently discriminate small objects, as indicated
by the abundance of such items as amphipods, mayfly nymphs, veliid
bugs, chaoborin midge larvae, ostracods, and ants. These groups, al-
though they occurred with a frequency comparable with the other taxa
listed, were not important items on a total volume basis. Average num-
bers of mayfly nymphs, ants, and limnebiid beetles indicate that they



46 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

were significant to certain individuals, but, when compared by this
method these items assume a greater apparent importance in the over-
all diet than is actually the case. Cyprinodontid fishes, cirolanid iso-
pods, pentatomid bugs, gomphid dragonfly nymphs, belostomatid bugs,
dytiscid beetle larvae, and grasshoppers were each found in only two
stomachs at most. These food groups were singularly important only
to a few individuals in which a single item frequently comprised a large
percentage of the individual volume in stomachs otherwise nearly lack-
ing any other food.

Little can be said regarding seasonal fluctuations in food habits of
T. coahuila because turtles were collected only in late summer and
spring. Frequency of occurrence, combined with the mean of the in-
dividual volume percentages for the July-August sample vs. the April
sample show the relative prominence of the various foods in the diet.
Curculionid and hydrophilid beetles were present in far greater numbers
in turtles taken in summer than in spring specimens. Ants were absent
in individuals taken in spring, but caddisily larvae and midge larvae
appeared more frequently in the spring sample. Stratiomyid larvae
occurred in more spring than in summer specimens, but made up less
of the stomach volume in spring.

More turtles in April had eaten large quantities of Eleocharis, prin-
cipally the seed heads. Intestines of four individuals were packed with
several hundred Eleocharis seeds. Barton and Price ( 1955) similarly
found a large number of Carex seeds in 11 Clemmys muhlenbergi they
examined from Pennsylvania.

Five individuals were collected in large, relatively deep ( 15-30
cm ) pools near Rio Mesquites ( Fig. 6). Of these, three had eaten one
fish each: two Cuprinodon atrorus and one Gambusia marshi. These
were the only fish found in the entire sample of 48 turtles. These and
other Bsh species were abundant in river pools where foraging box
turtles were often seen. In December 1964 at Posos de la Becerra,
Walter K. Taylor saw a male on land feeding on a dead cichlid fish,
Cichlasoma sp. Although Williams ( 1960) watched a captive T. coahuila
capture a live fish in shallow water, most fishes the turtles eat probably
are dead or dying. Only two regularly visited marshes contained fishes:
Gambusia marshi and G. longispinis in marsh 11; and G. marshi, G.
longispinis, C!/prinodon bifasciatus, and Cichlasoma spp. in marsh N-3.
Two turtles from marsh 11 preserved in July 1965 contained no trace of
fishes.

On several occasions T. coahuila were observed foraging in pools
along the river and in marshes while many small fishes ( Gambusia and
Cyprinodon) swam nearby, seemingly within their reach, but the turtles



1974 BROWN: Terrapene coahuila ECOLOGY 47

ignored them. My field notes on a turtle foraging in shallow water on
24 August 1965 are typical: Gambusia marshi swam to within 2 cm of
the animal's head, circling the turtle, and apparently searching for bits
of food material exposed in the mud stirred up by its activity. The
turtle paid no attention to the fsh." T. coahuila were never seen chasing
fishes.

Remains of a juvenile water snake, Natrix er!/throgaster, were in the
intestine of a female T. coahuila collected in August 1965. Aside from
fishes, this was the only record of predation on a vertebrate. The snake
seemed fresh and was probably eaten alive or right after having been
killed. None of the other reptiles and amphibians observed in the
marshes were present in any T. coahuila examined. T. ornata and T.
carolina sometimes feed on vertebrates, chiefly amphibians, lizards, and
snakes ( Babcock 1919, Eaton 1947, Norris and Zweifel 1950, Merhtens
and Hermann 1951, Klimstra and Newsome 1960, Hutchison and Vinegar
1963).

Of four species of emydid aquatic turtles Lagler ( 1943) examined
in Michigan, Emydoidea blandingi and Chrusemys picta showed food
preferences similar to T. coahuila in that the insects eaten were pri-
marily aquatic, immature stages of dragonflies and damselflies, aquatic
beetles and hemipterans. Insects accounted for 21.4% of the total vol-
ume of food in 66 E. blandingi examined, and 19.5% in 413 C. picta.
Chrusemys picta is more similar to T. coahuila in its extensive utilization
of various kinds of aquatic plants, which made up 61.5% by total volume,
whereas plants were a relatively insignifcant component ( 3.9%) in E.
blandingi.

Knight and Gibbons ( 1968) reported that a single river population
of C. picta in Michigan ate plants ( chiefly filamentous algae) in amounts
ranging from 30 to 40% of individual stomach volumes. This C. picta
population also ate large numbers of midge larvae and cladocerans,
demonstrating opportunism not unlike that shown by some T. coahuila
in their occasional extensive consumption of amphipods or Eleocharis
seeds. Webb ( 1961) recorded midge larvae, ants, caddisfly larvae, and
small hemipterans in stomachs of 8 map turtles, Graptemys pseudogeo-
graphica, in Oklahoma, and also found a specimen... gorged with
grasshoppers and one stomach filled with bermuda grass ( Cynodon)
and fogwort ( Lippia). As noted earlier, Clemmys muhlenbergi ap-
parently often feeds on seeds of sedges, and is also like T. coahuila in
consuming insects, but principally Lepidoptera larvae and beetles ( Bar-
ton and Price 1955).

Data of Klimstra and Newsome ( 1960) for Terrapene c. carolina in
Illinois ( plant material 34.2%, insects 19.6%) more closely resemble the



48 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

TABLE 9: MovEMENTs oF T. coahuila IN THE STUDY AREA.1

Within Marsh Between Marshes

n=14 n=7
Males 15.0 84.6

(3.0-50.0) ( 23.0 - 250.0)
n-27 n=6

Females 11.7 62.1
(2.5-51.0) ( 20.5 - 140.0)

n=41 n=13
Both Sexes 12.8 74.2

Combined (3.0-51.0) (20.5-250.0)

1 Measurements are mean straight-line distances in meters between successive points
of capture.

food habits of T. coahuila than do the data for T. o. ornata in Kansas
reported by Legler ( 196Ob). Scarabaeid and carabid beetles, noctuid
and arctiid caterpillars, and grasshoppers occurred most frequently,
with all insects accounting for an average volume of 88.6 % in stomachs
containing them; plant material from cattle dung ( in which T. ornata
foraged for food) averaged 20% ( Legler 196Ob).

T. coahuila is opportunistic and omnivorous, feeding extensively on
aquatic plants and insects. Variation in kinds and numbers of food
items from one turtle to another suggests that T. coahuila feeds on
whatever is available. The population studied more closely resembles
aquatic species of other genera in its food habits than it does the ter-
restrial species of Terrapene .

MOVEMENTS

Distance between 54 successive capture sites of T. coahuila were
measured. Field distances were recorded to the nearest meter and map
distances to the nearest half meter. Table 9 shows that 76% of recorded
movements were within the marsh where the animal was previously
marked, and 61% of the movements were by females. The mean dis-
tance for each movement of males and females in the same marsh is
not significantly different (P>0.30), so movements of the sexes were
combined to obtain a mean straight-line distance of 12.8 m between
points of capture.

Because a maximum of four movements was recorded for only two
individuals, and two movements for four individuals, the data do not
permit such refined home range calculations as the center of activity, the
mean recapture radius ( Hayne 1949a; Tinkle and Woodard 1967), or
the minimum area method ( Hayne 1949a). Fitch ( 1958) and Legler



1974 BROWN: Terrapene coahuita ECOLOGY 49

( 196Ob) both used a simpler ( and perhaps more vulnerable) method
to calculate home range size in T. o. ornata: the average distance be-
tween successive points of capture was assumed to represent the radius
of the home range. Using the same capture radii technique, Ernst
( 1970) found the home range of Clemmys guttata to be biased toward
a size over four times greater than the minimum area home range.

Despite the evident drawbacks of assuming the home range to be
circular, if 12.8 m is considered as the average home range radius of T.
coahuila, the mean diameter is 25.6 m. One factor that obviously affects
this assumption is the size of the marsh in which a turtle was recorded;
movements within a marsh would necessarily be restricted by its dimen-
sions. Straight-line movements of 10 individuals in the two smallest
marshes ranged from 3.5 to 15.0 m, mean 8.2 m. In the two largest
marshes 15 movements ranged from 4.0 to 28.0 m, mean 13.6 m. It is
possible for turtles to travel 50 to 130 m from opposite ends of the long
( north-south) axes in the largest marshes. The data show a slight dif-
ference, but the similarity of distances moved is more impressive and
indicates that T. coahuila in the study tract utilize areas of roughly
equal size regardless of the size of the marsh.

All marshes in the main study area are oriented in a northeast-
southwest direction ( Fig. 4) and so it is not surprising that 20 of 39
turtle movements within marshes were either northeast or southwest,
following the long axis of a marsh. Five were in the opposite directions,
northwest or southeast, nine movements were recorded as directly north
or south, while only five were east or west.

A fairly direct correlation exists between time separating captures
and distance traveled. Time between captures in the same marsh ranged
from 1 to 464 days, 61 percent of the intervals were less than 50 days,
and the mean distance of movement was 10.5 m. Of those animals
( n = 16) free for more than 50 days after marking ( average of 214 days),
the mean distance was 16.4 m.

Home range sizes have been estimated for several species of Terra-
pene. Stickel ( 1950) reported the average maximum diameter of the
home range for T. c. carolina in Maryland as 100.6 m for males and 112.8
m for females. The mean distance between successive points of capture
was 118.9 m for T. c. carolina in New York ( Nichols 1939c). Successive
capture distance of T. c. carolina in Indiana was 69.5 m, or an average
home range diameter of 139 m ( Williams 1961). Williams ( 1961) also
measured the maximum distance between any two farthest captures.
The result was 114.2 m, a mean home range diameter similar to that re-
ported by Stickel ( 1950). Mean distance traveled by T. carolina tri-
unguis in Oklahoma between successive hibernacula in successive years



50 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

was 49.4 m, and in the same year 51.2 m ( Carpenter 1957). If these
distances are considered as radii, the approximate home range diameter
is 100 m in the Oklahoma T. c. triunguis population. Distances between
captures of T. o. ornata in Kansas ranged from 22 to 278 m, mean 84.8
m; the mean home range diameter, then, becomes 166.5 m ( Legler
196Ob.)

The estimated home range size of T. coahuila is considerably smaller
than those of the terrestrial species of the genus that have been studied.
T. ornata, a prairie grassland species , utilizes larger areas than T. CaTo-
lina, a woodland species. Home range size and population density ( see
below) are seemingly closely correlated with habitat in the genus Terra-
pene.

Aquatic turtles, Chrusemys and Pseudemys. tend to remain in cer-
tain home areas within lakes or ponds, but may shift their ranges to more
favorable areas with changes in the immediate habitat ( Cagle 1944a;
Sexton 1959b; Emlen 1969). Few (less than 15%) of a population of
Chrysem!/s picta in a Michigan marsh moved farther than 100 m during
one summer ( Gibbons 1968c). Among a population of Pseudemys
scripta in Panama,. Moll and Legler ( 1971) reported mean lengths of
home ranges in hatchlings, juveniles, and adults as 34, 61, and 287 m,
respectively.

Foraging box turtles keep a web-like system of trails open in most
Cuatro Cidnegas marshes, thus maintaining a flow of water in the small
rivulets through the trails. As T. coahuila seldom move in a direct line,
but turn randomly within the rivulet reticulum, straight-line distances
between points of collection do not represent the animals' actual pat-
tern of movement.

T. coahuila occasionally enter large sinkholes ( Fig. 5), where they
easily elude capture by swimming along the bottom under 20 to 50 cm
of water and disappearing beneath the undercut banks. They remind
one of Sternotherus or Kinostemon in their rapid, elusive swimming
ability. Milstead ( 1967) referred to T. coahuila as an "awkward" swim-
mer, but I consider it remarkably agile.

Most T. coahuila tend to remain within a given marsh for relatively
long periods ( Table 10). Only 11 of 52 ( 21%) turtles in the main study
area moved from one marsh to another. If direct, the animals would
have crossed barren ground and most distances traveled would have
been less than 100 m. They could probably cross these stretches only at
times of day when temperatures would permit, as in the early morning
or late evening. Some turtles may have moved to new marshes by
following more indirect, connecting water courses. T. coahuila were



1974 BROWN : Terrapene coahuila ECOLOGY 51

TABLE 10. DURATION OF TIME SPENT IN ONE MARSH BY T. COalluita.1

Month in Number of Number Re- Mean Number of
Which Turtle New ( unmarked) Number captured in Months Elapsed
Captured for Turtles Captured Subsequently Same Marsh Between First &
First Time in Month Recaptured After 1 Yr. Last Captures

December 1964 13 9 ( 69%) 4 11.1
(4-19)

April 1965 11 5 (45% ) 5 9.6
(4-12)

July» 1965 61 20 ( 33%) 5 7.8
(1-12)

IData are from 34 turtles recaptured at least once in a later sampling period.

sometimes seen on land on overcast days. Webb et al. ( 1963) noted
that T. coahuila moved overland during rainy periods.

About one-fifth of the T. coa/mila recaptured in more than a year
and a half had made intermarsh movements. This suggests that some
individuals are either transients or shift their home ranges. Howard's
( 1960) hypothesis of innate vs. environmental dispersal could be readily
tested in this species. It should also be possible to determine what en-
vironmental factors are used in orientation during intermarsh dispersal.
Experiments of Gould ( 1957, 1959; see also Lemkau 1970) suggest that
T. carolina, when removed from their normal home range, employ sun
orientation possibly similar to the mechanism occurring in birds and
anurans. But Emlen ( 1969) reported that land-displaced Chrusemus
picta used visual recognition of local topographic landmarks to return
to their home pond; celestial navigation was all but totally discounted
over the short ( 100 m) experimental homing distances.

POPULATION

CoMposITIoN.-Through July 1966, 164 adult T. coahuila of known
sex were captured. Only three were juveniles, less than 2% of the
sample from the study tract population. This probably reRects their
cryptic coloration, small size, and possibly more secretive habits. For
example I discovered one juvenile only after seeing a slight movement
when it pulled its head into the mud beneath the water surface. Stickel
( 1950) and Legler ( 196Ob) found many fewer juveniles than adults in
populations of T. c. carolina in Maryland and T. o. ornata in Kansas.

Mean carapace length of 70 male T. coahuila ( 108.9 mm) is sig-
nificantly ( P <0.01) larger than that of 94 females ( 100.9 mm) ( Brown
1971 ). A comparison of carapace lengths of 8eld-caught turtles with



52 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

(30) ARTEM%WHNWi

15-

10 - - FEMALES

NU
M

BE
R 

OF
 

IN
DI

VI
DU

AL
S

0

(10) ES-*4/WdW
10 -

- MALES

5-

n n IM] 411, rr[-A n np n . .
80 90 ' 160 i,o 120 130 140 ISO

CARAPACE LENGTH (mm.)

FIGURE 12.-Frequency distributions of body size in 164 T. coahulla (70 males, 94
females) from study area. Bar diagrams show size of sexually mature individuals
examined in the laboratory. Horizontal and vertical lines represent range of ob-
served variation and mean, respectively; blocks represent 95% confidence limits.
Sample sizes in parentheses.

the size of sexually mature individuals of both sexes ( Fig. 12) shows a
close correspondence between the two: 73% of females and 77% of males
fall above the value of the lower 95% confidence limit for size at sexual
maturity ( 97.3 mm in females, 104.6 mm in males), and over 98% of
both sexes in the field were larger than the smallest sexually mature
individual in the preserved samples. Therefore most turtles captured
were either sexually mature or approaching maturity. Modes of both
sexes are within the 95% confidence ranges for size of sexual maturity
( females 95-105 mm, males 105-115 mm ). A majority of turtles, especi-
ally females, are clumped at the lower end of the confidence range ( Fig.
12). Cagle ( 1954) noted a similar concentration of Chrysemys picta
near the size of attainment of sexual maturity. Growth in C. picta is
apparently greatly reduced at that time ( Cagle 1954; Gibbons 1968b;
Ernst 197lb ). Legler ( 196Ob ) determined that growth in T. o . ornata
stops about 3 years after reaching sexual maturity in both sexes, and his
histogram of plastron lengths shows a close agreement with percentages
of sexually mature T. ornata; the greatest number of individuals fall
in the size groups having the largest proportion of mature individuals.

Of the 164 T. coahuila marked in the study area, 70 ( 43%) were
males and 94 ( 57%) wer females, a ratio of 1.00 male to 1.34 females.
The sex ratio may vary seasonally, although females did not significantly



1974 BROWN: Terrapene coahuila ECOLOGY 53

TABLE 11. SEX RATIOS OF T. coahuila MARKED IN THE STUDY AREA FROM
DECEMBER 1964 THROUGH APRIL 1966.

Males Females Male: Female
Month n % n % Ratio x2 P

December
&

January 14 50.0 14 50.0 1.00:1.00 - -
April 12 44.4 15 55.6 1.00:1.25 0.15 >0.70
July 28 43.7 36 56.3

 1.00:1.29 0.77 >0.30
August 16 35.6 29 64.4 1.00:1.81 3.20 >0.05
All Months

Combined 70 42.7 94 57.3 1.00:1.34 3.22 >0.05

outnumber males (as tested by chi square on the hypothesis of a 1:1
ratio) in any of the four monthly divisions of the season, or for the
entire span of collections ( Table 11).

In T. o. ornata Legler ( 196Ob) reported a male/ female ratio of
1.00: 1.69 among 164 adults in Kansas. Nichols ( 1939a) reports a male/
female ratio of 1.00:0.63 of 387 T. c. carolina from New York, but Stickel
( 1950) recorded a male/female ratio of 1.00: 1.09 in 245 adult T. c.
carolina from Maryland.

Gibbons ( 1970) reviewed sex ratios in some aquatic turtle popula-
tions. Apparent discrepancies in reports of Chrusemys picta, Malaclemys
terrapin, Pseudemys scripta, and Terrapene carolina are seemingly due
largely to one or more of the following factors: (1) seasonal changes in
activity, as in Malaclemys, where Cagle ( 1952) points out that the large
proportion of males ( 4.4 males/female) seemed to result from a move-
ment of females toward shore during the nesting season; (2) a misin-
terpretation of the sexual difference in size when secondary sex characters
appear or when the turtles become sexually mature; (3) different sam-
pling techniques employed ( Ream and Ream [1966] investigated this
factor, fnding significant differences between juvenile:male:female ratios
of C. picta in four of five sampling methods tested ); and (4) genetic
differences between ecologically isolated conspecific populations display-
ing little or no interpopulation gene flow ( Auffenberg and Weaver
[1969], for example, reported male/female ratios of 1.00:2.08, 1.00:2.00,
and 1.00: 1.15 in three populations of Gopherus berlandieri inhabiting
small, isolated hills [lomas] in extreme southern Texas).

Although the last factor may well be applicable to aquatic as well as
to terrestrial species, the second factor listed above appears to have
caused great disagreement in sex ratios reported in the literature, even
between populations of the same species not widely separated geograph-
ically. Cagle ( 1942) first pointed out that for species of the genera
Pseudemys, Chrysemys, and Graptemys, in which females reach ma-



54 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

turity at a much larger size than males, the sex ratio can be altered
( shifted upward in favor of males) by eliminating immature females
that overlap mature males in size. Thus, Sexton ( 1959b) reported an
"actual" male/female ratio of 1.00: 1.49 for 604 Chrysemys picta of known
sex in Michigan, but when he considered only sexually mature indi-
viduals, the ratio became 1.00: 0.76, closely approximating the 1.00: 0.78
ratio for a Minnesota population of the same species reported by Ream
and Ream ( 1966), who distinguished juveniles from adult males and
females throughout their study. Gibbons ( 1968b) reported a male/
female ratio of 1.00:0.89 for a Michigan population of C. picta, conclud-
ing that the slight divergence from a 1:1 ratio resulted chiefly from the
method of determining maturity in females. This may well be one rea-
son for the widely divergent results of Nichols ( 1939a) and Stickel
( 1950) on the sex ratio of T. c. carolina.

Most T. coahuila in the samples were sexually mature, so the sex
ratios given ( not considering juveniles) may show an actual difference
in adult population structure. Female T. coahuila in search of nesting
sites during the reproductive period may tend to travel more often and
farther than males, increasing the probability of their capture. The sex
ratios themselves ( Table 11) offer the only evidence for seasonal dif-
ferences in activity.

DENSITY.-The study tract was divided into two sections: a pri-
mary section ( main study area) of 11 marshes sampled regularly, and
a secondary section of marshes surrounding the main area not included
in daily sampling. Although the choice to limit the study area to a
given series of marshes was arbitrary, considerable expanses of unfavor-
able dry habitat separated marshes in the main area from outlying ones.
In some instances the distance from any marsh in the study area to an
outlying marsh was less than the extreme distance between two marshes
at opposite sides of the study area proper ( about 600 m).

Population estimates were probably affected by turtles moving be-
tween main and surrounding marshes, but the extent of dispersal was
difficult to estimate. The following factors suggest that recruitment of
the population by immigration, or loss of emigration, caused negligible
error in the census: (1) marshes are distinct communities with sharply-
defined borders, and box turtles are largely confined to them; (2) al-
though some overland movements do occur, salt grass communities in
surrounding dry, often bare, zones are effective barriers to T. coahuila
dispersal-the main study area is fairly well set off from other marshes
by these unfavorable habitats; (3) recaptures of T. coahuila show that
they have a tendency to remain in one marsh for long periods ( more
than 1 year in 41% of 34 recaptured turtles, and more than 1.5 years for



1974 BROWN: Terrapene coahuila ECOLOGY 55

two individuals ); (4) population size from December 1964 through
April 1966 in the main study area was relatively stable ( Table 12), and
the proportion of individuals recaptured increased from 8% to over 75%
as the study progressed. This suggests that replacement of the popula-
tion by unmarked animals was minimal ( see Hayne 1949b). The prob-
lem confronting Stickel ( 1950) of accounting for animals whose home
ranges overlapped a study area boundary in continuous favorable habi-
tat was not present in my study.

In the main study tract 114 T. coahuila were captured a total of 203
times; 31 ( 27%) were recaptured once, 9 ( 8%) twice, 9 ( 8%) three
times, 2 ( 2%) four times, and 1 individual ( 1%) was recaptured five
times, the maximum number of recaptures obtained, Slightly less than
half ( 46%) of the individuals were recaptured at least once. Four
turtles marked outside the main study area were subsequently recap-
tured in it and were counted as «new,  giving 118 first-capture indi-
viduals. Two turtles marked in December 1964 and in April 1965 were
found dead on 2 July 1965. Thus a total of 116 was used in all com-
putations on census samples taken after July 1965. Estimates of num-
bers and density of the population apply to more than 98% adults.

An important assumption of mark-recapture sampling is that the
balance between marked and unmarked animals remains undisturbed
between sampling periods ( Hayne 1949b; Stickel 1950; Ricker 1958).
As noted above, a few transient T. coahuila can be expected in collec-
tions from the study area. Stickel ( 1950) noted that only a large influx of
transients would be likely to disturb the ratio of marked to unmarked
animals signiBcantly.

A second major assumption in the sampling is that all animals in
the population, both marked and unmarked, have equal chances of being
collected. There is some evidence that marking had an adverse effect on
T. coahuila behavior, causing slight deviations in recapture frequencies
from those expected on the basis of random occurrence, Recapture
frequencies should follow a Poisson distribution if individuals recaptured
once, twice, three times, etc., are distributed throughout the population
at random and all have a random but equal chance to be caught. De-
partures from the Poisson distribution indicate that any recapture class
has a chance of capture greater or less than random expectation.

Four recapture classes ( 0 to 3 or more recaptures) showed highly
significant departures from the Poisson series ( P<0.0001). Calculated
differences between the observed and expected number of capture rec-
ords for each recapture class, with chi-square and probability values in
parentheses, are as follows: 0 recaptures, 9.8 greater than expected
< X2 -1.84, P>O. 10); 1 recapture, 9.8 less than expected ( X2 - 2.35,



56 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

P>0.10 );2 recaptures, 6,9 less than expected (X2 -3.02, P>0.05); and
3 or more recaptures, 6.9 greater than expected ( X2 -9.43, P -0.002).

Individuals were generally less susceptible to second and third cap-
tures, but were significantly prone to be captured four or more times.
In most marshes box turtle activity seemed to decline markedly after
several successive days of catching them. This was follciwed by a
period of the next several days, or even weeks in some marshes, without
captures. Marked box turtles apparently moved into seclusion. They
could also have become more wary because of my activity during July
and August 1965 when the study tract was visited daily, lowering second
or third captures. In subsequent sampling periods, after monthly inter-
vals with no human intrusion, normal activity resumed and recaptures
increased. Tinkle ( 1958a) noted that marked Pseu[len:gs and Graptemus
in rivers were warier than unmarked turtles. Sexton ( 1959b) found
marked Chrusemys picta difEcult to recapture, while unmarked indi-
viduals could be approached and netted with relative ease.

Three census techniques were used on the mark-recapture data to
estimate the size of the population. The first, a single census method
( Petersen or Lincoln index), has been widely used in population esti-
mates. A preliminary sample of animals is marked and released into the
population, and a later sample is examined for marked animals.

Fitch ( 1963, 1965), working with snakes, divided the season's rec-
ords into monthly intervals and then lengthened the preliminary sam-
pling periods successively, obtaining population estimates at different
points in time through a collecting season. Using the same technique
I sampled the T. coahuila population 10 times between December 1964
and October 1966. Each sampling period was separated by intervals of
2 to 4 months, but records were treated as seven units to obtain work-
able sample sizes. Successively increasing the length of time of the
first ( or preliminary) sample periods ( Table 12) gave six population
estimates by the single census ratio. For example, of the 83 turtles
marked from December 1964 through July 1965, 37 were recaptured
along with 33 new turtles in the period August 1965 through October

P 70
1966, and the Lincoln index formula is : - . or P= 157. The pop-

83 37
ulation estimated by single census ratios ranged from 146 to 171 indi-
viduals, or a density of 53.7 to 62.9 turtles per acre ( 132-155/ha).

Hayne ( 1949b) presented a modification of the Lincoln index
census. With continued sampling of the population, recapture ratios
steadily rise as the pool of marked individuals grows. The total popula-
tion can be estimated by projecting the trend of the increasing propor-



1974 BROWN: Terrapene coahuita ECOLOGY 57

tion of marked animals. Using the same census samples, percentages of
recaptures to total captures in each of the six follow-up periods were:
April 1965-October 1966, 8.0%; July 1965-October 1966, 14,0%; August
1965-October 1966, 52.9%; December 1965-October 1966, 64.0%; April
1966-October 1966, 65.9%; and July 1966-October 1966, 76.5%. Popula-
tion size calculated by the Hayne method was 149-a density of 54.8
turtles per acre ( 135/ha).

Another mark-recapture technique used was the multiple census or
Schnabel method, in which samples are taken and examined for re-
captures continuously over a considerable period. Each day's catch is
treated as a separate census. The method attempts to reduce errors of
random sampling encountered in single censuses by combining the data
from successive daily sampling of the population. I used Schnabel's
short formula ( in Ricker 1958). As turtles were caught in the main
study tract on 57 days, 57 samples were available. Prior to calculations,
corrections were made for known mortality and removals. The multiple
census indicated a population of 164, or 60.3 turtles per acre ( 149/ha).
As outlined in Ricker ( 1958), 95% conEdence limits were calculated for
P. The probability is 0.95 that 135 and 208 include the true population

TABLE 12. NUMBERS AND DENSITY OF T. coahuib IN MAIN STUDY AREA OF 11
MARSHES ( TOTAL AREA 2.72 ACRES ) FROM DECEMBER 1964 TO OCTOBER

1966 ( SEE TEXT FOR DETAILS).

Sampling Periods Estimated Population
Method Population Density

Preliminary Follow-up Size ( Turtles/Acre)

Dec. 1964 April 1965-
Oct. 1966 162 59.6

Dec. 1964- July 1965-
April 1965 Oct, 1966 171 62.9

Single Census: Dec. 1964- Aug. 1965-
Petersen Type, or July 1965 Oct. 1966 157 57.7

Lincoln index. Dec. 1964- Dec. 1965-Fitch's successive 153lengthening of Aug. 1965 Oct. 1966 56.3
sample periods Dec. 1964- April 1966-

Jan. 1966 Oct. 1966 155 57.0
Dec. 1964- July 1966-
April 1966 Oct. 1966 146 53.7

Hayne Method: Six census samples
increasing propor- used above.
tion of recaptures. 149 54.8

Multiple Census: daily 164 60.3
Schnabel Typel ( 57 capture-days) (135-208) ( 49.6-76.5)

195% confidence limits in parentheses.



58 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

size, and that between 49.6 and 76.5 turtles per acre ( 122-189/ha) in-
cludes the actual population density in the study area. Results from the
three census methods are summarized and compared in Table 12.

Considering all 11 marshes in the main study area as a unit, the
total ecological (i.e., marsh habitat) range is 2.72 acres ( 1.102 ha).
Population densities reported here for T. coahuila are based on this total
area. The main reason for doing so is that 21% of the turtles recaptured
in a sampling period after they were first marked had changed marshes.
Intermarsh dispersal might lead to extreme variations in density for a
single marsh; also sample sizes from individual marshes were too small
for precise census estimates. The population density calculated for T.
coahuila ( roughly 60 turtles per acre, or 148 per hectare) is, therefore,
an average density for all marshes in the main study area. It does not
take into account spatial relationships of turtles within marshes. Most
turtles were caught in open places, where they were more easily seen
than in dense vegetation.

Several careful population density estimates have been made for the
genus Terropene. Stickle ( 1950 ) calculated a density of 4.5 T. c. caro-
lina per acre in favorable habitat in Maryland, and gave between 4 and
5 adult turtles per acre as a reliable approximation of the true density
on her 30-acre study area. T. c. carolina occurred at a density of 3.6
turtles per acre in Indiana ( Williams 1961). Legler ( 196Ob) estimated
the average population density of T. o. ornata on 220 acres of grassland
in Kansas to be 1.3 turtles per acre; densities were higher, 2.6 to 6.3
turtles per acre, in the most favorable pasture habitats.

Population studies of aquatic turtles have often compared relative
abundance of different species from various habitats where obtaining
reliable estimates of numbers is dimcult ( Cagle and Chaney 1950;
Tinkle 1958a, 1959b). Other studies have provided data on population
densities in natural populations. Cagle ( 1942) estimated numbers of
Chrysemys picta and Pseudemys scripta in two small stock ponds in
Illinois. Densities calculated from his data are 142 and 556 C. picta per
acre, and 72 and 206 P. scripta per acre, Moll and Legler ( 1971) re-
port a density of 77 juvenile and young adult P. scripta per acre in a
Panama lagoon. Pearse ( 1923) reported a density of 5 C. picta per acre
in a shallow bay of 547 acres in Lake Mendota, Wisconsin; he estimated
densities of about 15 to 20 turtles per acre in vegetated areas. From
the size of the C. picta population Ream and Ream ( 1966) estimated
in the same bay, calculated density is 1.6 turtles per acre. Sexton
( 1959b) reported densities varying between 40 and 166 C. picta per
acre ( depending on surface levels) in five Michigan ponds. Gibbons
( 1968b) and Ernst ( 1971c) provide density figures of 233 and 239 C.



1974 BROWN: Terrapene coahuila ECOLOGY 59

picta per acre in open water of Michigan and Pennsylvania marshes,
respectively.

Probably 60 turtles per acre ( 148/ha) is representative of T. coahuila
population densities in most small marshes in the Cuatro Ci6negas basin,
as the study area does not appear atypical. The aquatic T. coahuila
thus occurs in densities roughly comparable to the aquatic species
Chrysemys picta and Pseudemys scripta, and at considerably higher den-
sities than its terrestrial congeners, T. carolina and T. ornata.

MORTALITY AND REPLACEMENT.-PrOper analysis of any population
requires data on age-specific survivorship and fecundity ( Deevey 1947;
Hutchinson and Deevey 1949; Andrewartha and Birch 1954; Slobodkin
1961). Birth rates and death rates depend strongly on the age distribu-
tion of the animals, and even density may mean little without some
knowledge of the population's age structure. Unfortunately no natural
size groups that might indicate age were apparent in T, coahuila.

Few mortality rates in any phase of the life cycle of turtle popula-
tions have been published. Probable reasons for the lack of information
are the comparatively low year-to-year mortality of adult turtles and
their relatively long life spans, making it dimcult to follow a population
for the many years that would be necessary to measure age-specific
mortality.

Prenatal mortality may take the greatest toll in many turtle popula-
tions. Moll and Legler ( 1971) reported that predators, chiefly lizards
( Ameiua) and armadillos ( Dasupus), robbed 213 of 231 nests of Pseu-
demys scripta in Panama. Predators destroyed many nests of Goph€rus
berlandieri in southern Texas ( Auffenberg and Weaver 1969). Only an
estimated 2 percent of an annual complement of 6,000 Chrysemys picta
eggs in Michigan survived to become part of the population ( Gibbons
1968b); egg predation was thought to be a major factor. Mortality curves
( Slobodkin 1961) of the C. picta population seemed to fit two general
patterns: type IV, heavy mortality in the young (Le., egg) stages; and
type III, constant mortality rate with age in the mature segment of the
C. picta population ( Gibbons 1968b). Mortality affecting immature
C. picta ( which comprised an estimated 60 percent of the population)
was judged to be constant but lower than that affecting adults, but Gib-
bons seemingly did not account for greatly reduced growth rates after
maturity. This appears to have had nearly as great, if not greater an
effect on the survivorship data than did the supposed greater exposure
to environmental hazards with increased activity associated with repro-
duction in mature turtles as Gibbons ( 1968b) suggested.

I found shells or old skeletal remains of 18 T. coahuila in the study



60 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

area ( mostly on land), but this gives little indication of the actual mor-
tality affecting the population. Several T. coahuila had serious cara-
pacial burn scars, and some mortality can probably be attributed to
fires. Several small patches of recent burning were noted in spring 1969
by M. A. Nickerson ( pers. comm. ), one of them within the study area.
Two dead marked T. coahuila Nickerson found had burn scars, circum-
stantial evidence that flre killed them.

The population of T. coahuila studied was cbmposed of approxi-
mately 57% adult females. Assuming that all were sexually mature, prob-
ably 90 females were capable of reproduction in the adult population of
around 160 individuals. The annual egg production of these females
may be about 400 eggs per season if (1) all 90 deposit at least a single
dutch averaging 2.7 eggs ( 240 eggs produced ); (2) if 47 ( 53%) deposit
a second clutch averaging 2.4 eggs ( 110 eggs produced);and (3) if 31
( 35%) deposit a third clutch averaging 1.7 eggs ( 50 eggs produced).
To maintain a stable population individuals dying each season must be
replaced. If adult mortality is low, a total annual complement of 400
eggs in the population studied could safely withstand rather high losses
from the time of deposition to the time sexual maturity is attained.

SOCIAL R~LATIONSHIPS

No aggressive encounters between T. coahuila were observed in the
field, although individuals frequently foraged near one another. Four
times I saw two or more T. coahuila in the same vicinity. Usually only
one turtle was initially seen and watched, but when I moved forward to
secure it, I found a second turtle nearby. Twice a male and a female
were involved; in another instance a male was caught but the second in-
dividual escaped. On one occasion three turtles, all females, were with-
in 3 meters of each other.

The limited evidence suggests no defense of a territory in nature, but
frequent fights between T. coahuila have been noted in an outdoor en-
closure at Arizona State University ( W. L. Minckley, pers. comm.)
Fighting in nature may not be as rare as suggested, but only difficult to
observe, as Evans ( 1961) pointed out.

Evans ( 1956a, 1956b) reported aggressiveness and social hierarchies
in captive T. c. carolina. Penn and Pottharst ( 1940) reported marked
aggressiveness and fghts among captive T. carolina major. In a labora-
tory study involving 13 T. c. carolina in two groups, Boice ( 1970) ob-
served: (1) stable social hierarchies resembling those of more commonly
studied vertebrates; ( 2) hierarchies accompanied by behaviors that
ranged from pushing to biting; and (3) a potential for territoriality



1974 BROWN: Terrapene coahuila ECOLOGY 61

exhibited by one sexually active male. These results were qualified,
however: "the box turtle seems to prosper as a lethargic and indepen-
dent animal ...A social hierarchy did emerge when the turtles were
encouraged to compete in unison but this is undoubtedly quite unlike
most natural situations for Terrapene ( Boice 1970 ). No defnite in-
stances of intraspecific aggression were observed in the lengthy field
studies of T. o. ornata ( Legler 196Ob), T. c. carolina ( Stickel 1950), or
Pseudemys scripta ( Moll and Legler 1971).

SURVIVAL STATUS

Canals carrying water for irrigation destroyed one extensive aquatic
habitat ( Posos de la Becerra) in the basin in December 1964. The origi-
nal surface area of this vast marsh-pool complex underwent an estimated
reduction from about 10 km2 to less than 0.2 km2 ( Cole and Minckley
1966), virtually eliminating the entire population of T. coahuila as well
as populations of the other two endemic turtles, Trionyx ater and
Pseudemus scripta ta!/lori. I estimate the loss of marsh habitat by
drainage at Posos de la Becerra to have been much less than 9.8 km2,
perhaps more realistically approaching ca. 0.25-0.50 km2 (25-50 ha,
as judged from maps of the basin and Fig. 16 in Minckley [1969] ). If
this revised surface area estimate is correct, and assuming a population
density of 60 adult T. coahuila per acre ( 148/ha), ca. 3,700 to 7,400 T.
coahuila may have died or emigrated.

The Survival Service Commission of the International Union for
Conservation of Nature and Natural Resources ( IUCN) defines a rare

"species as one not under immediate threat of extinction [ = endangered
species], but occurring in such small numbers and/or in such a restricted
or specialized habitat that it could quickly disappear." Although its
prospects of survival presently do not seem to be in immediate danger,
T. coahuila is certainly a potentially endangered species. On the basis
of its specialized aquatic habitat and small geographic range within the
surrounding Chihuahuan Desert, T. coahuila is best regarded currently
as a rare species according to the IUCN definition.

Although other basin turtle species are larger, T. coahuila is, none-
theless, the largest and most conspicuous element of the reptilian fauna
living in many of the small, shallow, spring-fed marsh communities in
the Cuatro Ci6negas basin ( where the other turtle species do not occur).
Yet its precise role in the ecology of these marshes will remain largely
unclariBed until population productivity, as it relates to other organisms
in the basin, both plant and animal, vertebrate and invertebrate, is better
understood. T. coahuila appears to be an important animal within the



62 BULLETIN FLORIDA STATE MUSEUM Vol. 19, No. 1

marsh community. Size, food habits, and population densities seem to
bear out this view. Thus with density decrease of these turtles, it is
reasonable to expect the shallow-water ecosystems of the basin to be-
come altered in regard to their energetics, as well as their composition.
More work is needed to clarify the ecological interrelationships existing
between the unique biota of this basin, the T. coahuila population, and
the physical environment.

Minckley ( 1969) warns that "acceleration of modification by man
adds some urgency to the situation [in the Cuatro Ci6negas basin]. The
biota is deEnitely under stress." Although habitat destruction appears
to represent the major threat to its existence, T. coahuila could also be
placed in jeopardy through callous exploitation by dealers in rare or
unusual reptiles, or even by some herpetologists. Adequate series of
T. coahuila have already been assembled; presently more than 100 pre-
served specimens are in U.S. museums alone. Hence there is very
little need for future collecting, except when living specimens are re-
quired for valid experimental purposes.

No protective measures have yet been taken. In view of its rare
status, I propose: (1) the adoption of measures to establish the feasi-
bility of any planned irrigation projects in regions immediately surround-
ing the prime aquatic habitats in the Cuatro Ci6negas basin; (2) the
establishment of restrictions against the indiscriminate construction of
canals that may result in the drainage of major aquatic habitats; and
( 3) special protection for the animal itself in the form of legislation
limiting collecting only for scientific purposes and by permission of the
proper governmental authorities. Permission should be given only to
investigators studying specific problems concerning the species.

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