






































Iguana 13.1 b&w text


IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 53HISTORICAL PERSPECTIVE

Introduction: At the present time there is little available infor-
mation on the influence that desert climates exert on ectotherm

activities. The small amount of data which are available has been
gathered chiefly as matter incidental to other problems in the
biology of reptiles. Even with this meager information, the poten-
tial significance of these data requires more elaborate studies than
those now available. It appears probable that temperature as a fac-
tor of the environment has a profound influence on reptilian dis-
tribution and ecology. Undoubtedly it has been fully as impor-
tant in the evolution and dispersal of reptiles.

Preface
Cowles and Bogert’s “Preliminary Study of the Thermal
Requirements of Desert Reptiles” is something of an anom-
aly in biology. It is a paper that introduced new concepts
concerning an entire vertebrate class and presented a rigor-
ous framework for experimental studies which has survived
with remarkably little change for three decades. A major
field of herpetological research can be traced directly to the
appearance of this work, and for more than a quarter of a
century nearly every paper dealing with the thermal rela-
tions of reptiles has referred to “Cowles and Bogert 1944.”

Cowles had long been interested in the ecology of the
reptiles of the deserts of southern California and in 1938 he
took advantage of sabbatical leave to set up a field station in
the Coachella Valley. His initial interest was the winter activ-
ities of reptiles and he gathered information by following a
bulldozer that was excavating clumps of mesquite and cre-
osote bush in preparation for irrigation. It was this interest
in cold reptiles that first brought him into the desert with a
thermometer in his hand. The resistance of reptiles to low
temperatures and their ability to resume activity when they
were warmed suggested to him that perhaps it was increas-
ing temperatures at the end of the Mesozoic that led to the
extinction of dinosaurs via heat-induced sterility. On this
basis he suggested that fur and feathers first evolved to keep
heat out, not in. From this hypothesis it was a natural step
to studying the reactions of living reptiles to high tempera-
tures. The prevailing view at the time held, with impeccable
logic, that if a lizard sits on a burning hot rock, the lizard
must be as hot as the rock. Biologists marveled at the heat
tolerance of reptiles, although there were some hints that
reptiles really preferred to be cooler.

The continuing value of the “Preliminary Study” rests
on its two major characteristics: it was an entirely new way
of looking at the biology of reptiles, and the methods and
their application were extraordinarily rigorous. Cowles and
Bogert discarded 80% of their body temperature measure-
ments because of uncertainty about the activity of the ani-
mal before a measurement was made or because they felt
the temperature might have changed during the process of

H I S T O R I C A L  P E R S P E C T I V E

Preliminary Study of the 
Thermal Requirements of Desert Reptiles1

Raymond Bridgman Cowles and Charles Mitchill Bogert

1 Bulletin of the American Museum of Natural History 83:261–296
(1944, reprinted in 1974 by the Society for the Study of
Amphibians and Reptiles, Miscellaneous Publications, Facsimile
Reprints in Herpetology).

Editor’s Note.—Early herpetological literature tended to be of
interest to a very limited audience — other herpetologists.
Much of the work was focused on listing the species found in
particular areas and describing their basic natural history, and
an evolutionary perspective, beyond the taxonomic focus, was
rare. Perhaps the first herpetological work to break into the
awareness of biologists at large was the 1944 publication of “A
Preliminary Study of the Thermal Requirements of Desert
Reptiles” by Cowles and Bogert, an eco-physiological study
with an evolutionary view. The importance of the work stems
from the demonstration that “cold-blooded” organisms did not
passively take on the temperature of their environment, as pre-
viously thought. Instead, Cowles and Bogert were the first to
document what we now take for granted: Reptiles have sophis-
ticated behavioral mechanisms that allow them to precisely reg-
ulate their body temperatures within a fairly wide range of
ambient conditions. This was not the last collaboration between
the two nor the final study of reptilian physiology. However,
whereas most of the work published that long ago is rarely ref-
erenced, the paper excerpted here is still frequently cited, show-
ing its enduring value — sixty years later, this work is still a
major milepost.

The original paper is too long to fully include here, as is
the Preface by Harvey Pough, added in the SSAR 1974 reprint.
I have selected excerpts from both the original work and
Pough’s preface in order to give readers a sense of the scope of
the work, selecting paragraphs that illustrate both substance and
style of the work. In the interest of brevity, the bibliography has
been omitted, although references were retained in the text (but
not in the preface). For those interested in reading more, the
SSAR reprint is still available for sale, along with some other
historically important herpetological publications. All can be
found at www.nbb.cornell.edu/neurobio/department/faculty/
adler/not%20using/ssar.html.

Gad Perry, Editor
Texas Tech University

Continued on page 54

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 53



54 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006

The great advantage inherent in studies of desert reptiles
lies in the extraordinarily high maximum temperatures, as well
as the greatly exaggerated temperature changes, so characteristic
of desert climates. Such environmental conditions may amplify
subtle details of thermal relationships that would otherwise
escape notice. The resulting accentuation of temperature
responses throws into relief temperature relations which in the
equable climate of the tropics might otherwise remain as imper-
ceptible, or at least unperceived, nuances in thermal adaptations.

Methods: The region wherein most of these experiments were
conducted presents typical aspects of the Coloradan subdivision
of the Lower Sonoran life zone. Cages and other equipment
were installed in the field, as described hereinafter, at a locality
near Indian Wells, Riverside County, California. This locality
lies on the floor of the Coachella Valley a few feet above sea level,
with drainage to the southeast into the sink of the Salton Sea.
The surface of this body of water is below sea level.

Experimental results (representative species, chosen for diversity
and availability of pictures): Coleonyx variegatus, Banded Gecko.
The thermal responses of this nocturnal lizard closely approxi-
mate those of the nocturnal snakes. It is able to crawl at tem-
peratures as low as 11° C., but in actual practice the animals do

COWLES AND BOGERT

measuring. The importance of that caution cannot be over-
stressed. Inanimate objects (in this instance beer cans) dis-
tributed through the environment show a distribution of
“body” temperatures which resembles the body tempera-
ture distribution measured from live lizards. In other words,
the observation of a negatively skewed temperature distri-
bution centered about a clearly-defined mean that is higher
than air temperature does not demonstrate thermoregula-
tion. It is essential to combine measurements of tempera-
tures with observations of the animals’ activities.

The scope of the field opened to investigation by the
view of reptiles as animals capable of a considerable degree
of homeothermy has not yet been fully realized. In the first
place it has made the classic division of animals into
homeotherms (warm-blooded) and poikilotherms (cold-
blooded) meaningless. During the course of the study
Cowles coined the words endotherm and ectotherm to
emphasize the source of the energy that warms the body.
The realization that reptiles can and do control their body
temperature has had ramifications in many areas of her-
petology. Ecologists have found that niches are defined in
part by thermal relationships, ethologists have discovered
that reptiles behave differently at different body tempera-
tures, and zoogeographers must consider the radiant energy
regimes available on postulated routes of migration. It is
scarcely possible to ask a question about the biology of rep-
tiles in which their thermoregulatory capacities are not
directly or indirectly involved. In particular, the very active
research in reptilian environmental physiology is built to a
great extent around thermal relations and thus stems from
the “Preliminary Study.”

There has been remarkably little modification of the
views embodied in the original paper during more than a
quarter century of extensive work. One of the first changes
was a de-emphasis of the concept of the “ecological opti-
mum” represented by the mean value of the body tempera-
tures measured during activity. Bogert found that the same
genera of lizards maintained very similar body temperatures
in different habitats while different genera maintained dif-
ferent temperatures in the same habitat, indicating an hered-
itary preference for a particular body temperature level. The
hereditary preference could be affected by environmental
conditions. For example, in Florida, which was frequently
cloudy, Cnemidophorus body temperatures were skewed
toward the lower end of the activity range while they were
skewed to the upper end in sunny Arizona. In other words,
the activity temperature should be viewed as a range of tem-
perature which is acceptable to a lizard species and allows it
to carry on the activities necessary for life. Indeed, a lizard
that is kept continuously at the average temperature it selects
in a temperature gradient loses weight and may even die.
Regal has shown that lizards voluntarily select low body
temperatures at night. Apparently different aspects of a
lizard’s internal economy function best at different temper-
atures and some temperature variation is essential.

F. Harvey Pough
(excerpted from the 1974 SSAR edition)

Preface continued from page 53

The thermal responses of nocturnal Banded Geckos (Coleonyx varie-
gatus) closely approximate those of nocturnal snakes.

L.
 L

EE
 G

R
IS

M
ER

This view from Santa Rosa Mountain illustrates several “typical” compo-
nents of the Coloradan subdivision of the Lower Sonoran life zone.

L.
 L

EE
 G

R
IS

M
ER

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 54



IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 55HISTORICAL PERSPECTIVE

not ordinarily expose themselves to temperatures below 16° C.
As a matter of fact, they are rarely found in the open below air
temperatures of 18° C., except when the ground is warm.
Whereas this gecko appears to be somewhat more resistant to
cold than the snakes, it is also capable of enduring somewhat
higher temperatures than these animals. For the snakes as well
as for this lizard, the activity range lies in the vicinity of 30° C.
This is well below that of the diurnal lizards. The small number
of available specimens has prevented a satisfactory determina-
tion of the critical maximum, but this point is reached at a body
temperature somewhat below 41° C. Under extreme conditions
of heat and with relative humidity at 11 per cent, respiratory
cooling of about 2.5° C. has been noted.

Dipsosaurus dorsalis dorsalis, Desert Iguana or Northern
Crested Lizard. After one night of exposure to temperatures
slightly below 8° C., a common night temperature in early
spring, these lizards are torpid and often so sluggish that they are
unable to move. As their body temperatures rise with increasing
warmth, they remain helpless up to 14° C. At 18° C. they are
unable to coordinate rapid movements, and they resort to
clumsy intimidation displays. A temperature of 21° C. permits
slow locomotion, and at 24° C. torpidity is still evident but the
animal can walk with well-coordinated movements. The tem-
perature of 27° C. constitutes a well- defined minimum volun-
tary tolerance. It is notable that at low temperatures this species
of lizard is more seriously discommoded than are some others
that have wider territorial ranges. Normal activity extends from
34°–41° C., and the mean for all records (38 observations) is

37.4° C. Retreat from high temperatures observed in 20 caged
animals over a period of two months indicates that they avoid
exposure to body temperatures of more than 41° C.
Nevertheless, lizards under apparently ideal conditions were
recorded with temperatures of 42° C. immediately after they
were shot, and on six occasions lizards captured alive had tem-
peratures of 43° C. Although the lizards that were shot did not
give any evidence of prolonged exposure resulting from fear of
the collector, the noosed lizards assumed the usual state of tonic
immobility attendant on the presence of danger. However, it did
not appear probable that there had been time for any abnormal
heat absorption. Until additional observations are possible, it
seems advisable to consider 41° C. the maximum normal tem-
perature tolerance. Desert iguanas taken near Indian Wells on
the Colorado Desert reach the critical maximum at 47.5° C. (6
trials; min., 47° C.; max., 48° C.). A like number from Saltdale,
a locality at a higher elevation on the Mojave Desert, under
experimental conditions reached the critical maximum at 47° C.
Three of the animals were again subjected to the same test and
at the end of 30 minutes collapsed and died at 47° C. Only two
tests for the so-called lethal temperature have been made, one
based on an individual from the Mojave Desert and another
from the Colorado Desert. The one from the cooler Mojave died
at 50° C., the other at 50.5° C. The difference is doubtfully sig-
nificant and probably not the result of climatic adaptation.

Crotaphytus wislizenii, Leopard Lizard. Only one individ-
ual was available for study. After repeated observations over a
period of two weeks it became evident that this animal invari-

Normal activity of Desert Iguanas (Dipsosaurus doralis dorsalis) occurs at temperatures ranging from 34–41 °C.

L.
 L

EE
 G

R
IS

M
ER

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 55



56 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 COWLES AND BOGERT

ably retreated to cover when lowering temperatures approached
23°C. Other observations did not yield conclusive data. 

Sauromalus obesus, Chuckawalla. At 14° C. these animals
are scarcely able to right themselves from a supine to normal
position. At 21 ° C. slow but effectual locomotion is possible,
while at 24° C. the fear response seems to be dulled, a distinct
contrast to the condition in Dipsosaurus where fear and alertness
seem to reach the normal pitch at this temperature. Fifteen
observations were made on wild individuals obtained on the
Mojave Desert in the course of a single cloudy day. Throughout
the period, air temperature remained at 24° C., while the tem-
perature in the crevices occupied by the animals was 25.5° C.
Although all the individuals were discovered in rock crevices,
none of the animals was more than halfway to the bottom of its
retreat. Reluctance to appear in the open at what seem to be
entirely tolerable conditions apparently endows these animals
with an excellent safeguard against capture by predators; appar-
ently chuckawallas do not expose themselves to view until their
body temperatures have risen to the point where celerity of
responses insures their ability to escape.

It was instructive to find that the lighter colored, but some-
what smaller individuals from the Colorado Desert absorbed
heat more slowly than the black-and-dark-red color phases more
characteristic of the cooler Mojave Desert. At maximum tem-
peratures attainable under moderate thermal conditions, a large
black and red individual reached a temperature 2° C. higher
than that of smaller, lighter colored individuals. In spite of its
darker color and more effective heat absorption, the larger lizard
required a longer time to reach its ultimate thermal level. This
is consonant with the change in surface area relative to the
changed mass of an organism. The influence of color on rate of
absorption can be determined to an exact degree only by the use
of two series of lizards of different color but having identical sur-
face areas and mass.

Chuckawallas should furnish an excellent source of infor-
mation on the importance of color as a physiological adaptation
versus its value as a means of concealment. The color variability
(but not metachromatism) that is found within any one species
of the horned lizard (Phrynosoma), fringe-footed lizard (Uma),

and many snakes seems to be a device of primary importance in
concealment. The same may be found true in the chuckawalla
which displays a high degree of color variability in different
localities, particularly in the Colorado Desert.

The darker coloration of the Mojave Desert form may reflect
a need for greater heat absorption consonant with its larger size
and the cooler climate of this higher desert. The various pattern
phases represented in the warmer Colorado Desert are lighter col-
ored on the body, although the head is nearly completely black. It
is pure speculation, but it seems possible that such a pattern
enables these Colorado Desert lizards to absorb heat rather rap-
idly when only the head is protruding from a crevice. On the
other hand, the yellow coloration of the trunk would not absorb
heat so rapidly as would the red and black body of the Mojave
Desert form when these lizards venture forth into direct sunlight.
Such a black and yellow pattern, therefore, may represent an evo-
lutionary compromise. On the other hand, such a pattern can be
interpreted as disruptive coloration. Klauber (1939) has pointed
out that black rock or the dark areas resulting from shadows in a
paler rock habitat may make it difficult to distinguish the body
outline of these dusky colored lizards. Hence their coloration may
be of protective value. It is noteworthy, however, that large diur-
nal lizards in all parts of the world tend to be dark colored. This
is particularly true of the crocodilians, the larger monitors
(Varanus), and of the larger iguanids (Ctenosaura and
Amblyrhynchus for examples). It is not impossible that the heat-
absorbing properties of dark skins are required to insure the intake
of sufficient heat to permit these large poikilotherms to attain tem-
peratures within the normal activity range.

To return to Sauromalus obesus, the average range for activ-
ity is 37.7° C. This figure was obtained from 25 observations
which were made as the animals retreated to shade, and 24 as
they retreated underground when shade temperatures became
too high. The two series agree to within 0.2° C. The highest
temperature recorded for voluntary tolerance was 42° C. The
critical maximum appeared at 44.5° C. in one individual, at 49°
C. in another, and in a third (under laboratory conditions) at
43.3° C. The reasons for the wide variations are not known. The
putative lethal was found to be 50°-51° C. with only four trials,
one of them conducted in the laboratory. 

Phrynosoma sps., the Horned Lizards. Little difference in ther-
mal preferences can be detected between the three southern
California horned lizards, although, to judge by their preferred habi-
tats, strong differences would be expected. One species, Phrynosoma
b. blainvillii, is confined to the coastal areas and the higher semi-
desert districts of the mountains. Another, Phrynosoma p. platyrhi-
nos, has a wide geographic range that includes considerable tem-
perature differences, and the third, Phrynosoma m’callii, is restricted
to the warmer areas of the Colorado Desert. P. p. platyrhinos is found
at elevations of 3000 feet, but it is also encountered in the Coachella
Valley at sea level or slightly below, where, as at the mouth of the
Box Canyon in Riverside County near Mecca, its range overlaps
that of P. m’callii.

In view of the slight differences in heat economy and the
considerable difference in temperature conditions in these habi-
tats, it seems probable that the necessary accommodation to
diverse climates is accomplished by means of habits. Coloration
and neurological adjustments may also be involved, and it is

Chuckwallas (Sauromalus ater, formerly S. obesus) do not expose them-
selves to view until their body temperatures have risen sufficiently to
insure an ability to escape a predator.

B
R

A
D

FO
R

D
 D

. H
O

LL
IN

G
SW

O
R

TH

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 56



IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 57HISTORICAL PERSPECTIVE

noteworthy that both of the desert dwelling species are paler in
coloration than the coastal form. A comparison of the averages
of temperature adjustment at the most significant levels is shown
below.

From these data it appears probable that a difference of
thermal adjustment amounting to only 2° C. distinguishes the
coastal species from P. m’callii, an inhabitant of the hottest desert
areas. This thermal similarity was further emphasized by the
observation that sand temperatures of 41° C. or more seemed
equally effective in restraining the sand-burrowing impulse.
When forced to submerge themselves, however, they all endured
short exposures to temperatures as high as 43° C.

As temperatures became unbearable all three species sub-
merged in the sand of their cages, and as the surface layers
became hotter they pushed downward until a hard-pan at a
depth of 75 to 80 mm. prevented them from penetrating deeper.
Under these conditions, usually by 2 to 3 P.M., they would rush
to the surface and retreat to the nearest shade, where by that
time temperatures had moderated sufficiently to be endurable.

Phrynosoma m’callii displayed a greater tendency to crepus-
cular activity than either of the other species, sometimes remain-
ing above ground for an hour or more after the others had

retreated and temperatures in the late dusk had dropped to 29°
C. Retreat below ground appeared to be due as much to the light
conditions as to the compulsion exerted by a falling temperature.

One of the difficulties involved in obtaining thoroughly reli-
able thermal records for any given reaction is revealed by the
inconsistent behavior of a single example selected at random from
among a large number of similar instances. While attempting to
obtain data on the minimum temperature at which Phrynosoma
m’callii leaves the sand on first emergence in the morning, it was
noted on one occasion that two individuals resting side by side
under apparently identical conditions ultimately emerged at body
temperatures respectively 22° and 36° C. Thus there was mani-
fest a difference of 14° C. for a single, apparently spontaneous
reaction. It should be emphasized that the lower temperature is
well below the level usually tolerated, while the higher figure lies
well within the range for normal activity and not far below the
limit of voluntary tolerance to high temperature.

Arizona elegans occidentalis, Western Glossy Snake.
Burrowing snakes are difficult creatures with which to work, and
consequently there is little available information on the activi-
ties of this digging species. Although it is one of the commoner
snakes collected while driving at night, in captivity it was secre-

Retreat from cold Activity range Maximum voluntary tolerance Number of observations

P. b. blainvillii 28.0° C 34.9° C 39.0° C 26
P. p. platyrhinos 29.0° 36.8° 39.0° 10
P. m’callii 29.3° 36.9° 41.0° 10

Desert Horned Lizards (Phrynosoma platyrhinos) have a broad geographic range that includes varying temperature regimes.

L.
 L

EE
 G

R
IS

M
ER

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 57



58 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 COWLES AND BOGERT

tive and seldom appeared above ground except for intervals of
relatively short duration. Like any true burrowing snake, it seems
capable of flowing into and out of the loose desert soil with very
little locomotory difficulty. Surplus heat acquired during active
periods on the surface is rapidly lost by direct conduction when
these animals burrow.

The lowest temperatures at which this species was observed
above ground were 14° and 18° C. These appearances were
exceptional, and surface activity was of very short duration,
probably not much in excess of a few seconds. At 19°-20° C.,
appearances become more frequent, and it is probable that this
constitutes a normal lower limit for voluntary surface activity.
As stated previously, strong stimuli such as fright or those
involved in sexual activities frequently result in modifications of
other normal responses.

When glossy snakes were confined in the same cage with
Crotalus cerastes and Rhinocheilus lecontei there were no marked
differences in their responses to temperature. With respect to
light, however, C. cerastes was more tolerant than the other two
species. Apparently these three snakes are all relatively cold-tol-
erant reptiles. Klauber (1939) lists C. cerastes and Rhinocheilus as
the snakes having the appearance of greatest tolerance to low
temperatures, and we would include Arizona in the same cate-
gory upon the basis of our observations.

One of the difficulties besetting the acquisition of exact
thermal data is the ability of the burrowing snakes to glide into
the soil with no preliminary warning of the approach of a
change in behavior. On one occasion, as temperatures fell rap-
idly, eight of the burrowing snakes were moving about the cage
shortly after dusk. As their disappearance was anticipated at air
temperatures of 19°–20° C., and body temperatures were
desired, the observer was ready to grasp the snakes as they started
to descend into the soil. As frequently happened, there was
almost simultaneous retreat, and before the snakes could be
extricated some seconds’ delay had occurred. Because they had
already been immersed in what at the time were the warmer,
deeper layers of the soil, and would be expected to yield erro-
neous data, the attempt was abandoned. However, a C. cerastes
in the same cage, which retreated at the same time and was
halfway down the mouth of its open burrow, was seized and the

temperature recorded as 20° C. It is probable that the somewhat
higher temperature represents heat absorption from the substra-
tum, and that the other snakes would have shown the same tem-
perature at the time of their departure.

Under laboratory conditions these snakes reached their crit-
ical maximum at 42, 42, 41, 41, 42, 43° C., mean 41.8° C.
Putative lethal temperatures were 43°–44° C. (six observations).

Despite the normal avoidance of both light and high tem-
peratures, these snakes occasionally bask on the surface of the
ground in winter, and in summer a large gravid female was cap-
tured in Coachella Valley, July 7, 1940, at 8 A.M. in full sun.
The day was cloudless and hot, probably well over 38° C. in the
shade. To judge by the tracks in the sand, the snake had been

Glossy Snakes (Arizona elegans) are burrowers; any surplus heat
acquired during periods when active on the surface is rapidly lost by
direct conduction when these animals burrow.

L.
 L

EE
 G

R
IS

M
ER

Sidewinders (Crotalus cerastes) and Long-nosed Snakes (Rhinocheilus
lecontei) are both relatively cold-tolerant, but the former is more toler-
ant of light than other nocturnally active snakes.

L.
 L

EE
 G

R
IS

M
ER

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 58



IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 59HISTORICAL PERSPECTIVE

very active, presumably during the night and early morning,
boring in and out of the ground near a large hummock of earth.
It was probably seeking a suitable spot for oviposition. She
deposited 23 eggs the following day, and these hatched after 68
days of incubation at room temperature, which fluctuated
between 25° and 32° C. At eight days of age, the young were
tested for the critical maximum and responded at 38°–39° C.
All recovered but showed varying degrees of partial caudal paral-
ysis. Thus the young appear to suffer from temperatures 3°–5°
C. lower than those which would be fatal to the adults. 

Crotalus atrox (C. cinereous auct.), Western Diamond
Rattlesnake. Freshly captured individuals have been observed
moving about at 14° C. but this was presumably the result of
abnormal stimuli; activity at this low temperature may be
ascribed to the presence of abnormal environmental factors. Even
at 18° C. there is some activity, but it is limited in frequency, and
it is not until temperatures of 27°–30° C. are encountered that
the snakes are persistently active. A temperature of 39° C. was
observed in the only test for the critical maximum, and this is
probably somewhat lower than should be expected.

Discussion: As suggested earlier, one of the seemingly important
requirements of reptiles is an effective extension of both their
daily and seasonal hours of activity. This necessity constitutes a
problem which is greatly accentuated in desert regions of the
temperate zone….

The acquisition and maintenance of necessary body tem-
peratures are the sine qua non of existence for terrestrial verte-
brate ectotherms. All other activities requisite for survival of
reptilian species seem ultimately to depend upon the mainte-
nance of necessary body temperatures which makes this ele-
ment one of basic importance. Successful predation by carniv-
orous reptiles is based on ‘alertness and agility’ and these
attributes reach maximum efficiency at or near optimum tem-
peratures. Even for the herbivorous types, the attainment of
suitable temperatures is imperative, for they must also maintain
high body temperatures if they are to function at maximum
efficiency during the crucial moments when they must escape
their enemies. “Survival of the fittest” among terrestrial reptiles
would appear to be the survival of the warmest in those
instances where other factors are equal. In other words, the
most successful reptiles probably are those which, by means of
their habits, are able to approach a state comparable to that
attained by homiothermic animals….

Presumably the less adaptable reptiles have failed to survive
in desert regions because of the rigorous climatic conditions.
Conversely, relicts are more often found on islands and penin-
sulas owing to the relatively slight temperature fluctuations char-
acteristic of maritime climates. Schmidt (1943) has recently dis-
cussed peninsular life and “paleopeninsulae,” pointing out that,
“The common faunal characteristic of the major peninsulas is
their accumulation of peculiar forms of life, frequently primi-
tive.” It has often been hypothecated that insular and peninsu-
lar relicts survive because they are relieved of “competition with
modern elements,” or because “biotic pressures are reduced.” On
the other hand other authors have spoken of the “extraordinary
congestion in species” on peninsulas. Can an abundance of
species lead to diminution of competition?...

Many modem distributions possibly can be interpreted in
terms of thermal and moisture requirements when these are bet-
ter understood. It must be borne in mind that the principal cli-
matic areas of today probably were in existence prior to the
Pliocene. During the late Quaternary seasonal changes presum-
ably approximated those of today. As the summer advances, the
temperatures of the surface soils rise above the optimum during
daylight hours, and most species included in the sand fauna, both
invertebrates as well as vertebrates, burrow increasingly deeper.
By so doing they follow the vertical drift of favorable tempera-
tures. The resultant concentration or stratification of fauna in
favorable thermal zones may be an important factor in the lives
of burrowing reptiles, since it results in a concentration of the
available food supply in a comparatively narrow thermal zone.

Summary and Conclusions: Twelve diurnal and seven noctur-
nal species of reptiles indigenous to the Southwest have been
studied. Data derived from field observations, from animals in
cages set up in the desert, and from supplementary laboratory
investigations provide the basis for the following statements:

1. The reptiles under observation were voluntarily active
only between the temperature extremes of 16° and 42° C. (cloa-
cal temperatures). Thus the maximum voluntary thermal toler-
ances of reptiles are somewhat less than those reported for birds.
Actually the ecological optimum or the mean for the “normal
activity range,” as defined herein, is somewhat lower than the
normal temperature of many mammals. Consequently even the
reptiles inhabiting one of the hottest regions in the world can-
not be considered notably thermophilic. 

2. Contrary to previous reports, nocturnal reptiles not only
tolerate but prefer temperatures somewhat lower than those of
diurnal reptiles. Provisionally the difference between mean crit-
ical thermal levels for diurnal arid nocturnal reptiles may be said
to approximate 6° or 7° C.

3. Under captive conditions approximating those in their
normal habitats the reptiles studied were able to avoid extensive
temperature fluctuations. Particularly noteworthy in this respect
was the ability of the sidewinder (Crotalus cerastes), while in a rel-
atively inactive or quiescent state, to maintain its body temper-
ature within the narrow limits of 31° and 32° C. The acuity of
temperature discrimination in this species under such conditions
is astonishing, more especially because the sidewinder proved to
be one of the least stenothermic reptiles investigated. Individuals
in an active state were noted with temperatures varying from 16°
to 34.5° C.

4. Observations recorded for one lizard (Sceloporus m. mag-
ister) indicate that defecation is most frequent at body tempera-
tures of 37° to 38° C. It is suggested that such temperatures are
necessary before peristalsis is possible, although this same lizard
commonly fed with the body temperature closer to 30° C. If such
precise requirements are widespread among reptiles, it may
account for the high mortality rate among captive reptiles in many
zoological gardens where animals are maintained under conditions
which prevent them from selecting preferred temperatures.

5. One of the notable facts is the close approximation of the
maximum temperatures tolerated voluntarily and the critical
maximum which immobilizes the animals. A difference of some-
what less than 6° C. between these levels is indicative of the tem-

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 59



perature hazards under which some of these animals would exist,
were it not for concomitant adaptations, particularly in habits.
The utter impossibility of prolonged activity in the full summer
sunshine of the desert is clearly indicated by the black-bulb tem-
perature of 87° C. observed as early as May.

6. It is suggested that reptilian relicts tend to survive on
islands or peninsulas owing to the relatively slight temperature
fluctuations characteristic of maritime climates. For similar rea-
sons such relicts would tend to survive in tropical regions rather
than in continental climates of temperate zones where a higher
degree of adaptability to temperature fluctuations would be a
prerequisite.

7. A notable characteristic of desert reptiles is the rapidity
with which these animals absorb heat. Changes are so rapid as
to exceed those of the thermometer in the case of small lizards
(Uta stansburiana). The rapid changes in reptilian temperatures
seem to be due primarily to: (1) their lack of effective surface
insulation; (2) their lack of hypodermal adipose tissue; (3) their
pigmentation, particularly the melanin; and (4) in the smaller
species, to the relatively small volume in proportion to the large
heat-conducting surface.

8. Since an important factor in the thermal adjustment of
desert reptiles is the surface-mass ratio of the body, it follows that
smaller lizards are capable of utilizing very short intervals of
favorable exposures (assuming other factors to be approximately
equal). In contrast, larger lizards, under favorable conditions of
heat, will require more time in which to reach the optimum, but
their activities at higher temperatures will be less restricted than
those of smaller individuals or species.

9. Although large lizards exposed to solar radiation or to the
warm substratum attain temperatures of maximum toleration
more slowly (and also dissipate heat more slowly) than small
lizards, there is no apparent correlation between body size and

the maximum temperature voluntarily tolerated. Even though
the largest diurnal lizard used in these experiments (Sauromalus
obesus) withstood a body temperature of 42° C. of its own voli-
tion, this maximum was closely approached by a much smaller
diurnal lizard (Phrynosoma m’callii) that voluntarily tolerated 41°
C. The data for nocturnal reptiles are not so extensive, but pre-
liminary investigations demonstrate a lower voluntary tolerance.

10. It is apparent that for diurnal lizards the effect of color
change (that is, the tint or shade rather than the hue) operates
to provide an extension of time at marginal thermal levels, but
the data assembled in these preliminary investigations throw lit-
tle light on the relative merits of protective coloration versus
physiological adaptation. However, a consideration of the size,
pattern, and coloration of Sauromalus obesus in various regions
suggests that some patterns may be interpreted in terms of
habits, and that an evolutionary compromise may exist when
antagonistic forces are involved.

11. Thermotaxis through behavior is one of the outstand-
ing characteristics of desert reptiles. Body temperatures within
the normal activity range are attained principally (1) by select-
ing positions in or on soil, or rock, where heat through direct
conduction can be absorbed, or (2) by basking, with all or only
part of the body exposed, to sources of solar heat. Conversely
temperatures above the critical maximum are avoided (1) by
retreating to cooler depths, in the ground (by burrowing or in
preempted burrows), in rock crevices, or beneath insulating
material, (2) or by respiratory cooling under extreme conditions.

12. Because the activities necessary for the survival of both
the species and the individual are dependent upon the acquisi-
tion and maintenance of suitable body temperatures, ther-
moregulation by means of behavior may be considered an ele-
ment of basic importance in the existence and evolution of
reptiles in continental climates.

60 IGUANA  •  VOLUME 13, NUMBER 1  •  MARCH 2006 COWLES AND BOGERT

Rapid changes in reptilian body temperatures, especially in smaller species like the Side-blotched Lizard (Uta stansburiana) are attributable, at least
in large part, to the relatively small volume in proportion to the large heat-conducting surface.

L.
 L

EE
 G

R
IS

M
ER

Iguana 13.1 b&w text.QX6  3/3/06  7:16 AM  Page 60




