BULLETIN
OF THE

FLORIDA STATE MUSEUM
BIOLOGICAL SCIENCES

Volume 4 Number 2

GROWTH AND DEVELOPMENT OF THE EASTERN
HARVEST MOUSE, REITHRODONTOMYS HUMULIS

James N. Layne

iF#.a~mik,PA

\=ly'

UNIVERSITY OF FLORIDA
Gainesville

1959



The numbers of THE BULLETIN OF THE FLORIDA STATE MUSEUM,
BIOLOGICAL ·SCIENCES, will be published at irregular intervals. Volumes
will contain about 300 pages and will not necessarily be completed in any one
calendar year.

OLIVER L. AUSTIN, Ja., Editor

The publication of this number of THE BUL-
LETIN has been made possible by a grant
from the Graduate School, University of Florida.

All communications cdncerning purchase. or exchange of the publication should
be addressed to the Curator of Biological Sciences, Florida State Museum, Seagle
Building, Gainesville, Florida. Manuscripts should be sent to the Editor of the
BULLETIN, Flint Hall, University of Florida, Gainesville, Florida.

Published 27 April 1959 Price for this issue $.30



GROWTH AND DEVELOPMENT OF THE EASTERN
HARVEST MOUSE, REITHRODONTOMYS HUMULIS

JAMES N. LAYNE 1

SYNOP*IS: Observations were made on the breeding bistory, growth,and
development of the young, and parental care in captive eastern harvest. mice,
Reithrodontomys h. humutis:

Breeding occurred throughout the year, gestation was presumably 24 days
~ or less, and mean litter size was 2.2. The sex ratio at birth was even.

The curve5 for body weight and total length became essentially asymptotic
by the 71:h week. Tail length increased only slightly after the 5th week, and
the curve for hind foot length leveled off at 8 weeks. Mean ear length 5howed
no significant increase after the 4th week. A marked, slowing ai 8 weeks of the
initially rdpid gtowth .in weight, total length, and tail length coincided with the
period of weaning.

Newborn young are pinkish and possess- both vibbssae and a sparse scattering
of hairs over the middorsal region. The pelage is sufficiently deve16ped by the
end of' -the first week to exhibit th6 typical juvenile coloration, and by the second
week the juvenile coat is apparently. fully grown. The postjuvenile molt takes
place between the 8rd and 9th weeks. Full adult pelage is not.,attained until at
least one further partial or complete molt.

The' eyes open from the 7th to the 9th  or 10th days of age. The pinnae
zinfold on the Ist or End day.of age, and the external auditory meatus becomes
patent from the 7th to 10th days. Claws are pre*ent at birth and are noticeably
better dbveloped by the 4th day.

Incisors were above ihe gums on the 4th.day in one mouse, but typically did
not appear until -the Bth of 7th day. They are faintly yellowish by the. end 6f
the. 2hd week. All m61ars were apparently functional in a specimen 4 weeks
of age.

Weaning occurs between the 2nd and 4th weeks. The vulvas of females
Opened between the 4th and 7th weeks, and two feniales became pregnant for
the first time at. 11 and 20 weeks of age. The testes and cremaster of. the males
enlar*ed by the 7th or Bth ~eek,

The young have little coordination at birth. When 4 days old they can sit.
upright, .and the prehensile nature of the tail is apparent. Week-old .young
scratbh and attempt to wash, With the opening of the eyes they become more
active  and exploratori and by 2 weeks of age their Behavior is similar to that
of adults. Most individuals were nervous and wild. The young and adults were
generally highly t61erant of one another, although some Eghting occurred among
males .in one cage.

Increased nest building  and food storing activities. may precede parturi(ion
in females. Mothers exhibited concern for their young through the Brd week
of age, although the intensity of parental cake. declined after the 2nd week. In-

1 The author is Assistant Professor of Biology at the Universitf of Florida, an
Associate of the Florida State Museum, and serves as curator of mammals for the
University of Florida Collections.



62 BULLETIN FLORIDA STATE MUSEUM Vol.· 4

stances of paternal care were observed, and  behavior assumed to represent a weak
retrieving response was noted in 4-week-old young.

Comparison of the growth and development of R. humulis and R. megalotis
indicates a close' similarity of postnatal developmental patterns, although megdto-
tis may have a somtwliat slower growth rate.

INTRODUCHON

Comparative data on postnatal growth and development in differ-
ent species and subspecies of mammals are often of interest from a
taxonomic,  genetic, physiological, behavioral, or ecological standpoint.
Knowledge of such aspects of the reproductiye biology of mice of the
genus Reithrodontomys is apparently limited to only 8 6f the 17 Recent
species. Two accounts (Svihla, 1981; Smith, 1986) apply to R. megalo-
tis. Svihla (1980) gives weights and a short description of the new-
born young of R. fuluescens, and Audubon and Bachman (1849) men-
tion the birth in captivity of two litters of the eastern harvest mouse,
R, humulis, noting that the young were blind and naked at birth,
became covered with hair in a few days,  and were' seen peeping out
of the nest when a week old. Holding and Royal (1952) present more
extensiye data 9n the development. of a single young R. humulis.

An adult female and juvenile male Reithrodontdmys h. humulis
(Audubon and Bachman) capturedby hand 16 December 1956, near
Gainesville, Alachua -County, Florida, serve-d as the hucleui of- a
small laboratory colony. This colony was studied actively for about
a year. During this time 11 litters were produced and observations
were made on breeding, growth and development of the young, and
parental care.

METHODS

The mice were first kept in a large aquarium with a hardware
cloth top. Several inches of dry sand served as a substrate, and

' dried grasses were supplied for cever and nesfing material. Later,
wire cages with a litter of sawdust or wood shavings and cotton for
nesting material were 1used. Shelled whole corn was the principal
food given. This diet was supplemented with sunflower seeds,
crimped oats and oat flakes, Purina lab chow, and various fresh fruits
and vegetables. Water was provided in open dishes or in standard
wat~r bottles.

When litters were expected, the nests were usually examined for
younk in the morning and evening. Young born during' the period
between the evening and morning checks  were considered as being
newborn on the day discovered, even though the exaet time of parturi-



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 68

tion was not known. No differences in the degree of development
of such litters as compared to those examined a short while aftef
birth were apparent.

The intervals at which different Iitters were examined varied.
The young of all litters, except two for which the date of 6irth was
doubtful, were examined on--the day of birth. In one litter the young
tere examined daily up to the 10th day and weekly to 11 weeks of.
age. One litter was examined at 11 to 8-day periods for the Brst 9
days and Weekly to the 4th week, while another was observed at 2-
day intervals for 2 weeks and once a week up to the 16th week 6f
age. Four litters were handled only at we6kly intervals up to 10,
11, 12, and 17 weeks, respectively. Two litters were examined only
at birth. In one of these the single young found shortly after parturi-
tion was missing from the· nest the next day. One young of the other
litter of three was preserved at birth as a permanent record, and the
mother destroyed its nest mates the following day. When examina-
tions at weekly intervals were discontinued, the mice were observed
at longer and m6re irregular intervals. For better observation of crit-
ical detailss. a stereoscopic microscope was used in examining the yoong
at early Ages.

Weights were ordinarily taken each time the litter was handled,
while measurements were generally taken only at weekly intervals
through the 10th week and usually more sporadically thereafter.
Three newborn young were sacrificed to obtain measurements at
birth. In all other cases measurements were taken on anesthetized
young. Mice 1 to 3 weeks of age were anesthetized by placing them
in a covered finger bowl containing a small piece of ether-soaked
60#6n. Older young were anesthetized in a similar fashion in a gallon
glass jar. The animals were removed from the dish or jar as soon as
movement ceased. If they became active during measuring or ex-
amination they were returned. to the container until unconscious
again. As the same state of relaxation was not always attained, this
method sometimes resulted in slight discrepancies in measurements
of the same, individual from week to week. However, as the errors
were ordinarily small and tended to cancel each. other out and the .
number of mea@urements for each age is fairly large, the values ob-
tained are considered sufficiently reliable to reflect , growth rate5
accuratelk

BREEDING HISTORY

All litters studied were the result of matings in captivity. Births
occurred in every month of the year except February, May, and



64 BULLETIN FLORIDA STATE MUSEUM Vol. 4

November. The original female produced 8 litters totaling 15 young
in an 11-month period. She gave birth to her Bist litter in captivity
81 March 1956, and six litters followed at 28-, 49-, 81-, 26-, and 24-day
intervals. The last litter was born in January 1957. Although she
was kept with he~ original mate for 15 months afterwards, this fe-
male did not breed agaih.. The three other litters wera born to her
offspring.

The 24-day minimum time between successive litters recorded
for the original female indicates l,hat the gestati6n period in Reithro-
dontomys humulis is 24 days or less and that mating may take place
shortly after parturition. Gravid females appeared noticeably swollen
in the abdominal region several days before parturition. The original
female, whose nonpregnant weight varied from about 6.5 to 7.0 grams,
weighed 9.5 grams 8 days before the birth of one litter of tw6 and
10.I grams 4 days prior to .the birth of another litter of two.

Litter sizes ranging from.2 to 5 have previously been reported
for the eastern harvest mouse (Audubon and Bachman, 1849; Brimley,
1923; Holding and Royal, 1952). In this study the exact number of
young at birth was known for nine litters; there were three litters
ef 3 young, five of 2, and one of 1, giving a mean litter size of 2.2.
The size of two other litters was questionable, for evidence suggested
that the mothers might have destroyed some of the young. _ The sex
ratio at ,birth was even.

GROWTH

The mean weight of 20 young at birth was 1.2 grams, with ex-
iremes of 0.8 and 1.6 grams. Figure 1 sh6ws the weights of individual
young plotted at weekly intervals from birth through 10 weeks; the
curve connects the average weights at each week. Two growth phases,
characterized 6y distinctly different relative growth rates, seem evi-
dent from. thesedata. During the initial phase, from birth to 8
weeks, weekly weight increments average 1.08 grams, or approxi-
mately 86 percent of the birth weight. Weight gain averages about '
.42 grams (85 percent of the birth weight) per week during the second
growth phase, which extends from the 8rd to the 7th week. By the
latter age, the curve for weight becomes essentially asymptotie.

The freshlj,-killed newborn mice had the following measurements
before being preserved: Total length, 37, 37,88 mm.; tail length,
10, 11, 11 mm.; and hind foot length, 5,5,6 mm.  Holding and Royal
(1952) give- measurements of 85 mm. total length and 11 mm. tail
length for a single R. humulis at birth.



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 65

Figure 2 shows measurements of young at weekly intervals from
birth to 10 weeks. Growth in total length and tail length shows
twa rather distinct phases, as in the case of weight. From birth to
8 weeks of age total length increases an average of approximately
20 mm. per week, or 54 percent of the length at birth. Growth from
the Srd to the 7th weeki averages only about 5 mm. (12 percent of
mean total length at birth) per week, and there is no significant in-
crease in total length after the 7th week. The initially high growth
rate of the tail froin birth to the 8rd week is approximately 118 per-
cent (12.6 mm. per week) of the tail length at birth per week. The
tail increases only about 1.2 mm. per week (10 percent of its length
at birth) from the 8rd to the Sth week. The mean tail length at 6
weeks is less than a millimeter longer than at 5 weeks and no further
growth is evident after the 6th week.

7.0 -

6.5-

6.0-53- .i o o~

'

.

.. -.., 5.0 -

..4.5- .....

W
EI

G
HT
 I

N 
G

RA
M

S .

4.0- :
. 1-3%- /4/3.0 -

05 3~ 6 -
-

10-+ -
0.5 - -

O 1 -1 -1 I 'lilli l
BIRTH 1 2 3 4 5 6 7 8 9 - 10

AGE' IN WEEKS

. Figure 1.-Growth in weight of Reithrodontomys humutis from birth to
10 weeks of age.

The growth of the hind foot levels off at 8 weeks of age at a
mean length of 14.8 mm. There is a slight increase, to an average
length of 14.7,mm., by the Gth week, after which no further change
is recorded. Increase in hind foot length during the first week is
approximately 88 percent of its size at birth, and it grows at a rate
of about 82 percent of.its length at birth per week in the next 2 weeks.



66 BULLETIN FLORIDA STATE MUSEUM Vol. 4

As the pinna is folded over at birth, the first ear measurements
(from notch) were made at one week of age (mean 4.4 mm.). The
ear is 84 percent larger at 2 weeks, and during the 8rd and,4th weeks
shows a weekly increase of approximately 19 percent of its length at
1 week of age. No significant difference in the weekly means for ear
length is apparent after the 4th week.

120 - ..
1...

110- . -

-

100 - ..
TOTAL LENGTH

90 - 4 :
80 -

70-

ME
AS
UR
EM
EN
TS
 
IN
 M

IL
LI
ME
TE
RS

60-

A
so- ·  At ·r X

40 TAIL LENGTH'

30-

20- HIND FOOT LENGTH

10-0 -~4 -_-an. m 610 Z T -s--r-+-r 4,9. EAR LENGTH
0lilllllIlll

BIRTH 123456 7 8 9 10

AGE IN WEEKS

Figure 2.-Growth in four linear measurements of Reithrodontomvs humutis
from birth to 10 weeks of age.

The termination 6f the pariod of rapid initial growth, which is
clearly evident at 8 weeks in the curves of weight, total length, and
tail length, is correlated with the period. of weaning.



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 67

The present data do not indicate any significant difFerences be-
tween intralitter and inteflitter variability in growth rates, nor any
apparent correlation betwee-n growth rates and litter size. In several
cases a young in a given litter was noticeably smaller than its litter
mates at the time of birth. Such individuals continued to lag behind
the others in size and development throughout the period of growth,
although the differences were most pronounced at the earlier ages.

PHYSICAL DEVELOPMENT

Pelage. Young examined within several hours of birth (Fig. BA)
are pinkish in color all over, although tiny, discrete, dark pigment
spots, apparently associated with, developing hair follicles, impart a
slightly dusky tinge to the middorsal region. The skiri is rather loose,
wrinkled, and somewhat transparent, particularly on the lower sides .
and venter. In these areas blood vessels are quite prominent, and
the lower portions of the ribs, the costal caftilages and sternum, and
the abdominal viscera are Visible through the skin, Nipple sites are
faintly evident on the venter.

The vibrissae are relatively well developed, the mystacials being
the most conspicuous. Fine, widely scattered, short hairs are also
present on the middorsal region and are most abundant on the head.
Short bristles are visible on the chin. otherwise the lower sides, limbs,
tail, and venter are devoid 6f hair. Although no young were examined
immediately after birth, the assumption is made that the pelage de-
velopment observed in mice several hours after parturition is similar
to that at birth. Auduben and Bachman (1849) and Holding and
Royal (1952) have stated that newborn humulis are hairless at birth.
The latter did not record the appearadce of hair until 5 days of age.
The discrepancies between these observations and the present are
most likely the result 6f differences in the manner in which the young
were examined at the earlier ages. I found that during the first few
days of development the hairs 'on the body could be seen clearly
only under magnification.

In day-old y6ung the dorsum is slightly darker and the hairs are
noticeably more abundant. Both short dark hairs and longer lighter
ones are now evident. The short dark hairs are denser than the widely
scattered long ones and are best developed on the head and shoulders.
Whitish hairs are visible on the sides of the body and on the limbs
to the Wrists and ankles; a few also appear widely spaced on the
venter. The tail is hairless but slightly bicolor from the developfnent
of pigmented follicles on the dorsal surface. Patches of dusky pig-



.
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f
1,

.,f a 0.

Ar % *
2 2.'

r*
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:ts.
94<t

h. :i~. -1=

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14



1959 , LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 69

ment are visible on the ankles; and the soles and plantar tubercIes of
the hind feet have become noticeably darker.

By the 2nd day of age the longest hairs of the developing pelage
are approximately 1 mm. in length. The dark hairs are still most
abundant 6n the crown and rostrum. The upper surface of the
fofe and hind feet, now have a covering of hair, but none is yet
visible on the digits. Although still sparse, hairs are more abundant
on the venter. The tips of the hairs have appeared above the skin
on the dorsal surface of the tail. The skin is definitely more opaque,
and blood vessels are visible only on the lower sides, belly, and limbs.

At 3 days of age, hairs appear on the digits and on the lower sur-
face of the tail. Dandruff-like scales are also visible on the dorsum.

By the 4th or 5th days (Fig. 8B) the dorsal pelage has assumed a
faintly brownish tinge, particularly on the head, and shoulders. A
patch of dark hairs has developed on the front of the wrists and
ankles. The livef is still faintly visible through the abdominal wall,
although the ventral pelage is better developed. Epidermal scales
are still conspicuous.

The pelage is sufficiently developed by the end of the first week
(Fig. 80) to exhibit the general juvenile color pattern, the upperparts
being dark grayish brown and the underparts grayish white. A faint
pale yellowish tinge is evident· on the cheeks, on the sides of the head
anteribr to the base of the ears, and along the extreme lower side5. 1
The wash along the latdral line is suggested in some individuals as
early as the 4th day, whereas the yellowish patch of hairs in front
of the base of the ear is apparent by the 6th day. Epidermal scales
are noticeably fewer on the 6th day and disappear a day ,or two
later.

Development of the pelage after the frst week chiefly involves
further growth in the length of th6 hairs (Fig. 4A). By the end- of
the 2nd week the juvenile pelage is essentially fully developed (Fig.
48). The coat has lost.its earlier sleek appeafance and is now looser
and more fuzzy.

Data on the postjuvenile molt were obtained from 6 litters com-
prising 11 young examined at weekly intervals (Fig. 5). Molting was
first observed at the 4th week, and all individuals had essentially
completed the molt by the 9th week of age. Although individual

Figure 8.-Stages in the development' of Reithrodontomus humulis. A. Young
at birth, B. Four days  of age. C. One week 9f age. Scale in each case equa15
1 cm.



70 BULLETIN FLORIDA STATE MUSEUM Vol. 4

variation in the timing and sequence of the replacement was rather
marked, the general pattern appeared to be for the molt to begin on
the renter and then spread to the upperparts, beginning first on the
lower sides, extending dorsally at midbody, and then proceeding an-
teriorly and posteriorly onto the head and lower back. In some in-
dividuals a patch of juvenile pelage persisted on the crown after the
remainder of the body had completed the molt. As the molt was
completed rather rapidly, the times at which the young were
examined were too far apart to provide a detailed picture of the
molt pattern. The sequence observed was, however, in general
agreement with that established by Hooper (1952) through the study
of conventional museum skins of various species of Reithrodontom!/s

*A

I '- -

*It .

42* E V-fy=

04 g. 1 61:LAGN,JE:-- F<{1~

1-1
B

Figure 4.-Stages in the development of Reithrodontomus hunmlis. A. Ten days
of age, B. Two weeks of age, Scale in each case equals 1 cm.



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 71

The postjuvenile or subadult pelage is browner above and more
distinctly buffy along the lower sides than the juvenile coat, but
not as bright or rich' as the full adult pelage. The venter of the sub-
adult animal also· appears more thickly haired and less grayish than
in the juvenile. Mice in the postjuvenile pelage frequently exhibited
a faint buffy wash on the breast and occasionally along the midline
onto the belly. ·It was not determined whether this coloration is due
to pigment in the hairs Or to a stain from some glandular secretion.

Five of 11 individuals completed the postjuvenile molt between
the 8rd and 6th weeks, while in 6 the molt ocburred in the interval
between the 5th and 9th weeks. The duration of the molt was cal-
culated from the last week that no molt was observed to the first
week tliat complete postjuvenile pelage was present (in61uding some
specimens that retained a small patch of juvenile pelage on the
crown). On this basis the maximum period of molting'' ranged from
1 to 3 weeks, with an average of 2 w6eks.

- Full adult pelage apparently .is not attained until one or more
additional complete or partial molts. Data obtained on molts after
the postjuvenile, though incomplete, do indicate another molt, and
perhaps two in some individuals, before 30 weeks of age. One
mouse was molting af 15 weeks and completed what was apparently
a second molt phase between 24 and 26 weeks. Another completed
a molt between 20 and 22 weeks, and a third was uhdergoing a molt
at 17 weeks. One individual of a litter of two began to molt at 13
weeks of age and was in an advanced stage of replacement of hair
when killed the following week. Its litter mate was molting when
examined at 17  weeks. The pattern of these molts Was not clearly , ,
evident. However, replacement seemed in general to proceed as in
the postjuvenile molt, as also indicated by Hooper (1952), although
it appeared to be even more diffuse and variable. The pelage fdllow-
ing these m61ts was grossly like that of the majerity of adults, being
a richer brown than the postjuvenile coat. Adult humulis are be-
lieved to undergo two partial br  complete molts a year (Hooper, 1948).

Eyes, ears, and claws. The eyes of the newborn young are cov-
ered with the sealed lids, which are so transparent that the iris is dis-
tinctly visible as a dark ring and the lens as a pinkish center. The
crease marking the 1jne of fusion of the upper and lower lids is not
well marked. By the 2nd day the eyes are less prominent, as pig-
ment is forming in the lids, and by the 4th day they are completely
obscured. At this time the slit between the lids. is considerably more
noticeable. The eyes opened between the 7th and. 10th days of age.



72 BULLETIN FLORIDA STATE MUSEUM Vol. 4

Of 9 young in which the opening Of the eyes was recorded, the eyes
were visible on the 7th day in 6, on the Sth day in 1, and on the 9th
or 10th day in 2. The eyes of the single humulis individual studied
by Holding and Royal (1952) opened between the 10th and 11th days
of hge.

11

8 
1 

1 
1

* 9-
4~ <it~:~2 <fi s, dE~~3 <S~~3 d<~3S

3Ffi 3;<=Z~=2==

5 CE CE CE CE <c=~ <~~~
oue«641414

Lr <f:r & <C <~0*4,&~~ (4~05=4~~ fbju

LI
TT

E
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 N

UM
BE

R

C=U Iii.::::,2, 42- <I~-
3

. F~~==Et:=55~

dc==t 2<====E <<* <63- 40 t===j==
2 FC N<===5 3-

14=3 4-41410<0
Ne-40== %- L

4 5 6 7 8 '9
AGE IN WEEKS

Figure 5.-Postjuvenile molt  patterns in six litters of Reithrodontomys hum#Zis
examined at weekly inter.vals. Molt on the tipper-parts ohly is shown. Clear
area indicates juvenile pelage, stippled area intermixture of juvenile and post-
juvenile pelage, and hatched area postj,ivenile pelage.



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 78

At birth the pinnae are tightly folded over and conceal the open=
ing of the external auditory meatus. The p6sterior margins are
slightly more elevated than the anteriof ones. The pinnae unfolded
in most of the young on the day following birth, though in some
this did not occur until the next day. At first the pinnae stand out
from' the side of the· head but within a day lie f[at against the head.
The pinnae are fleshy and but lightly pigmented during the first week
of development. Hairs appear on the anterior portion of the lateral
surface of the ears in some individuals by the 4th day of age, and by
the end of the first week a sparse coating of hair covers the entire
lateral 5urface.

The external auditory meatus remains tightly closed until at
least the 7th day of age. Five of 16 young had the meatus open on
the 7th day. In 2 it was open on the Sth or 9th day,'and in 8
others, on the 9th or 10th day. No reaction to sound was recorded
for any young before the meatus opened. In several instances the
typical reaction to a sharp sound, a convulsive start, was not elicited
until the day after the meatus appeared to be open.

The digits Qf the fore and hind feet are well separated at birth,
and short, blunt, more or less translucent claws are present. The
claws become noticeably sharper by the 8rd day and by the ,4th
are seeminglymore opaque and harder.

Dentition. In one young the lower incisors were visible on the
4th day of age, but in the rest the incisors were not clearly above /
the gum line until the 6th or 7th day. The lower incisors seemed
to erupt before the uppers in some individuals, whereas in others
both pairs appeared above the gum,line about the same time. The
incisors are white and chalky at first, but by the end of the 2nd week
have taken on a faintly yellowish tinge. At this age the upper in-
cisors are from .5 to .8 mm. in length and the lowers from 1.0 to 1.5
mm. At the end of the 8rd -week the upper incisors are about  1 mm.
long, the lowers 2.0 to 2.5 mm.. and both pairs are brighter yellow
than at 2 weeks of age. Little change in the size of the incisors is
apparelit aftef the 8rd week. The grooves in the anterior faces of
the upper incisors are evident as soon as the teeth have extended
beyond the gum.

In one mouse sacrificed at the end of the #th week the cheek
teeth were all in position, although the last molar in both upper and
lower jaws was slightly below the level of the others.

Weaning. One individual was observed still nursing at the end
of the 2nd week; another at this age was seen feeding on fresh corn



74 BULLETIN FLORIDA STATE MUSEUM Vol. 4

and was apparently fully weaned a week later. Other young were
noted outside the nest with the parents at night by the end of the
End week, and most were foraging regularly outside the nest at 8
or 4 weeks of age. A mouse killed at the end of its 4th week had
only solid food in the stomach. It thus appears that weaning occurs
between the 2nd and 4th weeks, and that most of the young probably ~
take mainly solid food by the end of the 8rd week of ag6.

Sexual maturity. Although the vulvas of females opened for the
first time between the 4th and 7th week, records of two pregnancies
in captive-born young suggest that full breeding status may not be
reached until several weeks later. Although caged with fully adult
males, these two young females did not become pregnant for the
first time until 11 and 20 weeks of age.· A female sacrificed at 4
weeks had a small, undeveloped genital tract showing no evidence
of reproductive activity.

The testes of males showed slight enlargement, on the basis of
external examination, by the end of the 4th week, but it was not
until the 7th or 8th week that the testes conspicuously filled out the
cremaster and the latter was sparsely haired. Two males killed at
10 and 14 weeks of age each had testes 5 mm. long and 8 mm. wide.
The accessories were fully -developed, and the cauda epididymal
tubules were enlarged and contained spermatozoa.

BEHAVIORAL DEVELOPMENT

The young are capable only of relatively uncoordinated move-
ments at birth. When disturbed they writhe and twist the b6dy
and move the limbs about aimlessly. If they chance to roll onto the
venter they can drag themselves a short distance, but soon t6pple
over. During the efforts to crawl the fore limbs seem to provide
most of the power and to be under better control than the hind limbs.
The young sleep or rest on their sides and at brief intervals give in-
voluntary convulsive twitches.' They utter very high-pitched, in-
sectlike peeping notes.

When 2 days old some young reacted quickly to a slight touch
on the side by giving a sudden jerk, sometimes throwing themselves
so violently that they landed on their backs; other individuals at
this age did not react similarly. It is not possible to say whether
this is b'ecause of individual variation or simply that the relhtively
shoFt periods of time the animals were out of the nest for examination
precluded the possibility of observing~ the full range of behavioral
capacities of any litter on a given date.



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 75

By the 4th day of age the young begin to utter a louder, more
insistent squeak when disturbed. At this age they are able to crawl
somewhat better and also begin to offer resistance when held in the
hand. As earlier, the propulsive force seems to be mostly ~ supplied
by the fore limbs which now exhibit better coordination than before.
At this age the tail begins to show its semiprehensile nature, moving
in a wormIike fashion when the animal is active and responding quick-
lyto ·a slight touch by bending towdrd the stimulus. The young
rest on the venter more frequently and are apparently able to as-
sume this position at will.

When 1 week old the mice show considerable improvement in
their crawling ability, although they still drag the venter on the
ground and their legs still splay out  to the sides. The hind limbs
now function more smoothly, but are still less well coordinat6d than
the fore limbs. The young can scratch the body with a hind leg
and attempt to sit up and wash but, although they support themselves
on the fore arms, are unsteady and fall over easily. When disturbed
in the nest they exhibit a defense reaction similar to that sometimes
observed in adults-lying on the back and kicking violently. Con-
vulsive twitches are still noted when the young are sleeping or 6ther-
wise motionless.

The opening bf the eyes and ears signals a marked change in be-
havior. The young become more active and responsive to their en-
vironment. At 10 days they walk more or less normally. They hold
the  legs fairly well beneath the body and no longer drag the venter.
They sit up to wash more steadily than before and. can lift the fore
limbs off the ground during the washing process, although they oc-
casionally lose their balance when doing so. They e*hibit a definite
tendency to remain together, crawling over and under one another
in a wriggling mass. They show more fear of the observer and strug-
gle hard to escape when grasped. Convulsive twitches are no longer
noted when they are still.

By 12 days of age the young have grown very active. If removed
from the nest they scamper about with the typical "hitching" gait
of the full grown animal and are difficult. to capture. They still
tend to remain together but wander about more. The use of the
tail for support and balance when the animal is climbing is quite
evident. The young ean now sit erect on the hind legs to. wash in
the manner typical of the adults. They wash the snout Brst, then the
fur behind the ears, bringing the fore feet  briskly forward over the-
backs of the pinnae. ·Next ihe flanks receive attention, the mouse



76 BULLETIN FLORIDA STATE MUSEUM Vol. 4

twisting from one side t6 the other and 'stretching down and back
to reach the pelage with mouth and paws. Finally, they groom the
entire tail from the base to the tip.

The young are venturing from the nest and look and behave like
miniature adults at 2 weeks of age. When in the nest they tend to be
difficult to flush and when outside run quickly back to the nest when
alarmed.

Most of the harvest mice composing the colony were nervous
and jumpy. in temperatment. If disturbed they usually dashed and
leaped wildly about the cage or aquarium in a frenzy of excitement.
Frequent handling appeared to have no effect on the disposition
of the young, and after the eyes and ears opened they soon became
as wild as the adults. The one exception to the generally nervous
temperament of the animals, at least at first, was the original female.
Immediately after capture she was extremely tame and would sit
on the hand and allow herself to be stroked gently without showing
any inclination to escape. Although her mate was seldom seen dur-
ing the day and remained hidden in the nest, she spent many hours
sitting in the top of a dried clump of brome sedge stalks with her
back pressed against the top of the aquarium. However, after sev-
eral months in captivity this female also grew quite wild and as diffi-
cut to catch and handle as the others. Coincident with this change
in temperament, she also abandoned h-er diurnal activity to a large
extent.

Both the young and adults were highly tolerant of other indi-
viduals. All animals in a cage or aquarium usually shared a com-
munal nest and newly introduced individuals were readily accepted
without apparent antagonism. The original female and her mate
often occupied a single nest with recently born young and those of
a previous litter or litters. However, several observations suggested
that the adult female might be somewhat antagonistic to other indi-
viduals· for a short period just before or after the birth of a litter.
In several cases the male or young of a previous litter who were
sharing a nest with a female in advanced pregnancy were noted out-
side the nest near the time when parturition was believed to have
occurred. Such aggressive behavior, if it actually was, was appar-
ently of short duration, for the other individuals would soon be found
back in the nest with the female and her young.

The only fighting among the captives was recorded in a cage
housing several males and females of older litters. Most of the
fighting apparently took place between the males. Evidence of strife,



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 77

such as torn ears, patches of hair missing, and tooth marks in the
skin, was not noted until the mice were Obviously sexually mature,
at about 11 or 12 weeks of age.

PARENTAL CARE

One observation suggests that the nest-building drive in pregnant
females may be intensified shortly before the young are born. A
gravid mouse housed in an aquarium for several days made no at-
tempt to build a typical nest until suddenly one night she was noted
actively so engaged. She bore her litter between 9 and 10 a.m.
the following day.

This female was also seen moving corn into the completed nest
the night before parturition, which suggests that pregnant females
may tend to provision the nest before bearing young. What might
possibly be further evidence of this was noted in the original female
before her first litter in captivity was born. About a week before
parturition, a cache· of corn was discovered in a small depression at
the base of a clump of dried grass. The kernels had- been carried
from a foot away. Although this female and her mate had been
kept in the same aquarium for 3 months, no food storage had been
noted previously.

Females exhibited concern for their young through the 3rd week
after birth. Mothers often were extremely reluctant to abandon the
nesf and attempted to,ward off fingers or forceps with boxing motions
when the young were being secured for examination. In one case
a female, using her mouth and fore feet, attempted to pull her week-
old young back into the nest as they were being lifted out. Some-
times a. mother would rapidly repair the nest wall from within as
the observer began to open it from the outside to get at the young.
On one occasion a female frequently interrupted her defense of 10-
day-old young to push them deeper into the bottom of the nest,
giving them a quick, nervous grooming each time. Mothers were
not recorded as actively defending the young after the 2nd week,
although during the 3rd week they often stayed around the nest
when the young were being handled and appeared to show concern.

On several occasions mothers flushed from the nest carried along
a 7 or 8 day-old young clinging to a nipple. In one instance the
y6ung mouse was dragged roughly about the cage for some 80 seconds
and even hung on when the mother ran upside down across the
wire cage top. Audubon and Bachman (1849) did not observe the
young being carried on the nipples in the captive harvest mice they



78 BULLETIN FLORIDA STATE MUSEUM Vol. 4

studied and believed this to be a behavioral difference 'between
Reithrodontomys and feromyscus.

Females readily retrieved small young placed up to 6 inches from
the nest. Before picking up the young, they performed a kitual of
thoroughly sniffing and wasbing it while vigorously rolling it around
with the fore feet. Females were seen to carry small young either
by the nape of the neck or the skin of the belly. Often when young
were removed from the nest to observe the retrieving response, the
mother, though the young might be in full view only a few inches
away, would seem to ignore their presence until they began to squeak.

A female was twice seen trying to get older and more active y6ung
back to the nest. Once she seized a 10-day-9ld individual moving
about near the nest by the shoulder and tried to push it through the
wall of the nest. Failing in this, she went into the nest and pulled
the young unceremoniously through the side. Several days later
when a young of the same litter was returned to the aquarium still
unsteady from anesthesia, the mother came up to it, straddled and
cleaned it, then went directly to the nest. The young followed her
and when it reached the nest began to push against it. The female
came partly through the side, seized the young with her mouth and
fore feet, pulled it inside, and then repaired the nest. ivall.

The orikinal pair was kept togetber throughout the observations
and the one male fathered the -8 litters the female- produced. Sev-
eral instances of apparent solicitude toward young in the nest by
this male were observed. Although he generally left the nest quickly
when it was disturbed by removing the young, he did occasionally
remain with the family and engage in defensive behavior, but was
always more easily driven away than his mate. On one occasion he
was seen to come part way out of the nest and retrieve a 4-day.old
young that had been placed about an inch away. After dragging
the young into the nest chamber he proceeded to clean it thoroughly
in. the same manner as the female and then set about repairing the
opening in the nest. Examples of paternal care in other genera of
cricetids (Baiomys and Peromyscus) have been given by Blair (1941)
and Horner (1947).

Twice, what appeared to represent a weak retrieving response in
young of an older litter to those of a younger litter was observed.
A pair of adults and litters of 2-day and 4-week-old young were
housed in the same cage and shared the same nest. One night when
the adults and older young were outside one of the babies was placed
a few inches away from the nest. Soon one of the older young came



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 79

over to it and sniffed and rolled it about to wash it in the same way
as the mother. A few moments later it was joined by its litter mate
and both washed and rolled the baby around but made no effort to
carry it back to the nest. One of these tw6 older young showed the
same behavior when the exDeriment was repeated the following
night.

COMPARISON OF THE DEVELOPMENT OF
REITHRODONTOMYS HUMULIS AND R. MEGALOTIS

- The descriptions given by Svihla (1981) and Smith (1986) of
the development of Reithrodontomys megalotis provide a basis for at
least a limited ,comparison of the developmental patterns of that -
species and R. humulis. As variations in certain aspects of growth
and development in different species or subspecies in, some mam-
malian genera, notably Peromyscus, have been shown to have taxo-
nomic, behavioial, or ecological implications, Such a comparison is
of interest.

The breeding seasons, gestation periods, and fecundity of the
two species appear to be comparable. Smith (1986) - indicates that
breeding occurs in wild R. m. longicaudus nearly every month of the
year, wit:h peaks in April and October. The present data on captive
eastern harvest mice suggest a similarly prolonged bfeeding cycle,
although this is not as yet well corroborated with field data. The
gestation period calculated for megalotis by Svihla (1981) was 28 or
24 days, whereas the evidence fer humulis indicates an equal, if not
shorter, period. Twenty-seyen litters reported for megalotis (Svihla's
and Smith's data combined) range from 1 to 7 with a mean, of 2.5,
while the averageof 11 humulis litters (present study and Audubon
and Bachinan, 1849) is 2.4 with extremes of 1 an'd 4. Brimley (1928) ,
gives a range of from 2 to 5 embryos for humulis in North Carolina
but does not include individual counts or the mean. These samples
show no significant difference in litter size between. the two species.
Adequate information on average and maximum fecundity in the two
species is lacking, but Svihla's- (1931) record of a female megalotis
producing 7 litters totaling 17 young in a year's time is equalled by a
humulis female *ho gave birth to 8 litters totaling 11 young in an
interval of 11 months.

The weights of newborn megalotis and humulis agree closely,
ranging from 1.0 to 1.5 grams in the former and fr6m .8 to 1.6 grams
in the latter. Weights of 1.07 and 1.15 grams -have been given for
the young at birth of a third species, R. .fuluescens (Svihla, 1980).



80 BULLETIN FLORIDA STATE MUSEUM Vol. 4

There is some disagreement in the descriptions of the newborn
young of fuluescens, humulis, and two subspecies of megalotis and
of the time of first appearance of body hair in the last two species.
Svihla (1930) describes R. fuluescens as being naked at birth. R.
humulis has also been considered to be hairless at birth, with the first
body hair not appearing until the 5th day of age. Svihla (1981) states
that the newly born young of R. m. megalotis are without hair, which
first becomes visible on the head at 5 days, while Smith (1936) claims
that vibrissae are present at birth in the subspecies longicaudus and
that the first body hair is visible at 3 days. As mentioned above, the
present.study found beth vibrissae and scattered body hairs in humults
examined shortly after. births and the lack of agreement with previous
accounts is considered as possibly the result of differences in observa-
tional techniques. A similar explanation may also account, at least in
part, for the alleged variations reported in the other species.

Data given on further pelage development in megalotis by Smith
can probably be compared more confidently with those obtained for
humulis. In the former, the first hair on the tail and the first sug-
gestion of coloration of the body hair was noted on the 4th day, while
in humulis hairs were barely visible on the tail at 2 days of age, and
coloration was evident by the 4th or 5th days.

In Svihla's (1981) study the eyes of R.' m. megalotis young opened
at 12 days of age, whereas the progeny of R. m. megalqtis-R. m.
nigrescens crosses opened their eyes a day earlier. Although the
latter form has since been synonymized with megalotis, these data,
if consistent, may be indicative of at least some degree of genetic di-
vergence between the two populations. Smith noted eyes opening in
one litter of R. m. longicaudus at 10 days. There appears to be an
average difference of about 3 days between the opening of the eyes
in megalotis and humulis, in which most of the young observed had
their eyes open on the 7th day.

Only Smith gives data on the development of the ears in megalotis.
He notes that the pinnae became erect on the 2nd day and the audi-
tory meatus opened on the 9th day in one litter, each event about a
day later than the average for humulis.

<Smith notes that the lower incisors were visible in megalotis at
4 days, whereas in humulis the eruption of the incisor teeth was
not recorded in most individuals until the 6th or 7th days. The dis-
crepancy here may be more apparent than real, since in the present
study the incisors were not considered as erupted until they appeared



1959 LAYNE: DEVELOPMENT OF EASTERN HARVEST MOUSE 81

above the gum line„ although they may have been visible at the .gum '
lihe a day or so earliet.

The completion of weaning at about 3 weeks in inegalotig falls
well. within the· Deriod of weaning in humulia. Thus; there fippears
to be no significant difference in this aspect of the development of

- these species,
The two subspecies of megalotis that have been studied appear

to differ in temperament. Svihia (1981) indicates that mice of- the ,
nominate subspecies had generally pugna6ious dispositions, whereas
Sinith (1936) found longicdudus individuals highly tolerant of one an-
other. In this respect, humulis seemingly agrees with the latter.
Similar differences in temperament between various species and sub-
spedies of deer mice, Parbrngscu# have been noted by Svihla (1982),
King (1958), McCabe and Blanchard (1950); and 6thers.

Comparison of behavidral development on the basis of general
written descriptions is difficult. However, the ' chronology of be.
havioral development described by Smith for R. m.· longicaudus ap-
pears to correspond rather well to that 9f humulis.

In summary, to the extent that the data available permit  dompari-
son, development is closely similar-in. the species humulis and megalo-
tis, although the latter may develop at a slightly slower rate, perhaps,
in keeping with its somewhat greater. size. The apparently c16se
similarities in developmental patterns and chronologies in the two
species are in aecordance with their morphological likenesses, on the
basis of which the two forms afe placed in the same .species group
under the subgenus Reithrodontbmys.

LITERATURE (ITED

Audubon, John James, and John Bachmah
1849. The quadrupeds of North Americas ·vol. 2. New York: G. A. Lockwood,

884 pp

Blair, W. F.rank
1941. Observations on the.life history of Baiom!* tal/lori subater. Jour. Marn-

Smal., vol. 22, no. 2, pp. 878-883.

Brimley, C. S.
1928. - Breeding dates of small mammals at Raleigh, North Carolina. Jour.

Mammal.,.vol. 4, no. 4, pp  268-264.

Holding, B. F.., and O. L. Royal ' ,
1952. The development of a young harvest ,mouse, Reithrodontomys. Jour.

Mammal., v61.38, no. 8, p. 388.
Hooper, Emmet T.

1948. Geographic variation in harvest mice of the species Reit4rodontomys
humulis. Otc. Pap. Mus. Zool. Univ. Mich:, no. 477, 19 pp



82 - BULLETIN FLORIDA STATE MUSEUM Vol. 4

i952. A systematic review of the, harvest mice (Genus Reithrodontomys) of
Latin America. Misc. Publ, Mus. Zool. Unix, Mich., no, 77,255 ·pp.,
9 pls.

Horner, Elizabeth B. ,
1947. Parefital care of young mice of the genus Perom!/scus. Jour. Mammal.,

vol. 28, ·no. 1, pp. 31-86.

King, John A. . ,
1958. Materdal behavior and behavioral development in two subspecies of

Peromykeus maniculatus. Jour. Mammal:, vol, 89, no, 2, pA 177-190.

McCabe, Thomas T., and Barbara D. Blanchard
1950. Three species of Peromyscus. Calif.: Rood Ass6ciatds, 9 + 186 pp.

Smith, Clarenqe F.
1986. Notes on the habits of the long-tailed harvest mouse. Jour. Mammal.,

vbl. 17, no. 8, pA 274-278.

Svihla, Arthur
1932. A comparative life history study of the mice· of the genus Reromvous.

Misc. Plibl. Mus. Zoal. Univ. Mich., na. 24,89 pA

Svilila, Ruth Dowell '
1980. Notes on the g6lden harvest mouse. Jour. K{aminal., vol. 11, no. 1,

pp. 58-54.
1931. Notes on desert and dusky harvest mice (Reithrodontomys megalotis

megalotis and R. m. nigrescens). four. Mammal; vol. 12\ ,no. 4,  pA
868-865.



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