Hispid Cotton Rat 
45 
lacking well-developed mammary glands and nipples); primiparous 
females with one set of placental scars and corpora albicantia or with 
embryos and corpora lutea (but not placental scars or corpora albicantia); 
and multiparous females with more than one set of placental scars or 
with embryos, corpora albicantia, and placental scars. In the analysis of 
body mass, the mass of the uterus was subtracted so that pregnancy 
would not confound the results. 
Additional data were recorded for males: (1) testes position (scrotal 
or abdominal), (2) paired testicular mass (mg), and (3) condition of 
epididymal tubules (looped or convoluted). Males were considered to be 
breeding if the epididymal tubules were convoluted (Jameson 1950). 
Data are presented as x ± SE. 
RESULTS 
Females 
The most reliable indicator of female reproductive state is pregnancy. 
Of 148 females, 48% were pregnant. However, no pregnant females were 
collected from November through February (Fig. 1). The level of 
breeding in females was high from March through October, when the 
average pregnancy rate was 68.7% (including October of both 1983 and 
1984). 
Using the Chi-square test, we found no differences (x^ - 0.76, 2 df, 
P > 0.50) in the proportions of females that were pregnant in the April- 
May, June-July, and September-October bimonthly periods (not sampled 
in August). Thus, as measured by pregnancy, females bred at a uniform 
rate during these months of peak activity. 
Overall, litter size averaged 5.00 ± 0.284 SE. However, during the 
peak breeding months of April to October, females averaged 5.18 ± 
0.274 embryos per female. There was significant variation in litter size {- 
embryo counts) throughout the months of the breeding seson (ANOVA: 
F - 30.46, df = 4,62, P < 0.005) with largest litters (a = 7.83 ± 0.984) in 
May. In contrast, females in April averaged only 4. 18 + 0.652 embryos 
per female, and the two pregnant females in October 1983 had one and 
two embryos, unusually small litters for the cotton rat. 
The pregnant females were divided into primiparous (those in their 
first reproductive experience) and multiparous (experienced breeders) 
groups to determine whether a difference in litter size was attributable 
to reproductive experience. Although there was a trend toward larger 
litters in multiparous females, there was no significant difference between 
the litter sizes of primiparous (x = 4.76 ± 0.378 SE) and multiparous (x 
= 5.72 ± 0.371) females during the “peak” breeding months (/ s = 1.74, df 
= 65, 0. 1 > P > 0.05). Thus, season had greater influence on litter size 
than age of the female. 
