Hispid Cotton Rat 
55 
but only 5.10 ± 0.37 in coastal Texas (Houston). Nutrition may also 
contribute to these differences within this subspecies (Cameron and 
McClure 1988; table 6). Although litters of S. hispidus virginianus , the 
subspecies in Virginia, were significantly larger than those of Mexican 
and Central American subspecies, they were significantly smaller than 
those of S. h. texianus (Cameron and McClure 1988). 
The litter size of 5.00 for Virginia cotton rats lies in the range of 
values reported from other studies (Cameron and McClure 1988: table 
2), although on the low side for “northern populations.” Populations 
from Tennessee averaged 6.1 embryos per litter (Dunaway and Kaye 
1961), from Oklahoma 6.0 (Goertz 1965), from western Kansas 6.7 
(Fleharty and Choate 1973), and from eastern Kansas 9.0 (McClenaghan 
and Gaines 1978). Furthermore, laboratory animals derived from 
Houston, Kansas, and Tennessee populations and raised by McClure at 
Indiana University remained significantly different in average litter size 
even after 16-28 generations and 8-12 years in the laboratory (Cameron 
and McClure 1988: figure 2). Thus, the determination of litter size in 
Sigmodon hispidus is complex, involving both genetic and environmental 
factors. 
Although Lawrence, Kan., and Portsmouth, Va., are both near 37° 
N latitude, the Kansas winters are longer and colder (average 2° C), in 
the absence of moderating oceanic effects. In coastal Virginia, snow falls 
only once or twice a year and periods of freezing weather rarely last 
more than a few days. Despite the more moderate conditions in Virginia 
(Cameron and McClure 1988: table 4), the Virginia cotton rats did not 
breed longer than the Kansas cotton rats, and Virginia litter sizes as well 
as body sizes were significantly smaller. However, female cotton rats in 
Virginia were pregnant at nearly maximum levels throughout the breeding 
season (Fig. 1), and there was a trend (0. 1 > P> 0.05) for multiparous 
females to have larger litters than primiparous females. Thus, differences 
in age of onset of breeding and in longevity (neither of which was 
measured in these studies) may be important in affecting geographic 
differences in the dynamics of these populations. 
Male Breeding 
The breeding season of males began in February and lasted to 
November (Fig. 2). Based on the breeding criterion of convolutions in 
the cauda epididymides, 73.3% of males were in breeding condition in 
February and 100% were fertile from March through June. McClenaghan 
and Gaines (1978), who also used epididymal convolutions to determine 
breeding condition in males, did not find 100% breeding in any month. 
Their highest monthly rates were just under 90% in June and August; 
and in all other months during the breeding season except May, fertility 
