57 
Hispid Cotton Rat 
near the northern limit of distribution for hispid cotton rats; therefore, 
it is not surprising to find some similarities. Differences also were noted. 
Although the breeding season in Virginia started a month earlier in both 
sexes, it lasted until October for females in both Kansas and Virginia. 
Another similarity in the breeding of Sigmodon in these two studies is 
the percentage of breeding females; the rate was relatively low during 
the first month in which pregnancies occurred and then rose sharply to 
near the maximal rate in the following month. In McClenaghan and 
Gaines (1978), this trend must be inferred because their study ended in 
April. However, if the high May level at the start of their study can be 
extrapolated to the preceding April, a large increase in breeding level 
occurred at the same time in Kansas and Virginia. 
In Texas (Haines 1961), the breeding season began in the same 
month as in Virginia, but it ended one month sooner. This is an 
unexpected result if we assume that the breeding season has been 
shortened at more northerly locations because of the constraints of 
winter on the energy budgets of mammals. We would expect the 
breeding season to be longer at more southerly locations. Goertz (1965), 
who found pregnant females in some winter months, believed that 
breeding was possible in Oklahoma under the favorable conditions of 
mild winters. 
Effects of Body Size 
Several factors affect patterns of body size in Sigmodon hispidus, 
including sex, latitude, subspecies, and nutrition. In some, but not all, 
populations males are larger than females, and northern populations 
tend to have larger skeletal sizes and, in some seasons, higher fat 
content. Sigmodon h. texianus is significantly larger than all other 
subspecies (Cameron and McClure 1988). McClenaghan (1977) found 
generally larger skull and skeletal variables for cotton rats from northern 
populations (Kansas and Virginia) than from southern localities (Mexico), 
but Kansas and Virginia populations differed in only one skeletal 
feature. However, the seasonal pattern of body growth of Virginia 
cotton rats differs from that of Kansas cotton rats, in which Slade et al. 
(1984) found that the large adult animals lost weight over the winter. By 
contrast, in Virginia, the males in particular gained body mass steadily 
throughout the winter months (Fig. 4). This pattern of winter increase is 
evident but less well defined for body length (Fig. 5), although males did 
increase in length nearly every month from October to May or June. 
Mean body mass for females was low in October (79.7 ± 6.77 g) 
but it rose sharply to 98.6 ± 10.19 g in November. The mean values for 
December and February were low, indicating that females were not 
gaining weight during this time. Because there was no breeding during 
these months, the lower mean mass in winter cannot be a result of the 
