54 
Robert K. Rose and Michael H. Mitchell 
Randolph et al. (1977) found that the fat accumulated during the last 
half of pregnancy was used during lactation, when energy demands 
outstripped the female’s speed in processing food. It is plausible that 
females in the later stages of pregnancy would be increasingly attracted 
to the high-energy food source (mixed seeds) that was used as bait. 
Although Dunaway and Kaye (1964) did not calculate monthly 
pregnancy rates, they did observe low levels and apparently sporadic 
breeding throughout what they judged to be a relatively mild Tennessee 
winter. In Oklahoma, Goertz (1965) found no pregnant females during a 
severe winter, but he did find pregnant females during November, 
December, and February of a milder winter. Goertz reported highest 
pregnancy rates during May to September. Haines (1961) recorded no 
embryos in Texas cotton rats from October to February, but he did 
record corpora lutea throughout the year. Haines observed the highest 
pregnancy rates between February and July, with low rates after 
September and the lowest rates in December. Thus, the breeding season 
seems to be somewhat earlier in Texas compared with Tennessee, 
Oklahoma, or Virginia. 
In duration, methods, and analysis, our study most closely parallels 
that of McClenaghan and Gaines (1978), conducted near Lawrence, 
Kan. They found no breeding from November through March, which is 
similar to what we observed in the Virginia population. The pregnancy 
rate in Kansas was low (30%) in April, highest (over 80%) in May, and 
generally greater than 70% from June through October. Overall, the 
patterns of breeding in Virginia and Kansas were similar for both sexes. 
Litter Size 
Within a species, litter size is affected by several interacting factors, 
including age, parity, body weight, and nutritional state (Sadleir 1969). 
In a recent exhaustive review, Cameron and McClure (1988) examined 
the patterns of breeding in female Sigmodon hispidus by evaluating 
published and unpublished laboratory and field data. Using a stepwise 
multiple regression analysis on data from 18 studies, Cameron and 
McClure (1988: table 2) examined the patterns of geographic variation 
in litter size and the effects of body size on litter size. By finding 
latitude, longitude, and body length to be significantly associated with 
mean litter size, their analysis “confirmed the existence of both north- 
south and east-west variation in litter size.” Largest litters were reported 
for the large females of the north-central states. 
A further analysis “indicated that latitudinal and longitudinal 
variation in litter size were due primarily to differences among subspecies” 
(Cameron and McClure 1988). Specifically, S. hispidus texianus, which 
had the largest litters at 7.20 ± 0.23 SE, averaged 8.35 ± 0.35 in Kansas 
