58 
Robert K. Rose and Michael H. Mitchell 
recruitment of young, lightweight animals; therefore, we can assume 
that females either lose mass or fail to gain significant mass during the 
winter months. From February through July, the monthly mean mass 
of females rose steadily, indicating a real increase in body mass during 
this time. Then, when the young of the year finally entered the trappable 
population, the monthly mean mass of females (and males) declined. 
Patterns of body length were similar to those of body mass. 
Although both length and mass are measures of body size, length may 
be a more reliable index of body growth in S. hispidus because (1) 
animals lose mass but not length during starvation or during winter, (2) 
both sexes divert resources away from growth and towards reproduction 
during the breeding season, and (3) females store up body fat during 
pregnancy in preparation for the greater energy demands during lactation 
(Randolph et al. 1977). 
In conclusion, cotton rats in southeastern Virginia seem to be well 
adapted to the northern limit of their present distribution on the East 
Coast; their March-to-October breeding season and the sustained growth 
of overwintering individuals suggest high survival rates of both young 
and adults during the winter months. The modest litter size may 
indicate that, unlike Kansas populations, Virginia populations have not 
been selected for larger litter sizes to compensate for winter mortality. 
ACKNOWLEDGMENTS.— We thank Patricia Hopkins, Sean Priest, 
and David Wade for field work and other assistance with this project. 
LITERATURE CITED 
Cameron, G. N., and P. A. McClure. 1988. Geographic variation in life history 
traits of the hispid cotton rat ( Sigmodon hispidus). In Evolution of Life 
Histories of Mammals: Theory and Patterns (M. Boyce, editor). Yale Univ. 
Press. 
Dunaway, P. B., and S. V. Kaye. 1961. Studies of small mammal populations 
on the radioactive White Oak Lake bed. Trans. 26th N. Am. Resource 
Conf.: 167-1 85. 
Dunaway, P. B., and S. V. Kaye. 1964. Weights of cotton rats in relation to 
season, breeding, and environmental radioactive contamination. Am. Midi. 
Nat. 71:141-155. 
Fleharty, E. D., and J. R. Choate. 1973. Bioenergetic strategies of the cotton 
rat, Sigmodon hispidus. J. Mammal. 54:680-692. 
Genoways, H. H., and D. A. Schlitter. 1967. Northward dispersal of the hispid 
cotton rat in Nebraska and Missouri. Trans., Kans. Acad. Sci. 69:356-357. 
Goertz, J. W. 1965. Reproductive variation in cotton rats. Am. Midi. Nat. 
74:329-340. 
