Northward dispersal in Sigmodon hispidus has not significantly affected early growth rate of young or size of young at birth. But there has been a significant increase in litter size in the northern population. Northern S. hispidus have the same size young, but more young per litter than the southern population. Maintenance of the size of young at birth in northern populations may be related, in part, to the development of thermoregulatory capabilities. In the northern population there was seasonal variation in litter size, growth rate (tail length), and size of young at birth (tail length and body weight). Litters of S. hispidus produced in cold weather months are significantly smaller than those produced in warm weather months. INTRODUCTION The northward dispersal of the hispid cotton rat (Sigmodon hispidus spp.) has been well documented over most of its geographical range (Patton, 1941; Rinker, 1942; Cockrum, 1948; Anderson and Berg, 1959; Jones, 1960, 1964; Mohlhenrich, 1961; Genoways and Schlitter, 1966). With increasing latitude, populations of S. hispidus encounter increased seasonal climatic variability and longer cold periods. Populations of S. hispidus show both seasonal and irregular long-term fluctuations in density. Irregular fluctuations in population, frequently to plague proportions, have been reported in both southern and northern populations (for a review of the literature see Goertz, 1964). Odum (1955) demonstrated three well-marked long-term cycles in a southern population of S. hispidus hispidus, although in all years the autumn density of cotton rats was markedly greater than the spring density. He considered the latter type of variation a northern-type seasonal cycle in these populations. Northern-type in density have also been observed in other southern populations of S. hispidus (Erickson, 1949; Dunaway and Kaye, 1964). In northern populations of S. hispidus, seasonal in population are better defined and more regular as a result of decreased temperature (and increased duration of low temperatures) associated with increased latitude (Goertz, 1964; unpublished data). Variation in S. hispidus populations appears to be regulated by temperature (Baumgartner, 1945; Sealander and Walker, 1955; Dunaway and Kaye, 1964; Goertz, 1964). Goertz (1964) found that a long period of below-freezing weather, snow cover, and frozen ground coincided with a population decline. Similar declines had four factors in common: (1) high summer and autumn population density; (2)