An Examination of Seasonal Growth and Survivorship of Sigmodon hispidus in Southeastern Virginia

A reexamination of specific population dynamic aspects of Sigmodon hispidus, the hispid cotton rat, is necessary in order to gain additional knowledge and perspective on this species in its northernmost distribution on the east coast of the United States. Previous studies of Virginia cotton rats were based on data from necropsies, which do not allow for the determination of certain population characteristics, such as density, growth, and survival. General population trends and rates of growth and survival in males and females throughout the seasons in Virginia will also provide a basis for comparison with populations of cotton rats in other parts of its range. A monthly mark-and-recapture study was conducted in an old field in Chesapeake, Virginia, between December 2002 and March 2005 to elucidate these aspects of the population, as well as to confirm previous reproductive patterns for this geographic region. Annual population density in this study is similar to that observed in other northern populations of cotton rats. However, slight bimodal distributions also began to surface, which are more common in southern distributions of this species. Current patterns of reproduction were not significantly different from those previously reported in Virginia and were similar to patterns in northern portions of the species' range. Highest rates of growth were present in the summer, while winter exhibited the lowest rates of growth. Previous Virginia studies have suggested males have the ability to gain mass over the winter months. Although positive winter growth in males was observed in this study, nil or negative growth was just as common. Higher rates of survival were present in the spring and autumn, while lowest rates of survival were in summer and winter and were influenced by energetic and environmental stresses, as well as other intrinsic population factors. Newly collected winter data reveal that winter growth and survivorship in cotton rats are very similar to trends observed in Kansas populations located at the same latitude. The new information from this study helps to better understand why different geographic populations exhibit differences in body size and other population characteristics and the relevant forces of selection promoting these patterns.