Comparative Life Histories of Georgia and Virginia Cotton Rats

Adult hispid cotton rats (Sigmodon hispidus) were collected from the field monthly for .2 years from populations near the northern edge of their range in Virginia and contemporaneously from south-central Georgia. Body measurements and weights were taken at capture, and after dissection embryos, corpora lutea, and placental scars were counted and measured; testes and seminal vesicles were dissected out, measured, and weighed. This allowed comparison of several life-history parameters between the populations and tests of several life-history hypotheses. The breeding season was up to 2 months longer in Georgia than in Virginia, where there was typically a 3-month or longer winter inactive period. Some reproductive activity was observed among Georgia females in all 12 calendar months, whereas pregnancies were never observed in Virginia during November– February. Average litter sizes were significantly higher in Virginia (5.91 6 1.41, up to 13) than in Georgia (5.16 6 1.79, up to 9); this difference may partly result from a higher incidence of embryo resorption and prenatal mortality in the Georgia population, primarily in the cooler 6 months of the year. Virginia rats averaged significantly smaller for both sexes, but this was likely the result of a younger age distribution. Among reproductive males and females, no body-size differences were found between populations except that pregnant females from Virginia averaged significantly longer. Fifty percent and 75% of the random sample of adult females and males, respectively, were reproductively active in Georgia, whereas only 35% and 40% were reproductively active in Virginia. Spermatogenically active males in Virginia had significantly greater relative gonadal mass than their Georgia counterparts. Overwinter survival of parous females was lower in Virginia. Virginia populations, in a more seasonal environment, displayed a more r-selected life history, with greater reproductive allocation, faster growth (except over winter), higher mortality, and less iteroparity.