THE EFFECT OF MATERNAL MASS ON LITTER SIZE AND OFFSPRING SURVIVAL IN THE HISPID COTTON RAT (SIGMODON HISPIDUS)

We tested the proposition that maternal body mass affects litter production and recruitment in the cotton rat (Sigmodon hispidus) in Kansas. We also addressed hypotheses about why cotton rats in Kansas are larger and more fecund than conspecifics in Texas. We livetrapped cotton rats on both control and food-supplemented grids. Pregnant females were briefly confined until parturition; their pups were weighed and toe-clipped, then mother and young were returned to the site of capture. Subsequent trapping revealed survival of the offspring, i.e., ability of mothers to recruit pups into the population. Heavier mothers on both sets of grids gave birth to larger, heavier litters and had more pups survive to recruitment. The percentage of pups surviving was not related to maternal mass or to pup mass, so the increased recruitment resulted primarily from heavier mothers' having more rather than heavier pups. Mothers on the supplemented grids bore larger and heavier litters than their same-mass counterparts on the control grids. Despite their larger litters, lighter (ca. 100 g) mothers on the supplemented grids recruited fewer pups than their same-mass counterparts on control grids. Because heavier female cotton rats are more fecund, our results are consistent with the hypothesis that cotton rats in Kansas are larger than in those in Texas because of selection for increased litter size in Kansas. Seasonality in Kansas restricts the number of breeding opportunities and provides a nutrient pulse that supports rapid reproduction in the growing season.