EFFECT OF MASS ON SEASONAL SURVIVORSHIP OF NORTHERN COTTON RATS

We tested the possibility that mass differentially affects seasonal su rvivorship of cotton rats (Sigmodon hispidus) in Kansas. Because Kansa s is on the northern margin of the range of this subtropical species, we expected that heavier animals might survive better, especially in w inter. We calculated seasonal survivorship using the Jolly-Seber model for a population of S. hispidus residing in northeastern Kansas and l ivetrapped since 1973. Gender did not affect survivorship, but year, s eason, and mass did. We found that the heaviest animals survived best only during spring and that cotton rats of intermediate mass survived best during winter. There was broad overlap in summer survivorship, an d the lightest cotton rats survived best during autumn. Energy require ments may explain the patterns of survivorship, particularly during wi nter.