Seasonal Variation in Field-Determined Growth Rates of the Hispid Cotton Rat (Sigmodon hispidus)

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Abstract
Growth rates are estimated for a population of hispid cotton rats near Lawrence, Kansas. Seasonal and sexual differences exist in growth, with all cotton rats showing higher growth rates in summer than in winter. In fall and winter the heavier animals tended to lose mass whereas lighter cotton rats gained, reducing the variability within the population. This phenomenon might account for seasonal differences in mass distributions within rodent populations. Seasonal variation in growth is related to cotton rat environment and energetics. Growth rates of cotton rats from Kansas are compared with published growth rates for cotton rats in Florida. Some of the growth rates we found are the highest yet reported for cotton rats. If ecologically meaningful growth rates can be determined for free-living animals, mass classes may be useful as categories for stage-based studies of cotton rat population dynamics.