Mass-Based Demography of a Hispid Cotton Rat (Sigmodon hispidus) Population

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Abstract
Determination of age in rodents is difficult after the animals reach sexual maturity, and biologists often classify mature animals as “adults,” considering this possibly heterogeneous group as a unit. However, many small mammals differ in probability of survival and reproduction as adults, and these differences often correlate with body mass. We provide a rationale for using body mass as stages in Lefkovitch transition matrices, and model the monthly population dynamics of cotton rats in eastern Kansas using mass-specific estimates of survival and reproduction. These monthly transition matrices illustrate seasonal differences in both patterns of individual growth and population dynamics. Sensitivity analyses indicate that natural selection may favor rapid growth in small northern cotton rats in summer and survival of all size classes in winter. Although these transition matrices are composite, they are quite accurate models of the observed population dynamics. Mass-based demography may be an effective method of modelling the population dynamics of short-lived animals that show variation in survival and reproduction related to body mass.