Litter sex ratios in a food-supplemented population of Peromyscus mexicanus

Abstract

We examined the tertiary sex ratio of litters in a wild population of the tropical mouse Peromyscus mexicanus for evidence of facultative adjustment of the sex ratio as a function of (i) three maternal characteristics, mass at conception, litter parity, and access to a food supplement, and (ii) rainfall and temperature around the time of early gestation and around the time of birth. We examined unreduced litters (litters that had not experienced losses during gestation or lactation, usually 3 pups) separately from litters of all sizes in order to distinguish between prenatal (unreduced litters) and postnatal (all litters) shifts in litter composition. We also examined a sample of litters that were enumerated at birth for further evidence of prenatal adjustment of the sex ratio. None of the independent variables were significant predictors of the frequency of male pups. Furthermore, we found no evidence that litters of any size deviate from a sex ratio of 1.0, whether enumerated at birth or at weaning. Sex-ratio shifts occur in other species of Peromyscus, but P. mexicanus has a low reproductive potential, and adjustment of litter composition may not occur because such a strategy is too costly and too risky.