Female investment strategies and reproductive success in the Uinta ground squirrel, Spermophilus armatus.

Abstract

To understand the evolution of female investment strategies and social structure in ground squirrels, I studied the reproductive consequences of litter size and mass, the timing of reproduction, and social organization in the Uinta ground squirrel, Spermophilus armatus. Litter size and litter mass correlated positively with maternal body mass. Juvenile female survival varied positively with juvenile weaning weight and negatively with weaning date; mothers appeared to compensate for late weaning date by producing smaller litters containing heavier offspring. Juvenile survival did not vary with litter size, so larger litters produced more surviving yearlings. Thus, the "optimum litter size" exceeded the mean litter size. The iteroparity/longevity and the bad year hypotheses are probably not adequate explanations for this anomaly; the individual optimum hypothesis seems most appropriate. I studied the demographic cost of reproduction by experimentally removing litters early in lactation. Females with litters removed gained more weight before hibernation and emerged from hibernation the following spring heavier than if they had weaned litters, suggesting a capacity to wean larger litters subsequently. These females also hibernated earlier in the season than unmanipulated females, which probably improved their chances of survival by decreasing their exposure to predators. Removal of litters tended to increase the mothers' survival rates and subsequent litter sizes. Most females become permanently established as yearlings in burrow systems near their place of birth, creating "kin clusters" of related females. Individuals as closely related as siblings or grandmother/granddaughter treated each other preferentially. Increasing numbers of these relatives in a kin cluster accompanied poorer reproductive success even in this relatively asocial species, suggesting that a reproductive cost opposed kin clustering very early in ground squirrel social evolution. Close proximity of yearling daughters did not depress the reproductive success of their mothers. Yearlings that inherited a dead mother's burrow system had better reproductive success than yearlings that lived peripherally to a living mother or that emigrated to a new area, and peripheral yearlings tended to have better reproductive success than emigrants. Therefore, kin clustering appears to result from direct fitness benefits to yearlings and inclusive fitness benefits to their mothers.