53.5 Saturday, Jan. 5 The costs of current reproduction are not traded against maternal survival or subsequent reproductive performance in the Columbian Ground Squirrel SKIBIEL, A.L.; SPEAKMAN, J.; HOOD, W.R.*; Harvard University; University of Aberdeen; Auburn University firstname.lastname@example.org
Life history evolution is contingent upon proximate and ultimate costs of reproductive effort. Allocating a greater amount of limited resources, such as energy, to current reproduction can reduce the amount of energy available for somatic maintenance and in turn ultimately impair future breeding success or maternal survival (i.e. cost of reproduction hypothesis). Although there is some support for the cost of reproduction hypothesis in birds, few empirical studies of mammals have demonstrated a tradeoff between current and future reproduction. Furthermore, most studies testing ultimate costs neglect to confirm that the proximate costs of reproduction are high. We experimentally manipulated litter size in a wild population of Columbian ground squirrels for 2 years to examine the proximate energetic and ultimate fitness (i.e. survival and breeding) costs of reproduction. Although females raising augmented litters had field metabolic rates that were almost 1.5 times greater than females raising control or reduced litters, there were no negative impacts on the probability of maternal survival or future reproduction. However, pups from augmented litters grew more slowly during the lactation period, were smaller at weaning and had a lower probability of survival over-winter. Thus, although females are capable of raising more young than they give birth to, our observations suggest that it is not an energy allocation tradeoff that restricts litter size, but rather the reduced offspring survival associated with raising larger litters.