Is Mean Litter Size the Most Productive? A Test in Columbian Ground Squirrels

Abstract

We studied the evolution of litter size in natural and experimentally manipulated populations of Columbian ground squirrels (Spermophilus columbianus) in eastern Washington state and southwest Alberta. Litter size at weaning in a large natural population (mean = 3.51 pups/litter, 248 litters) was significantly lower than the litter size (6 pups) that produced the most offspring surviving to yearling age class. This evidence contradicted Lack's "optimal litter size" hypothesis, which predicts that the most productive litter size should approximate the mean. Litter size had no significant effect on the subsequent survival or reproduction of mothers, contrary to the negative effects predicted by the "cost of reproduction" hypothesis. Litter success varied among years, and good and bad years for reproduction could be experimentally simulated with food supplementation. However, small samples of the largest litters rendered the "bad—years" hypothesis inappropriate for application to our data. Proportional survival of offspring was relatively constant among different litter sizes, contrary to the necessary condition of the "cliff edge" hypothesis of a dramatic decrease in survival of young from the largest litters. The data supported the "individual optimization" hypothesis most strongly. As predicted, the number of surviving offspring increased with litter size, although in one population the six largest litters (2.4% of 248 litters) suffered reduced success. In natural and food—supplemented populations, changes in maternal body mass were associated with changes in litter size (r = 0.205 to 0.926). Because survival of young remained relatively constant as litter size increased, these correlations resulted in greater offspring success for mothers in better body condition.