On the Evolution of Reproductive Rates in Microtine Rodents

Abstract

The purpose of this study was to evaluate the effects of various patterns of population dynamics on the evolution of reproductive rates of voles. Population sizes of both field voles (Microtus agrestis) and bank voles (Clethrionomys glareolus) show different degrees of fluctuation in different regions; the degree of cyclicity increases northwards. Animals caught in both optimal and suboptimal habitats in three regions of Sweden were examined regarding litter size and length of reproductive season. The litter size was greater in optimal than in suboptimal habitats during the increase phase of cyclic populations. The breeding season was shorter during the peak than during the increase phase. In both species, during low—density phase, the reproductive rate of overwintered females increased northwards. Young—of—the—year females of the field vole, but not of the bank vole, showed a decrease in the reproductive rate with distance northwards. These results were compared with predictions based on three hypotheses on the evolution of reproductive rates, emphasizing, respectively, (1) food supply, (2) the pattern of density fluctuations, and (3) a combination of both food supply and the pattern of fluctuation. Observed between—habitat differences in reproductive patterns could be explained by hypothesis. 1. However, hypothesis 2 could explain part of the remaining variation among regions. In addition, when data from optimal habitats in the present study were combined with similar data from the literature, a high correlation was found between litter size and degree of cyclicity, confirming the predictions of hypothesis 2.