Why breed early? A study of reproductive tactics in Peromyscus

Abstract

Female deer mice (Peromyscus maniculatus) near Vancouver, British Columbia, were found to have a bimodal pattern of breeding. This paper compares the success of females that attempted to breed in the early peak of breeding with that of females that did not breed until the later breeding peak. While the average success was about the same for the two tactics, early breeding had a much higher variance. It is suggested that this difference in variances would result in eventual elimination of the early-breeding phenotype, unless it were favored by other factors. Differences in the selection pressures on males and females, and the relatively frequent occurrence of years of good reproductive success for early-breeding females, may contribute to the maintenance of this phenotype in the population. A hypothesis is presented to explain both the bimodality of the breeding pattern, and the dominance of the second breeding peak.