Is the Female Biased Sex Ratio in Wood Lemming Myopus schisticolor Maintained by Cyclic Inbreeding?

Abstract

The hypothesis is developed where the female biased sex ratio in wood lemming Myopus schisticolor (Lillj.), a cyclic microtine, is maintained through evolutionary time by recurrent inbreeding. If the effects, with respect to sex ratio, of inbreeding are stronger than the effects of outbreeding, then the optimal strategy (being favoured by individual selection) would be to produce as many females as possible; the condition is that a sufficient number of males are present to ensure fertilization of all (or most) daughters. This is ensured by producing enough sons and/or by fathers mating with daughters. Some evidence supporting the occurrence of cyclic inbreeding (both sister-brother and daughter-father matings) during large fractions of microtine cycles in general, is outlined. Female biased sex ratio is predicted to be more common among exaggerated cyclic microtines than appreciated at present. One known case that seems to be similar to the situation in the wood lemming is briefly discussed: The arctic lemming Dicrostonyx torquatus (Pall.) in arctic USSR and North America. Studies aimed at producing data for testing the hypothesized occurrence of inbreeding are sorely needed: Microtines should be studied with respect to breeding structure during low and early increase phases. In addition, breeding experiments for analysing the genetic mechanism are needed.