Friends and Strangers in Vole Population Cycles

Abstract

During the last decade, several studies have attempted to test the Charnov and Finerty (1980) model for population cycles in small mammals (Kawata 1990). The model basically states that at low population densities, neighbors are primarily kin who behave amicably toward one another ("friends"), whereas at high densities neighbors are primarily nonkin who behave aggressively toward one another ("strangers"). Kin and nonkin are defined as individuals with high and low coefficients of relationship, respectively. At low densities, populations increase rapidly due to nepotistic behavior within kin groups and as densities increase, dispersal results in an influx of nonkin resulting in increased aggression among neighbors and a breakdown of the kin groups. The ultimate result of this influx of strangers is a decrease in population growth, primarily through reduction in reproduction and juvenile recruitment. Tests of this model have produced mixed results, but in general do not support the model (Kawata 1990 Pugh and Tamarin 1990, Stenseth and Lomnicki 1990). The discrepancies in the results are due in part to three problems with the model and its interpretations: one is the lack of a definition of what constitutes "friends", a second is the lack of a stated mechanism for decreased reproduction and juvenile recruitment at high densities, and the third has to do with the relationship between density and dispersal. In this paper, I suggest that friends must include males and not just related females, the primary mechanism for decreased juvenile recruitment at high densities is infanticide, and that female kin groups are more apt to form at high than at low densities, the opposite of what the Charnov-Finerty model assumes (see also Pugh and Tamarin 1990, Lambin and Krebs 1991). Without a clear understanding of the mechanisms that retard population growth at high densities, the role of relatedness or familiarity in population growth cannot be discerned. Tests of the kin selection hypothesis