Viability of Large- and Small-Sized Adults in Fluctuating Vole Populations STOR

Peak populations of many species of lemmings and voles contain adults 20-30% larger than adults found in low populations (the Chitty effect). What is the adaptive advantage of being large? We try to answer this question by analyzing the survival rates of largeand small-size adults of 4 Microtus species. Two alternative hypotheses are proposed to explain the adaptive value of large size. Large adults may be r-selected genotypes at an adaptive advantage in increasing populations. Microtus pennsylvanicus and male M. townsendii fit this alternative because the survival advantage of large size is positively related to population growth rate and negatively related to density. Alternatively, large adults may be a-selected genotypes at an adaptive advantage in high density populations when aggression is important. Microtus caltfornicus and M. ochrogaster fit this hypothesis because in these species the survival advantage of large size is positively related to density and negatively related to population growth rate. Thus large body size may be adaptive for quite contrary reasons in different species of cyclic voles and lemmings.