DIFFERENTIAL INBREEDING TOLERANCE IN 2 GEOGRAPHICALLY DISTINCT STRAINS OF ROOT VOLES MICROTUS-OECONOMUS

Two geographically distinct strains of Microtus oeconomus, each consis ting of an inbred (sibling mating) and an outbred treatment group, wer e bred in the laboratory over three generations to determine the effec ts of inbreeding on reproductive parameters, growth rates of young and paternal behavior, The southern strain (originating from southern Nor way) suffered from depressed reproductive rate (litter size and pregna ncy rates) most likely due to inbreeding, while no effects of inbreedi ng were detected in the northern strain (originating from northern Nor way). This result questions previous generalizations about inbreeding tolerance at the species level for Microtus. Growth rates and paternal behavior did not differ significantly between inbred and outbred vole s in either strain. inbreeding depression rather than inbreeding avoid ance is the most likely mechanism behind the depression in reproductiv e parameters of inbred southern voles. This is suggested by the decrea se in the proportion of breeding pairs with the number of generations of inbreeding, and by the reduced litter size of inbred compared with outbred pairs. Field and laboratory studies have shown that behavioral and demographic traits, possibly related to the degree of inbreeding, differ between the two strains which suggests that inbreeding toleran ce might be a life history adaptation.