Differential inbreeding tolerance in two geographically distinct strains of root voles Microtus oeconomus

Two geographically distinct strains of Microtus oeconomus, each consisting of an inbred (sibling mating) and an outbred treatment group, were bred in the laboratory over three generations to determine the effects of inbreeding on reproductive parameters, growth rates of young and paternal behavior The southern strain (orginating from southern Norway) suffered from depressed reproductive rate (litter size and pregnancy rates) most likely due to inbreeding, while no effects of inbreeding were detected m the northern strain (originating from northern Norway) 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 voles in either strain Inbreeding depression rather than inbreeding avoidance is the most likely mechanism behind the depression in reproductive parameters of inbred southern voles This is suggested by the decrease 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 tolerance might be a life history adaptation