Natal Philopatry, Competition for Resources, and Inbreeding Avoidance in Townsend's Voles (Microtus Townsendii)

Individuals may leave their birthsite before reproducing or they may postpone reproduction because they are at risk of engaging in inbred matings with their close relatives or because they are exposed to severe competition for resources or for mates. I studied whether avoidance of inbreeding and intrasexual competition determine the seasonal changes in the rate of philopatry and the fate of individual Townsend's voles (Microtus townsendii). By marking most nestlings before weaning, I knew which recruits were related to one another and to which adult female. There was a pronounced sexual bias in the tendency to remain close to the birth site: 9% of all juvenile males 33% of all juvenile females entered the breeding population on their natal grid. Males settled farther from their birth site than females did. The proportion of males and females reproducing on their natal grid changed markedly over the breeding season. More males and females reproduced if they were born at the beginning of the breeding season or when breeding activity was depressed by botfly infestation. Seasonal changes in female philopatry were consistent with the hypothesis that females disperse in response to competition for resources with breeding females. Females born at high adult female density were less likely to be recruited than those born at other times. They also dispersed farther at low than at high density, probably in response to habitat saturation. Avoidance of extreme inbreeding influenced the dispersal tendency of maturing males. Males were less likely to join the breeding population if their mother or a littermate sister was present in their home range when they matured than if they had no female close relatives in their home range. Males with their mother alive at sexual maturation settled farther than males whose mother had disappeared by that time. I concluded that inbreeding avoidance influenced natal philopatry in males and that competition for resources influenced natal philopatry in females.