NATAL PHILOPATRY, COMPETITION FOR RESOURCES, AND INBREEDING AVOIDANCEIN TOWNSENDS VOLES (MICROTUS-TOWNSENDII)

Individuals may leave their birthsite before reproducing or they may p ostpone reproduction because they are at risk of engaging in inbred ma tings with their close relatives or because they are exposed to severe competition for resources or for mates. I studied whether avoidance o f inbreeding and intrasexual competition determine the seasonal change s in the rate of philopatry and the fate of individual Townsend's vole s (Microtus townsendii). By marking most nestlings before weaning, I k new which recruits were related to one another and to which adult fema le. There was a pronounced sexual bias in the tendency to remain close to the birth site: 9% of all juvenile males and 33% of all juvenile f emales entered the breeding population on their natal grid. Males sett led farther from their birth site than females did. The proportion of males and females reproducing on their natal grid changed markedly ove r 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 philo patry were consistent with the hypothesis that females disperse in res ponse to competition for resources with breeding females. Females born at high adult female density were less likely to be recruited than th ose born at other times. They also dispersed farther at low than at hi gh density, probably in response to habitat saturation. Avoidance of e xtreme inbreeding influenced the dispersal tendency of maturing males. Males were less likely to join the breeding population if their mothe r or a littermate sister was present in their home range when they mat ured than if they had no female close relatives in their home range. M ales with their mother alive at sexual maturation settled farther than males whose mother had disappeared by that time. I conclude that inbr eeding avoidance influenced natal philopatry in males and that competi tion for resources influenced natal philopatry in females.