Balancing Costs and Opportunities: Dispersal in Male Baboons

Abstract

Young male baboons typically disperse from their group of birth as they near adult size and may continue to migrate between social groups throughout their lives. Long-term data on dispersal and residence patterns of male baboons in Amboseli National Park, Kenya, were available for 110 males in the population, including 43 that were monitored during their natal dispersal. These data enabled us to provide not only a detailed evaluation of the effects of reproductive competition on dispersal but also the first direct estimates of the costs of dispersal in male primates and one of the few direct estimates of fitness costs associated with breeding in the natal group. Males underwent natal dispersal at a median age of 8.5 yr (range, 6.8-13.4 yr) and subsequently remained in nonnatal groups for a median tenure of 24 mo (range, 1-138 mo). Half of the males in the study engaged in moderate to extensive reproductive activity before natal dispersal. Reproductive costs associated with breeding in the natal group were suggested by the high mortality of offspring for whom natal males were their likely fathers, even though maternal relatives avoided mating with each other. Dispersal involved considerable time spent alone, and therefore the costs of dispersal were substantial, because of mortality risks and missed reproductive opportunities during dispersal. Female availability and male mating success apparently affected both natal and secondary dispersal patterns. We present a model of dispersal tendency in order to explicate the ways in which differences in population density, predation risk, and the distribution of mating opportunities among groups might result in complex dispersal patterns that are consistent with both the results of the present study and the disparate empirical reports in the literature.