Nonrandom Dispersal in Free-Ranging Vervet Monkeys: Social and Genetic Consequences

Male vervet monkeys in Amboseli National Park, Kenya, disperse nonrandomly from their natal groups at sexual maturity, and migrate to specific neighboring groups with their brothers or peers. Nonrandom dispersal in the company of allies appears to benefit young males by minimizing the risk of predation and reducing the probability of attack by resident males and females. Nonrandom dispersal also decreases the likelihood of mating with close female kin. Persistent nonrandom transfer, however, increases the risk of inbreeding depression. We emphasize the importance of considering social, as well as genetic, factors when evaluating the costs and benefits of any pattern of dispersal. In certain cases, there may be social advantages to a particular dispersal pattern even though it appears initially to carry some genetic costs. In other cases, a dispersal pattern that at first appears to increase the risk of inbreeding may in fact permit individuals to avoid mating with close kin. Finally, the benefits of a given pattern of dispersal may change markedly during an individual's life. While males seem to benefit by transferring nonrandomly with their peers when young, it may be more advantageous for older males to disperse alone. Age-related changes in the social benefits of nonrandom transfer appear to have important genetic consequences for the population as a whole.