Costs and benefits of home range shifts among vervet monkeys (Cercopithecus aethiops) in Amboseli National Park,Kenya

Summary A decline in the density of trees and the deterioration of the habitat of vervet monkeys (Cercopithecus aethiops) in Amboseli National Park, Kenya, have been accompanied by movement of vervet groups into unfamiliar areas. These home range shifts were documented over a period of 26 months. The largest group moved into the home ranges of neighboring groups and acquired more trees, a resource that had become more limited. The groups whose home ranges were encroached upon did not share their home ranges with the intruders, but instead moved into areas that were unoccupied and had a lower density of trees. The largest group was thus more successful than smaller groups in competition for limited resources. For all but one group, movement into new areas was directly associated with the disappearances of female and immature monkeys. Most of the disappearances were attributable to predation. These results suggest that the cost of predation in unfamiliar areas may contribute to the evolution of philopatry in vervets. However, under unstable conditions, resource competition forces these animals to disperse (in groups) despite the high risk of predation. Offprint requests to: L.A. Isbell