Changes in ranging and agonistic behavior of vervet monkeys (Cercopithecus aethiops) after predator-induced group fusion.

Socio-ecological theory predicts that group fusion in female-philopatric primate species will be rare because females experience increased costs by associating with non-relatives. Indeed, fusion has been reported only 14 times in only 4 female-philopatric cercopithecines despite many years of observation. Here, we describe changes in ranging and agonistic behavior of vervet monkeys (Cercopithecus aethiops) after the fusion of two groups, the sole group fusion during 11 years of observation, induced by a brief but intense period of apparent leopard predation. Before fusion, both groups made few incursions into the other group's territory and spent most of the time in their own territories. After the fusion, the amalgamated group shifted its activities and used both territories in similar proportion. Rates of female agonism increased after fusion, particularly in the 2 weeks following fusion, and the small group females assumed the lowest ranks in the female dominance hierarchy. Rates of agonism returned to prefusion rates a month later. Although rates of high-intensity interactions (i.e., chases) did not increase after fusion, small group females were more likely to be the recipients of, and lose, agonistic interactions than large group females; a small group female and her infant were attacked and wounded by a coalition of large group females shortly after the fusion. The observations presented here reveal that the circumstances surrounding group fusions are more variable than previously realized, but are still in accordance with expectations from socio-ecological theory that predation can favor the formation of larger groups. In this case, under threat of severe predation, individuals may have surrendered group autonomy for the greater security of larger numbers.