Friendship among adult female blue monkeys (Cercopithecus mitis)

A study group of blue monkeys in the Kakamega Forest, Kenya, provides data on friendly relationships between adult females. Females are invariably antagonistic toward members of other groups, and collaborate with their own groupmates in defending territorial boundaries. Females are primarily responsible for these aggressive intergroup encounters, which occur every other day on average. Encounters are often immediately followed by intense grooming among adult females, which gives the impression of reinforcing the collaborative team. In some lower-density populations, intergroup aggression is rare; in these populations, and at Kakamega as well, female groupmates may provide other or additional services, such as protection against predators or aggressive males. Friendships among females of a single group are differentiated. Individuals interact with certain grooming and proximity partners much more than others. Despite low rates of agonism, blue monkey females also show stable if shallow dominance relationships, and linear hierarchies. Rank is not correlated with diet, feeding behavior or reproductive rate. Coalition formation is rare, and these females do not groom up the hierarchy. Grooming may simply be traded for itself, leading to highly symmetrical grooming bouts. Blue monkeys have been misclassified in theoretical papers on socioecology. Their behavior seems not to fit these models well, because certain features of their social system, related to within-group contest competition, do not co-occur as expected. Their hierarchical dominance relationships are also surprising, because rank seems to be uncorrelated with fitness-related variables. Finally, the rarity of affiliative behavior exhibited by these monkeys suggests that new ways of measuring friendships may be appropriate. The possible importance of, and reasons for, rare within-group coalitions are discussed.