Versatility in a loud call: Dual affiliative and agonistic functions in the blue monkey boom

To investigate the selective agents that shape signals, we examined function (adaptive benefit to signaler) in the boom loud call by male blue monkeys. Using natural observation and playbacks in a wild population in Kenya, we characterized boom function from conspecifics' behavioral responses and also variation in call usage relating to context and reproductive season. Booms occurred in several contexts and after varied stimuli, including falling branches, yet were strongly associated with intragroup activity and, in particular, affiliative interactions between callers and females. Males produced more booms during the mating season and, congruently, when more females in their groups were sexually active. After hearing booms, compared to no call, females spent more time near groupmates and the caller and were more likely to approach and have mating interactions with him. Males tended to move away after hearing a boom. In the aggregate, results indicate that booms achieve multiple functions relating to facilitating group cohesion, affiliative interactions, and mating opportunities, while also repelling rival males. An observed association with falling branches is puzzlingly distinct from these social functions. We explore the hypothesis that booms enable affiliative encounters by acting as signals of benign intent, and discuss the functional versatility of signals perceived by multiple receivers that vary in age, sex, and relationship to signalers.