A non-human primate combinatorial system for long-distance communication.

Complex vocal systems are thought to evolve if individuals are regularly challenged by complex social decision-making, the social complexity hypothesis. We tested this idea on a West African forest non-human primate, the Olive colobus monkey, a highly cryptic species with very little social behavior and very small group sizes, factors unlikely to favor the evolution of complex communication. The species also has an unusual fission-fusion social system, with group members regularly spending considerable amounts of time with neighboring groups. As predicted by the social complexity hypothesis, we only found a very basic repertoire of two call types in this species, produced by both males and females. However, the calls were astonishingly loud, never uttered alone but in syntactically structured sequences assembled along a set of rules. We concluded that the Olive colobus monkeys have evolved a combinatorial system to interact with distant group members.