Mandrill visual gestures

Apes have rich gestural communication systems. But do monkeys? If so, what forms and functions do monkey gestures have, and how does the relationship between form and function originate developmentally? This chapter reports intensive observations aimed at answering these questions in the largest of all monkeys, the mandrill (Mandrillus sphinx). Nearly thirty captive mandrill groups distributed across three continents were studied over the course of a decade, some groups for three consecutive generations. Mandrills performed three visual gestures: slap ground, hand extension, and eye covering. While slap ground was distributed uniformly across individuals and groups, hand extension and eye covering – the two focal gestures of this chapter – showed restricted distributions that were consistent with their being learned rather than instinctive. Contrasting hand extension against eye covering for fifteen different signal parameters revealed that these two gestures had antithetical forms and functions but had similar developmental histories. Within each gesture the close correspondence between form and function suggested that ontogenetic ritualization contributed to both hand extension’s and eye covering’s origin. Furthermore, for eye covering the clustering of all known performers within a single group suggested this gesture was culturally transmitted. Broadly, these results suggest that multiple interacting factors, including instinct, learning, and culture may contribute to the emergence of monkey gestures. Follow-up experiments, however, remain critical for rigorously testing the developmental origins and potential cultural transmission of non-human gestures. Keywords: development; gestural acquisition; ontogenetic ritualization; cultural transmission; monkeys