Vocal recruitment for joint travel in wild chimpanzees.

Joint travel is a common social activity of many group-living animals, which requires some degree of coordination, sometimes through communication signals. Here, we studied the use of an acoustically distinct vocalisation in chimpanzees, the 'travel hoo', a signal given specifically in the travel context. We were interested in how this call type was produced to coordinate travel, whether it was aimed at specific individuals and how recipients responded. We found that 'travel hoos' were regularly given prior to impending departures and that silent travel initiations were less successful in recruiting than vocal initiations. Other behaviours associated with departure were unrelated to recruitment, suggesting that 'travel hoos' facilitated joint travel. Crucially, 'travel hoos' were more often produced in the presence of allies than other individuals, with high rates of recruitment success. We discuss these findings as evidence for how motivation to perform a specific social activity can lead to the production of a vocal signal that qualifies as 'intentional' according to most definitions, suggesting that a key psychological component of human language may have already been present in the common ancestor of chimpanzees and humans.