Contextual variation in chimpanzee pant hoots and its implications for referential communication

There have been several previous studies of the loud, long-distance call of chimpanzees, termed a 'pant hoot'. Some have explored the possibility that there are acoustically distinct subtypes of pant hoots that communicate to distant listeners different information about the caller's behaviour, or ecological and social circumstances. However, results to date have been either inconclusive or conflicting. To help resolve these issues, we undertook research on pant hoots produced by wild chimpanzees, Pan troglodytes schwienfurthii, living in the Budongo Forest, Uganda. In this paper, we report the results of acoustic analysis of 201 pant hoot series produced by seven adult males. Principal components analysis (PCA) was used to identify key dimensions of structural variation in the calls, which were then related to a large set of behavioural, social and ecological variables associated with call production. Although there was little evidence for distinctive pant hoot subtypes according to many of the social and ecological dimensions traditionally recognized, a number of significant patterns were identified. Specifically, pant hoots were produced with higher probability at abundant food sources, and they were more likely to contain a 'let-down' phase when produced in specific behavioural contexts, such as travelling and upon arrival at a food source. In addition, pant hoots produced while travelling along the ground in small parties prior to joining-up with other community members were consistently different from all other pant hoots, varying reliably in the fundamental frequency of their build-up elements, the tonal quality of climax elements and in the presence of a let-down phase. We discuss the possible mechanistic bases for these differences in the pattern of calling and detailed spectral structure of the calls and consider the implications for referential communication.