Baboon responses to graded bark variants

We studied chacma baboons', Papio cynocephalus ursinus, responses to conspecific 'barks' in a free-ranging population in the Okavango Delta of Botswana. These barks grade from tonal, harmonically rich calls to calls having a noisier, harsher structure. Tonal or clear barks are typically given when the signaller is at risk of losing contact with the group or when a mother and infant have become separated ('contact barks'). The harsher variants are given in response to predators ('alarm barks'). In both contexts, however, we also observed acoustically intermediate forms. Using the habituation-recovery method we tested whether baboons discriminated between (1) clear contact barks versus harsh alarm barks, and (2) clear contact barks versus intermediate alarm barks. Calls were selected according to the results of a discriminant function analysis conducted on a suite of acoustic parameters. In these experiments, animals showed a significant recovery of response only after playback of the harsh alarm call. We performed another set of experiments using single exemplars of clear and intermediate contact barks, or intermediate and harsh alarm barks. Animals responded only to the playback of a harsh alarm bark. Apparently, only this harsh variant was placed in a category that warranted a response. We hypothesize that baboons' responses were to a large degree influenced by their assessment of context. (C) 2001 The Association for the Study Animal Behaviour.