The function and mechanisms underlying baboon ‘contact’ barks

Free-ranging baboons, Papio cynocephalus ursinus, in the Okavango Delta, Botswana, often give loud ‘contact’ barks when separated from other group members. Although these calls appear to function to maintain contact between widely dispersed animals, individuals apparently do not give contact barks with the intent of informing others of their location. Females are more likely to give a contact bark in the 5 min after they themselves have called than in the 5 min after another female has called. Playback experiments suggest that females primarily ‘answer’ their close relatives’ contact barks when they themselves are separated from other females or at the end of the group progression. ? 1996 The Association for the Study of Animal Behaviour When moving through wooded areas, free-ranging chacma baboons, Papio cynocephalus ursinus, in the Okavango Delta, Botswana, often give loud, tonal barks (Fig. 1) that can be heard up to 200 m away. These barks are acoustically distinctive and can easily be distinguished, both spectrographically and by ear, from other calls in the baboons’ vocal repertoire (R. Seyfarth & D. Cheney, unpublished data). The barks appear to function as ‘contact’ calls, because they often seem to be exchanged between widely dispersed sub-groups or individuals. Although not previously described for other populations of savanna baboons in East or Southern Africa, the calls seem functionally and acoustically similar to the contact barks of forest Guinea baboons, P. c. papio (Byrne 1981), and mandrills, P. sphynx (Kudo 1987). Analogous loud calls occur in other primate species. In squirrel monkeys, Saimiri sciureus (Boinski 1991), and white-faced capuchins, Cebus capucinus (Boinski 1993), for example, loud trills seems to function not only to maintain group cohesion but also to initiate and set the direction of the group’s movement. In some species, long distance calls may also serve as conditional recruitment signals that attract others to food. Foraging subgroups of both spider monkeys, Ateles geoffroyi (Chapman & Levebre 1990), and chimpanzees, Pan troglodytes (Wrangham 1977; Clark & Wrangham 1994), often give loud calls upon arriving at food resources. Typically, more calls are given at large food patches than at small ones. Despite these observations, there is some doubt about whether loud calls have evolved specifically to alert others to food or to maintain contact between separated group members. For example, although calling sub-groups of spider monkeys are joined by other monkeys three times as often as sub-groups that remain silent, even calling sub-groups are joined only 17% of the time (Chapman & Levebre 1990). In the case of chimpanzees, parties that call are not joined more than parties that remain silent (Clark & Wrangham 1994). Furthermore, chimpanzees do not call more at times of high fruit production than at other times, and individuals that fail to alert others to food are not obviously punished. Observing that high-ranking males give more loud calls (or pant hoots) than other individuals, Clark & Wrangham (1994) hypothesized that these calls function to signal the caller’s status rather than to alert others to food (see also Mitani & Nishida 1993). Analyses of chimpanzee pant hoots highlight a problem that is common to all studies of long range ‘contact’ or ‘food’ calls: although listeners can potentially use the calls to maintain contact with signallers or to locate food resources, signallers may not call with the intent Correspondence: D. L. Cheney, Department of Biology, University of Pennsylvania, Philadelphia, PA 19104, U.S.A. (email: [email protected]). 0003–3472/96/090507+12 $18.00/0 ? 1996 The Association for the Study of Animal Behaviour