Individual Recognition, Intragroup Cohesion and Intergroup Spacing: Evidence From Sound Playback To Forest Monkeys
Authors: Waser Peter M. 
Published: April 2008

The contexts and functions of several loud mangabey vocalizations, particularly the "whoopgobble", were investigated observationally and experimentally. Whoopgobbles are notable for their audibility and distinctiveness over long distances, their temporal pattern of delivery, and particularly their stereotypy and individual distinctiveness. On the other hand, contexts of and responses to these vocalizations are variable and sometimes nonobvious. In order to control context and more systematically investigate response, an experimental method involving playback of recorded vocalizations was developed. Although precautions against habituation were necessary, mangabey responses to playbacks were clearcut and repeatable. Answering vocalizations, changes in group movement, and changes in the dispersion of individuals within a group occurred only in response to mangabey vocalizations. Whoopgobble playbacks provoked a pattern of response, including most notably the rapid approach of one adult male (the "RA" male) from each group, which was specific to this call. Playback of whoopgobbles between 100 and 600m from mangabey groups indicated that this call does transmit information regarding the identity of the vocalizing individual and group over these distances. Test groups moved away from neighboring- and unknown-group calls, but towards those of their own males - particularly those of RA males. RA males, on the other hand, do not approach calls of other males from their own groups. Within a group, whoopgobbles may thus increase cohesion and influence the direction of movements. Characteristics of whoopgobble form and context are discussed with regard to hypothesized functions of these and other forest monkey loud calls. Responses by free-ranging mangabeys to playback of the whoopgobble confirm its role in maintaining distance between groups. Response was found to be independent of group size, despite the fact that whoopgobble rate is closely related to this variable and thus could transmit such information. Since responses were also found to be independent of location within the home range, intergroup spacing among mangabeys appears not be be "territorial", site defense does not occur. Nevertheless, the central areas in at least some mangabey groups' home ranges were never penetrated by neighbors. Playback tests with black-and-white colobus and blue monkeys, among which territorial spacing has been reported, indicate that responses to loud calls have some degree of site-specificity among these species. But the mangabey pattern of intergroup spacing appears to result from a combination of low group density, site attachment within groups, and site-independent avoidance between groups. These results emphasize that spacing "system" and "pattern" are not necessarily equivalent; a given set of spacing behaviors can result in different spacing patterns under different ecological conditions, while a given pattern may be obtained by any of several behavioral means. Evidence for site-independent spacing in other primate species is discussed.