Distance Vocalisations of Guinea Baboons (Papio Papio) in Senegal: an Analysis of Function
Guinea baboons (Papio papio) were studied in the field at Mont Assirik, Senegal; visibility is poor, baboon predators are frequent and baboon population density is high relative to other prey species at this site. Data were collected on the rates of occurrence of vocalisations in relation to baboon ranging and group fission-fusion, to habitat, and to activities and circumstances of use. Factor analysis, with varimax rotation of orthogonal axes, was used to identify reliable clusters of calls whose rates covaried. For each, a weighted index was constructed to examine variation with social and environmental context. Nocturnal volleys of adult male loud calls were found to be provoked by distant sounds of major predators and by similar volleys from other sleeping sites, and playback confirmed this "triggering" of volleys. Sleeping sites are restricted to branches of the tallest trees, and are so secure as to be difficult to access; baboons do not react to volleys except by calling. This behaviour was interpreted as imparting reliable information to potential predators about numbers of adult male baboons at a site; predators would be expected to hunt, at dawn when baboons descend to the ground, only in the vicinity of sites with fewest large males. Baboons at each site should then compete to deter predators by nocturnal calling. Diurnal use of single and double-phased barks was strongly associated with intergroup coordination. Barks are used to prevent separation of animals when a group travels rapidly in dense vegetation, and as a "lost call" by isolated groups. When a troop splits into large subgroups during feeding, barks enable contact to be maintained and allow efficient reunion before any open area is crossed; open laterite plateaux are crossed silently, in large compact groups and only with circumspection. Groups which meet during ranging typically make auditory contact first, and often fuse; presumably such groups belong to the same social network. When feeding in grassland, groups break up into many small parties (of the order of 5-10 animals) which forage independently. Reaggregation is rapidly achieved when contagious barking of each party allows it to be located and to locate others despite the low visibility. This prompt reaggregation, and in general behaviours which promote and maintain large group size, are interpreted as adaptations to predator pressure. Loud calls, particularly of adult males, were also found to be associated with intragroup agonism; however, lack of individual recognition precluded useful study in this case. Direct reactions to potential predators and their sounds showed fine tuning to the particular nature of danger presented. The single, sharp bark is used to signal the initial perception of possible danger, including unfamiliar humans. Predators which rely on stealth and surprise are either mobbed, or watched and double-phased barks used to mark any movement or sudden change in their behaviour; their danger is thus neutralised. Cursorial predators, on the other hand, are watched carefully and in silence, from the vicinity of trees.