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Functions of Wild Gorilla 'Close' Calls. I. Repertoire, Context, and Interspecific Comparison

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Individuals in social groups of a number of species produce and exchange among themselves frequent, quiet vocalisations. The function of most such vocalisations, here termed 'close' calls, remains obscure, because of the lack of any obvious context of behaviour associated with their production. In this first of two papers that attempt to determine the function of these calls in wild girilla groups, we describe the call repertoire, the age-sex distribution of frequencies of call-types, the contexts in which the calls are given, compare the above measures with available descriptions for the other African great apes, the common and bonobo chimpanzees, and briefly speculate on possible functions. We provide sonograms and 21 acoustical measures of 15 calls in total, including eight 'close' calls, recorded from identified individuals of two habituated heterosexual groups of wild gorillas (Gorilla gorilla) of average composition for the region. In so doing, we provide the most detailed description yet of any great ape's within-group vocalisations. The age-sex classes differed in their use of 'close' calls: adult males called the most in total, immatures the least; and while most classes gave all calls, they gave them at differing frequencies. The differences were consistent across the two-year study period. We suggest two main contexts of production, namely situations of potential separation, and potentially agonistic situations. Thus animals called when far from others and before changes in group activity, and they called when unusually close to one another, especially while feeding. In comparison to the closely related common and bonobo chimpanzees, gorillas apparently had no calls specific to subordinate individuals behaving submissively: subordinate gorillas gave calls in such a situation, but they and dominants gave the same calls (at our current level of analysis) in other situations also. Acoustically and functionally, the gorilla's 'close' calls can be separated into 'syllabled' grunts and non-syllabled' longer calls, we suggest. 'Syllabled' calls might function to maintain contact between animals, to coordinate activity, and to act as mild threats; 'non-syllabled' calls might function as appeasement signals, and to cohere and coordinate group movement when given at good feeding sites, both by attracting animals to the site, and by inhibiting competition at it.

Affiliations: 1: Dept. Anthropology, University of California, Davis, California 95616, U.S.A.; 2: Animal Communications Lab., Dept. Zoology, University of California, Davis, California 95616, U.S.A.

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