Function and Meaning of Wild Gorilla 'Close' Calls

Abstract

We investigate the social significance of the within group ('close') calls of gorillas by examining correlates of calling with dominance rank and with relatedness of adults, and by examining whether the outcomes of interactions between adults differ depending on the calls given during the interactions. In two wild gorilla groups, the majority of adults give most of their calls when near another adult and fully in sight of them. Thus gorillas' use 'close' calls as more than mere contact calls. An adult gorilla's use of 'close' calls correlates with its own and with its partner's dominance rank, with effects being most obvious for the most dominant animals, the fully adult males. Thus the proportion of 'double grunts', the most common 'close' call, in an individual's repertoire correlated consistently with dominance rank; all non-silverback adults gave a higher percent of double grunts in the presence of subordinates than they did near dominant animals; individuals were most likely to give 'non-syllabled' grunts in the adult male's presence; and they consistently exchanged calls at a higher rate with the adult males than with other group members. An adult's type of calls did not obviously differ depending on whether their neighbour was kin or non-kin, but kin were overall more likely to give calls in the presence of kin, and to exchange calls with kin than with non-kin, although the association was not consistent throughout the two years of the study. With regard to the consequences of calling, subordinates were less likely to be feeding one minute after an approach by a fully adult male during which calls were exchanged than during silent approaches. In contrast to findings from studies of some other species, calling did not correlate with duration of grooming. We suggest that, among other functions, gorillas' 'close' calls mediate social interactions. One form of calls, 'non-syllabled' calls, are interpretable as appeasement signals. The broadest interpretation of the 'double grunt' is that it is an exaggerated announcement of presence, whose function is to attract attention to the caller, and to signal conditional future activity.