The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

The function of long calls in western gorillas (Gorilla gorilla): behavioral flexibility in ape communication


1Interdepartmental Doctoral Program in Anthropological Sciences, Stony Brook University, 2Department of Anthropology, Stony Brook University

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Animals often use long calls to regain contact and reestablish proximity with group members. Although both gorilla species use long calls, only among western gorillas are individuals occasionally separated by more than 200 m from other group members, thereby requiring some mechanism to restore group cohesion. Here we test whether their “hoots” function as long contact calls, in which case they should be individually distinct, given by both males and females when separated, and result in reduced inter-individual distances. We collected behavioral (1,458hr), spatial (simultaneous GPS points = 268) and acoustic data (643 call segments), during simultaneous 2h-focal follows of the male and an adult female (rotating among 4 females) in one group of wild western gorillas, at Mondika Research Center, Republic of Congo. Hooting bouts (sequence of hoots, n = 89) were given every third day on average and lasted nearly an hour. Discriminant function analysis confirmed individual distinctiveness of hoots. When hooting bouts began, at least one female (or the male) was absent from the main party, and the average distance between the separated individuals (216±83 m) was significantly greater than their mean daily proximity (51±15 m). Both the male and females initiated and responded to hoots, and the distance between separated individuals decreased significantly by the end of the hooting bouts. We conclude that hoots, used only by males during inter-group encounters in mountain gorillas, are used by both sexes in western gorillas to promote group cohesion, suggesting flexible usage of homologous calls by closely related species.

This study was funded by The Leakey Foundation, Primate Conservation Inc., Conservation International Primate Action Fund, Richard Leakey and Wildlife Direct, Stony Brook University.

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