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


Individual and group level factors shape the social sphere of individual mountain gorillas (Gorilla b. beringei)

DAMIEN CAILLAUD1,3, FELIX NDAGIJIMANA1, VERONICA VECELLIO1 and TARA S. STOINSKI1,2.

1The Dian Fossey Gorilla Fund International, 2Zoo Atlanta, 3Integrative Biology, The University of Texas at Austin

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Research on primate sociality has emphasized important variations in social relationships between primate populations or taxa, but the proximate mechanisms responsible for these variations operate at a finer scale: the individual level. Using data collected by the Karisoke Research Center between 2003-2012, we generated social networks and individual measures of social centrality using agonistic and affiliative behavior data for nine mountain gorilla groups (group size: 2-64). We then examined how individual traits (e.g., sex, age) and group composition shape the social sphere of individual gorillas. Although mountain gorilla society is generally considered to be egalitarian, we found that the degree of sociality varies considerably across individual gorillas. The number of individuals a gorilla regularly interacts with, or social centrality, was mainly explained by the individual’s age and sex, the number of adult males and females in its group and the number of related individuals in its group. Over their life spans, we found that gorillas experience sharp changes in social centrality at approximately a year after weaning and again around the age of ten. The maximum number of group members an individual gorilla interacts with does not increase linearly with group size, but reaches an asymptotic value of around 10 individuals, which causes the largest groups to exhibit a modular network structure. Our analyses also revealed the importance of social inertia in female gorillas, which maintain their social centrality for extended periods of time. We discuss these findings in the context of the evolution of social systems in long-lived species.

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