Networking Silverbacks?

Much of what we know about the behaviour of gorillas in the wild comes from studies of the habituated mountain gorillas (Gorilla beringei beringei) at Karisoke Research Center in Rwanda. The lives of the Karisoke gorillas have been monitored for over three decades and study subjects cross multiple gorilla generations (Robbins et al. 2001). By comparison, we know rather little about the social behaviour of the wild western gorillas (Gorilla gorilla) that live in the dense lowland forests of western equatorial Africa. In such forests it is difficult to monitor gorillas from a distance, and western gorillas have been notoriously difficult to habituate to the presence of human observers.
However, after years of effort, researchers at several sites have finally been successful in habituating some western gorilla groups to sustained observation. Another successful strategy has been a 'wait and watch' approach in which observers on platforms at the edge of forest clearings (or bais) monitor the comings and goings of multiple groups (e.g. Parnell 2002). Both types of study have highlighted something rather unusual: western gorilla groups frequently react more calmly to the presence of other gorillas than do mountain gorillas (Doran et al. 2004). These peaceful interactions between different western gorilla groups puzzle researchers because in many primates, when different social groups cross paths, interactions are often hostile. This is especially true in species like gorillas in which females can transfer between groups during inter-group interactions. Male mountain gorillas have been observed to aggressively defend their females against males from other groups by chest beating, charging and actively herding females (Sicotte 1993).
Why then are some western gorilla males more nonchalant in their interactions with other gorilla males? Recent analyses of genetic relationships among 14 silverbacks at Mondika Research Center (Central African Republic and Republic of Congo) indicate that the majority of silverbacks ranging in the study site are related (father-son or brothers) to one or more males leading nearby groups (Bradley et al. 2004). This results in what appear to be clusters or networks of related males. So perhaps these oddly peaceful interactions between western gorilla groups are simply silverback family reunions.
But how might these males recognize each other as relatives? Not all interactions between western gorilla groups are peaceful, in fact some have involved violent interactions between silverbacks that have led to fatal injuries. This suggests that silverbacks might know which males are relatives and which are not and respond accordingly.
A possible clue to how male relatives might recognize each other is also found in the genetic data. Virtually all western gorilla groups observed to-date have only one silverback, and DNA-based paternity analyses indicate that the group silverback sires all of the group offspring, that is, there is no evidence that females produce offspring with immature males or with males outside of the group. Thus, the silverback can be fairly certain that an infant is his and an infant can be fairly certain that the group silverback is his father, and the time together in the same group gives them a chance to learn to recognize each other.
But how would brothers recognize each other? If some silverbacks can maintain leadership of their groups for long periods of time, as is indicated from observational and genetic data, then all gorillas growing up together in the same group will be paternal half-siblings. Lengthy periods of infant and juvenile development (~8 years) allow for years of interaction among paternal siblings. These siblings could then later recognize each other long after leaving the natal group. Social familiarity has been demonstrated as an important mechanism for kin recognition in other primates, such as macaques (Widdig et al. 2001) and baboons (Smith et al. 2003). Thus, it is plausible that such paternal-kin recognition could also play an important role in the social lives and social bonds of gorillas. The gorilla male network hypothesis suggests that paternal kin ties among silverbacks are a fundamental component of western gorilla social structure.
However, the idea that social relations among groups vary with the degree of relatedness among males is based upon consideration of genetic, but not behavioural data, from some groups and behavioural, but not genetic data, from others. Testing of the hypothesis requires determining whether peacefully interacting silverbacks are really relatives, or if their laid back attitudes are based on something other than kinship. This research, combining observations and genetic characterization of the exact same groups, is underway at Mondika.
If this ongoing genetic study finds additional support for our hypothesis that western gorilla silverbacks are forming social networks with male relatives, these findings could have broad importance for our understanding of ape and human social evolution. Indeed, these results suggest that a "patrilocal" social system, that is, a social system in which males stay in the region of their birth and potentially benefit from male kin associations while females leave, is a feature uniting African ape and human societies.

Brenda Bradley, Diane Doran-Sheehy and Linda Vigilant

References
Bradley, B. J. et al. (2004) Dispersed male networks in western gorillas. Current Biology 14:510-513.
Doran, D. M. et al. (2004) Ecological and social impact on ranging in western gorillas. Amer. J. Primatol., in press.
Parnell, R. J. (2002) Group size and structure in western lowland gorillas (Gorilla gorilla gorilla) at Mbeli Bai, Republic of Congo. Amer. J. Primatol. 56:193-206.
Robbins, M. M. et al. (eds.) (2001) Mountain Gorillas. Cambridge (Cambridge University Press).
Sicotte, P. (1993) Inter-group encounters and female transfer in mountain gorillas: Influence of group composition on male behavior. Amer. J. Primatol. 30:21-36.
Smith, K. et al. (2003) Wild female baboons bias their social behaviour toward paternal half-sisters. Proc. R. Soc. Lond. B Biol. Sci. 270:503-510.
Widdig, A. et al. (2001) Paternal relatedness and age proximity regulate social relationships among adult female rhesus macaques. Proc. Natl. Acad. Sci. USA 98:13769-13773.

Dr. Brenda Bradley did her dissertation at Stony Brook University on the molecular ecology of wild gorillas and is now a postdoc at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.
Prof. Diane Doran-Sheehy works at SUNY at Stony Brook. She was the director of the Karisoke Research Center (1989-1991), prior to establishing the Mondika Research Center for the study of western gorillas.
Dr. Linda Vigilant works at the for MPI Evolutionary Anthropology, Leipzig, and runs a research laboratory in which tools of genetic analysis are applied to questions of the reproductive strategies, kinship, dispersal, population histories of wild primates.

Western gorilla overview

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