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Yamagiwa / Oral

Social flexibility of gorillas: some implications for the origin of human family

Juichi Yamagiwa (Kyoto University, Japan)

     The formation of family is one of the peculiar features characterizing modern humans along with the language.  The family is not found among non-human primates, but is cross-culturally universal in human societies.  However, taking recent findings into consideration, most characteristics of human family have been inherited from non-human primates, especially from the great apes, and it should be regarded as one of the most primitive invention by human culture.  The social structures of non-human primates lack "sociological father" and the organization of community, in which more than two different social units support each other through frequent interactions.  Gorillas have the primitive form of both, and could be regarded as having common social features with the early hominids.  From a comparison of social structure and demographic history between the Virunga and the Kahuzi gorilla populations, I propose that the social organization of gorillas is based on tolerance among related males and prolonged association among unrelated females.  The degree of such tolerance and association may vary with environmental conditions.  Both populations show the similar tendencies in female's natal transfer and reproduction in the group into which they transfer.  Males tend to emigrate from their natal groups and to spend a solitary life before establishing their reproductive groups in both populations.  However, patterns of female transfer and association among related males are different between Kahuzi and Virunga.  The Kahuzi females tend to transfer with another female and their offspring, while the Virunga females usually transfer alone.  After the death of leading silverback, the Kahuzi females associated with each other for prolonged periods, while the Virunga females subsequently transferred into other groups or solitary males.  These differences may be related to presence or absence of infanticide.  The Kahuzi males tend to establish their groups beside their natal groups, and they occasionally took females from their natal groups.  The Virunga males tend to remain their natal groups after maturity.  The higher proportion of multi-male groups in the Virunga population reflect such association among mature males within a group.  The tolerance among related males appears between social units in Kahuzi, while it appears within social units in Virunga.  Both ecological and social factors may influence these association patterns among females and males, and frequent inter-unit encounters enable them to adopt both options .  Under the severer environmental conditions than those of the present gorilla's habitats, the early hominids may have promoted the stronger association and cooperation among related males in various ways with keeping cohesiveness of group and tendency of natal female transfer.

Yamagiwa, J., 1987.  Male life history and social structure of wild mountain gorillas (Gorilla gorilla beringei).  In: Evolution and Coadaptation in Biotic Communities, S. Kawano, J.H. Connell & T. Hidata (eds.), University of Tokyo Press, Tokyo, pp. 31-51.Yamagiwa, J., 1999.  Socioecological factors influencing population structure of gorillas and chimpanzees.  Primates, 40: 87-104.
Yamagiwa, J., Maruhashi, T., Yumoto, T. & Mwanza, N., 1996.  Dietary and ranging overlap in sympatric gorillas and chimpanzees in Kahuzi-Biega National Park, Zaire.  In: Great Ape Societies, W.C. McGrew, L.F. Marchant & T. Nishida (eds.), Cambridge University Press, Cambridge, pp. 82-98.