Reproductive behaviors of Chimpanzees and Bonobos

The study of cooperation and competition is fundamental to the field of social behaviour (Harcourt, A.H., & de Waal, F., Eds., 1992, Oxford University Press). Many of these publications, particularly in the field of primatology, have addressed coalitions and alliances, especially among males who were assumed until relatively recently to be the strategic sex. A new report by Surbeck et al. provides additional insights into the costs and benefits of coalitions and other close associations for males. In particular, these authors studied polygynandrous bonobos (Pan paniscus) finding that maternal support increases a son's reproductive success with the potential to decrease the reproductive success of dominants. Presumably, mothers gain via indirect fitness. Another important aspect of the new report is the finding that males exhibit a steep, linear dominance hierarchy. Surbeck et al. suggest that bonobos are intermediary between chimpanzees (exhibiting coercive social traits) and polygynandrous muruquis (characterized in the literature as "egalitarian") on a putative continuum of aggression. Of further significance is the authors' insight that agonistic (and, possibly, other) support modifies spatial dynamics among males so that females are no longer defensible for the dominant. It seems, also, that support may decrease the benefits and/or increase the costs of aggression. It is widely known that dispersal in mammals is primarily male biased (i.e., in most species, males disperse from their natal groups before reproducing, often termed "female-philopatry"). In these species, support of daughters by mothers is known to enhance daughters' reproductive success. Surbeck et al. hypothesized that the same pattern would be found in species in which dispersal is female biased (e.g., bonobos), and their hypothesis was supported. In primates, dispersal in prosimians and cercopithecine monkeys is primarily male biased; in apes and many Neotropical monkeys, dispersal is female biased. Where kinship was known in studies of coalitions and alliances across the order, it is notable that, in polygynandrous chimpanzees (Pan troglodytes), a congeneric of the bonobo, coalitions have been found to be independent of kinship. Unlike chimpanzees, bonobo females are dominant over males, a trait that may, in part, explain Surbeck et al.'s findings. Many other features of mammalian individuals and groups remain to be investigated before the differential benefits and costs of coalitionary behavior are understood. For example, few studies have reported alternative reproductive behaviors such as cryptic responses that might enhance male reproductive success. Related to this, few studies have tested Fisher's hypothesis that, over the long term, frequency-dependent selection should equalize fitness of alternative phenotypes. It would be helpful to view dominant and subordinate organisms and their differential traits as displays of alternate phenotypes (rather than "individuals" in use in the "social sciences") and to study these as products of condition-dependent genetic processes (e.g., switches). Phenotypes would, then, be amenable to mapping over space. Comparative studies might investigate the sociosexual and ecological causes for female-philopatry as an ancestral trait in mammals, including primates. For some of the many questions remaining, species exhibiting bisexual dispersal (e.g., mantled howler monkeys: Jones, C.B., 1980, Primates 21: 389-405, in which all age/sex combinations of coalitions were observed to occur) may yield interesting comparative data.

Surbeck, M., Mundry, R. & Hohmann, G. 2011. Mothers matter! Maternal support, dominance status, and mating success in male bonobos. Proceedings of the Royal Society B 278: 590-598.
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