Ryan S.J., Starks P.T., Milton K., Getz W.M.
Department of Anthropology, Stanford University, Stanford, CA 94305, United States; Department of Anthropology, McGill School of Environment, McGill University, Montreal, QC H3A 2T7, Canada; Department of Biology, Tufts University, Medford, MA, United States; Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA 94720, United States; Mammal Research Institute, University of Pretoria, 0002 Pretoria, South Africa
Ryan, S.J., Department of Anthropology, Stanford University, Stanford, CA 94305, United States, Department of Anthropology, McGill School of Environment, McGill University, Montreal, QC H3A 2T7, Canada; Starks, P.T., Department of Biology, Tufts University, Medford, MA, United States; Milton, K., Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA 94720, United States; Getz, W.M., Department of Environmental Science, Policy and Management, University of California at Berkeley, Berkeley, CA 94720, United States, Mammal Research Institute, University of Pretoria, 0002 Pretoria, South Africa
Models of optimal primate group size suggest that group formation and growth arise to benefit individual fitness, but that size is limited by costs. The ecological constraints hypothesis posits that group formation and growth is driven by protection from predation or the advantages of group foraging, while an upper limit on group size is constrained by travel costs and intragroup competition for food or other critical resources. Socioecological models also predict that individual reproductive success, hypothesized to decrease with increasing group size, also places an upper limit on the number of individuals in a group. Our analysis of 23 yr of group composition data on mantled howlers (Alouatta palliata) from a single Panamanian study site on Barro Colorado Island not only corroborates the socioecological model but also shows that female reproductive success increased, whereas that of males decreased, with the less female-biased sex ratios in larger groups. We suggest that the conflict of interest between the sexes over adult sex ratio, particularly the male proportion in a group, in combination with ecological factors, is an important determinant of group size and composition. © 2007 Springer Science+Business Media, LLC.