International Journal of Primatology

, Volume 29, Issue 2, pp 405–420 | Cite as

Intersexual Conflict and Group Size in Alouatta palliata: A 23-year Evaluation

  • Sadie J. Ryan
  • Philip T. Starks
  • Katharine Milton
  • Wayne M. Getz


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.


Alouatta palliata Barro Colorado Island group size reproductive success sex ratio 



A Miller Fellowship from the Miller Institute for Basic Research in Science supported P. T. Starks and an EPA-STAR fellowship (FP-916382) and an NSF Bioinformatics post-doctoral fellowship (0630709) supported S. J. Ryan, with additional support under an NSF/NIH EID Grant DEB-0090323 and James S. McDonnell Foundation 21st Century Science Initiative Award to W. M. Getz. We thank C. A. Blackie, M. E. Hauber, C. L. Nunn, J. V. Redfern, and J. M. Reed for comments on early versions of the manuscript. We also thank C. A. Chapman for constructive comments and edits on later versions. We gathered all data provided in this article observationally. The authors do not have any financial, personal, or professional interests that could be construed to have influenced this article.


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Copyright information

© Springer Science+Business Media, LLC 2007

Authors and Affiliations

  • Sadie J. Ryan
    • 1
    • 2
  • Philip T. Starks
    • 3
  • Katharine Milton
    • 4
  • Wayne M. Getz
    • 4
    • 5
  1. 1.Department of AnthropologyStanford UniversityStanfordUSA
  2. 2.Department of Anthropology and McGill School of EnvironmentMcGill UniversityMontrealCanada
  3. 3.Department of BiologyTufts UniversityMedfordUSA
  4. 4.Department of Environmental Science, Policy and ManagementUniversity of California at BerkeleyBerkeleyUSA
  5. 5.Mammal Research InstituteUniversity of PretoriaPretoriaRepublic of South Africa

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