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Oecologia

, Volume 151, Issue 1, pp 140–149 | Cite as

Network metrics reveal differences in social organization between two fission–fusion species, Grevy’s zebra and onager

  • Siva R. Sundaresan
  • Ilya R. Fischhoff
  • Jonathan Dushoff
  • Daniel I. Rubenstein
Behavioral Ecology

Abstract

For species in which group membership frequently changes, it has been a challenge to characterize variation in individual interactions and social structure. Quantifying this variation is necessary to test hypotheses about ecological determinants of social patterns and to make predictions about how group dynamics affect the development of cooperative relationships and transmission processes. Network models have recently become popular for analyzing individual contacts within a population context. We use network metrics to compare populations of Grevy’s zebra (Equus grevyi) and onagers (Equus hemionus khur). These closely related equids, previously described as having the same social system, inhabit environments differing in the distribution of food, water, and predators. Grevy’s zebra and onagers are one example of many sets of coarsely similar fission–fusion species and populations, observed elsewhere in other ungulates, primates, and cetaceans. Our analysis of the population association networks reveals contrasts consistent with their distinctive environments. Grevy’s zebra individuals are more selective in their association choices. Grevy’s zebra form stable cliques, while onager associations are more fluid. We find evidence that females associate assortatively by reproductive state in Grevy’s zebra but not in onagers. The current approach demonstrates the utility of network metrics for identifying fine-grained variation among individuals and populations in association patterns. From our analysis, we can make testable predictions about behavioral mechanisms underlying social structure and its effects on transmission processes.

Keywords

Individual associations Equids Animal groups Social structure 

Notes

Acknowledgments

We thank Gujarat Forest Department and Kenya Ministry of Education for permission to work in India and Kenya, respectively. Tanya Berger-Wolf, Stephen Pratt, Simon Levin, David Lusseau, Marc Mangel, and one anonymous reviewer gave us valuable comments on drafts of the manuscript. Patrick Akilong, Jayanti Degama, and Yaseen Malek provided field assistance. Mpala Research Center offered logistical field support. For financial support, we acknowledge Wildlife Conservation Society Research Fellowship Program (SRS), Teresa Heinz Environmental Scholars program (IRF), Pew Charitable Trusts award 2000-0002558 “Program in Biocomplexity” to Princeton University Department of Ecology and Evolutionary Biology (SRS, DIR, IRF), NSF grant CNS-025214 (DIR, IRF), NSF grant IOB-9874523 (DIR) and NSF grant IBN-0309233 (DIR, SRS). All research presented here complies with laws in Kenya, India, and the United State of America.

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • Siva R. Sundaresan
    • 1
    • 2
  • Ilya R. Fischhoff
    • 1
    • 2
  • Jonathan Dushoff
    • 1
  • Daniel I. Rubenstein
    • 1
    • 2
  1. 1.Department of Ecology and Evolutionary BiologyPrinceton UniversityPrincetonUSA
  2. 2.Mpala Research CenterNanyukiKenya

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