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A new classification of mammalian uni-male multi-female groups based on the fundamental principles governing inter- and intrasexual relationships


Polygyny is the most common mating system in mammals, and many species form uni-male multi-female groups (UM-MF units). Polygynous systems are traditionally distinguished according to male reproductive strategies, such as “resource defense” or “female defense,” both of which are often described in the literature as forming “harems.” However, this focus on male strategies, and the use of umbrella terms to describe them, lumps together societies that fundamentally differ in their ontogeny, stability, and relationships. Integrating foundational theories of mating strategies with the principles governing relationship dynamics, driven by both male and female strategies and modulated by male-female conflicts of interest, we propose a new framework for classifying the diversity of UM-MF units. We differentiate UM-MF groups in terms of average female kinship within the group and length of male tenure to define general classes with distinct predictions for the nature of inter- and intrasexual relationships. We propose a narrower definition for the “true harem” along with new terminology to describe the other three classes: “benign consortship,” “coterie,” and “coercive consortship.” Using socioecological data for 40 mammalian species from 27 families, we found our framework was able to successfully predict patterns of female-female cooperation and the presence of coercive male-female relationships. Finally, we refine our framework, identifying subclasses of the main four classes and propose hypotheses about the underlying causes of observed patterns. By focusing on the nature of within group relationships, this framework provides a powerful lens for asking broad, comparative evolutionary questions about social evolution and socioecology.

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We apologize to all the colleagues whose work we were not able to include. We thank Drs. Carsten Schradin, Konstantin Rogovin, Jan Randell, Dan Blumstein, Dina Dechmann, Jenna Kohles, and Eileen Lacey for generously sharing their insights and data to fill in key information gaps for Table 1. We also thank Madeleine Andrews, Christopher Lawrence, and Drs. Andrew Gersick, Jaqueline Kaarithi, Andrew Carlson, in addition to the editor, Dr. Fritz Trillmich, and three anonymous reviewers for their thoughtful feedback on the study. This work was funded by the National Science Foundation.

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All data presented in this manuscript are tabulated in Table 1.


This research was funded by the National Science Foundation (IBN-9874523, CNS- 025214, and IOB-9874523).

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Conceptualization, SH, KT, and DR. Methodology, SH and KT. Investigation SH and KT. Formal analysis, data curation, and visualization, SH and KT. Illustrations, SH. Writing–original draft, SH. Writing–review and editing, all authors. Funding acquisition, DR.

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Correspondence to Severine B. S. W. Hex.

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This article does not contain any studies with human participants or animals performed by any of the authors.

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The authors declare no competing interests.

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Communicated by F. Trillmich

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Hex, S.B.S.W., Tombak, K. & Rubenstein, D.I. A new classification of mammalian uni-male multi-female groups based on the fundamental principles governing inter- and intrasexual relationships. Behav Ecol Sociobiol 75, 157 (2021).

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  • Harem
  • Mammals
  • Mating system
  • Polygyny
  • Social behavior
  • Uni-male multi-female units