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.