Group Size and Polygamy in Social Mammals

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Abstract
Understanding the evolution of polygamy in social mammals requires explanations of why females form social groups and how males compete for mates once those groups are formed. A model is described for relating group size to female survival and reproductive rates and several tentative hypotheses concerning male reproductive strategies for social mammals are proposed. The group size model is derived from the Euler-Lotka life table equation by assuming that group size influences female survival and reproduction. It depends on the assumption that female choice rather than male herding behavior is responsible for the evolution of sociality in mammals. Complicating factors not included in the explicit model are considered, and the relationship between the group size model and the Orians-Verner model for polygyny is discussed. Female survival and reproductive rates are influenced by group size, as required by the model. Predictions concerning the effects of group size on female survival and reproduction are made by attempting to classify the best studied social mammals into the 4 general cases generated by the model. The evolution of male reproductive strategies has been poorly studied. The number of males associated with groups and their mating behavior within those groups may be influenced by group size, vulnerability of solitary males to predation, group cohesiveness and group mobility. Additional factors may also be important but have not been identified.