Costs and benefits of joining South American sea lion breeding groups: testing the assumptions of a model of female breeding dispersion

Abstract

A recent cost-benefit model has been proposed (M.H. Cassini. 1999. Behav. Ecol. 10: 612–616; M.H. Cassini. 2000. Behav. Processes, 51: 93–99) to predict the dispersion of female mammals when breeding resources are distributed in fixed and predictable patches. The benefit of the model is a reduction in male harassment when females join breeding groups, and the cost is an increase in female–female competition for breeding resources. We tested the main assumptions of this model in a breeding colony of South American sea lions (Otaria flavescens), a sexually dimorphic, polygynous pinniped. The rate of female–female agonistic interactions increased with the number of females, which suggests that higher levels of female–female competition in denser breeding groups could reduce pup survival, owing to mother–pup separation effects. The rate of male–female interactions per female decreased with the number of females defended by a male, the trend being nonlinear, and males did not modify the frequency of interaction with females according to variations in the size of breeding groups. This evidence supports the advantage of female gregariousness in reducing the reproductive costs of interacting with males. We concluded that avoidance of male disturbance through dilution effects may have played an important role in the evolution of this species' mating system.