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Description

The implications of population structures are well-studied in many ecological contexts, but surprisingly little theory has been developed on the implications of social structure in population ecology. The dynamics of socially structured populations are unique because social groups themselves may split, fuse, and compete. Here, I analyze a model that clarifies the important role such “between-group processes” play in the dynamics of social populations. The effect of between-group processes is mediated by how they influence the stable group-size distribution. I show that density dependence in social groups does not lead to density dependence at the population level. However, between-group processes can lead to either negative or positive density dependence at the population level, even if birth and death rates do not directly depend on population size. After building intuition regarding the role of social structure in population ecology, I explore the evolutionary implications of a specific aspect of social behavior: the immigration of infanticidal males. Exposure to such males results in male-mediated early sexual maturation (MMM) in many female mammals. The ‘optimal timing hypothesis’ posits that MMM may be an adaptive counterstrategy to infanticide; maturing immediately after a new male immigrates may maximize a female’s chances of weaning her first offspring before the next infanticidal male immigrates. On the other hand, the non-adaptive ‘Bruce effect byproduct hypothesis’ posits that MMM is triggered by a suite of physiological changes that occur in female mammals after exposure to an infanticidal male, which, in pregnant females, produces spontaneous abortion and the resumption of sexual cycling (the Bruce effect). I use data from wild primate populations, individual-based simulations, and analytical modeling to argue that MMM is most likely a non-adaptive byproduct of the Bruce effect.