A trait-based approach to understand the evolution of complex coalitions in male mammals

Coalitions occur when multiple individuals cooperate against a common opponent or for a common goal. Coalition formation is a complex behavior, typically described in highly social and cognitively complex species. Surprisingly, we know little about the social and environmental factors that may select for the evolution of coalitions. We studied the evolution of coalitionary behavior by first redefining it in a continuous way that acknowledges variation in the degree to which animals collaboratively work toward a common goal. We then examined the evolutionary association of coalition complexity with 3 social factors (estrous duration, group size, and presence of a dominance hierarchy) and 3 environmental factors (habitat type, diurnality, and diet type). We found that estrous duration, group size, and dominance hierarchy were significantly correlated with coalition complexity and thus conclude that social factors are relatively more important in the evolution of complex coalitions than are environmental factors. From these results, we infer that complex coalitions may be the product of social factors that reduce female monopolizability and encourage the aggregation of multiple males. Copyright 2009, Oxford University Press.