Dispersal and the evolution of sex differences in cooperation in cooperatively breeding birds and mammals

Sex differences in cooperation are widespread, but their evolution remains poorly understood. Here, we use comparative analyses of the cooperatively breeding birds and mammals to formally test the leading Dispersal Hypothesis for the evolution of sex differences in cooperation. The Dispersal Hypothesis predicts that, where both sexes delay dispersal from their natal group, individuals of the more dispersive sex should contribute to natal cooperation at lower rates (either because leaving the natal group earlier reduces the downstream direct benefit from natal cooperation or because dispersal activities trade-off against natal cooperation). Our comparative analyses reveal support for the Dispersal Hypothesis; sex biases in dispersal predict sex biases in helper contributions to cooperative care within the natal group across cooperative birds and mammals. Strikingly, in every species that showed significant sex biases in both dispersal and natal helping, the direction of sex bias in dispersal predicted that in natal helping in the manner predicted by the Dispersal Hypothesis. Our analyses also suggest that these patterns cannot be readily attributed instead to alternative hypothesized drivers of sex differences in cooperation (kin selection, heterogamety, paternity uncertainty, patterns of parental care, or differences between birds and mammals). These findings help to clarify the evolutionary drivers of sex differences in cooperation and highlight the need for single-species studies to tease apart whether sex differences in dispersal predict sex differences in natal cooperation because dispersal impacts the direct benefits of natal cooperation (as is often proposed) or because activities that promote dispersal trade off against natal cooperation.