Sex differences in dispersal predict sex differences in helping across cooperative birds and mammals

Sex differences in cooperation are widespread, but their evolution remains poorly understood. Here we use phylogenetic comparative methods to test the Dispersal hypothesis for the evolution of sex differences in contributions to cooperative care across the cooperatively breeding birds and mammals. The Dispersal hypothesis predicts that, where non-breeding individuals of both sexes help to rear offspring within their natal group, the more dispersive sex will contribute less (either because leaving the natal group earlier reduces the downstream direct benefit from cooperation or because dispersal activities trade-off against cooperation). Our analyses reveal (i) support for the Dispersal hypothesis (sex biases in dispersal predict sex biases in natal cooperation across taxa), and (ii) that this pattern cannot be readily attributed to alternative hypothesized drivers of sex differences in cooperation (kin selection, heterogamety, paternity uncertainty, patterns of parental care or differences between birds and mammals). Our findings help to clarify the evolutionary drivers of sex differences in cooperation and highlight the need for single-species studies to now 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.