Evolution of sex differences in cooperation: the role of trade-offs with dispersal

Abstract

Explaining the evolution of sex differences in cooperation remains a major challenge. Comparative studies highlight that offspring of the more philopatric sex tend to be more cooperative within their family groups than those of the more dispersive sex but we do not understand why. The leading Philopatry hypothesis proposes that the more philopatric sex cooperates more because their higher likelihood of natal breeding increases the direct fitness benefits of natal cooperation. However, the Dispersal trade-off hypothesis proposes that the more dispersive sex cooperates less because preparations for dispersal, such as extra-territorial prospecting, trade-off against natal cooperation. Here, we test both hypotheses in cooperatively breeding white-browed sparrow weavers (Plocepasser mahali), using a novel high-resolution automated radio-tracking method. First, we show that males are the more dispersive sex (a rare reversal of the typical avian sex-difference in dispersal) and that, consistent with the predictions of both hypotheses, females contribute substantially more than males to cooperative care while within the natal group. However, the Philopatry hypothesis cannot readily explain this female-biased cooperation, as females are not more likely than males to breed within their natal group. Instead, our radio-tracking findings support the Dispersal trade-off hypothesis: males conduct pre-dispersal extra-territorial prospecting forays at higher rates than females and prospecting appears to trade-off against natal cooperation. Our findings thus highlight that the evolution of sex differences in cooperation could be widely attributable to trade-offs between cooperation and dispersal; a potentially general explanation that does not demand that cooperation yields direct fitness benefits.

Prolactin and the shared regulation of parental care and cooperative helping in white-browed sparrow weaver societies

“…White-browed sparrow weavers are rain-dependent breeders that live in year-round territorial groups throughout the semi-arid regions of sub-Saharan Africa (Lewis, 1982; Wood et al, 2021). Within each social group, a single dominant male and female completely monopolize within-group reproduction and up to 10 non-breeding subordinates of both sexes help to feed their nestlings (Harrison et al, 2013a; 2013b; Capilla-Lasheras et al, 2021; 2023). Subordinates are typically offspring from previous broods that have delayed dispersal from their natal group (and so are helping to rear their parents’ young), but subordinate immigrants of both sexes do also occur (Harrison et al, 2013a; 2013b; Harrison et al, 2014).…”

“…Subordinates are typically offspring from previous broods that have delayed dispersal from their natal group (and so are helping to rear their parents’ young), but subordinate immigrants of both sexes do also occur (Harrison et al, 2013a; 2013b; Harrison et al, 2014). Subordinates contribute to several cooperative activities year-round, including territorial defense, roost construction and anti-predator vigilance (Lewis, 1982; Walker et al, 2016; York et al, 2019), and during breeding periods they contribute substantially to nestling provisioning (Cram et al, 2015a; Capilla-Lasheras et al, 2021; 2023). Helping behavior by subordinates in this species appears to have positive effects on both the helped offspring being fed and their parents.…”