Sex differences in helping effort reveal the effect of future reproduction on cooperative behaviour in birds

Abstract

The evolution of helping behaviour in species that breed cooperatively in family groups is typically attributed to kin selection alone. However, in many species, helpers go on to inherit breeding positions in their natal groups, but the extent to which this contributes to selection for helping is unclear as the future reproductive success of helpers is often unknown. To quantify the role of future reproduction in the evolution of helping, we compared the helping effort of female and male retained offspring across cooperative birds. The kin selected benefits of helping are equivalent between female and male helpers-they are equally related to the younger siblings they help raise-but the future reproductive benefits of helping differ because of sex differences in the likelihood of breeding in the natal group. We found that the sex which is more likely to breed in its natal group invests more in helping, suggesting that in addition to kin selection, helping in family groups is shaped by future reproduction.

Ten recent insights for our understanding of cooperation

“…In birds, the sex that remains, and therefore has a chance of inheriting the nest, helps at higher levels than the sex that disperses, and so is less likely to gain a direct benefit from helping 74 . A role of direct fitness benefits in birds is further supported by the observation that longer subordinate lifespans, which can increase the chance of inheriting a patch, have favoured the evolution of cooperative breeding, but especially in species where rates of polyandry are high, and so kin selection will be weaker 75 .…”

“…• Which sex cooperates? Across different species, both the likelihood of helping and the effort put into helping varies between the sexes 74,139 . • Why do individuals preferentially direct help towards relatives (kin discrimination) in some species, such as long-tailed tits, but not others, such as superb fairy-wrens 55 ?…”

Context-dependent social benefits drive cooperative predator defense in a bird

“…Our findings thereby support recent evidence from meta-analyses on the importance of direct benefits deriving from breeding position inheritance in driving helping behavior, even when controlling for sex differences in dispersal. 9,49 Indirect benefits did not predict subordinate investment in nest defense (''kin selection'' hypothesis). 10,11,13,30,33 In contrast to our predictions, subordinates that were more related to the brood did not defend the nest more often than less related ones (p R 0.08; Table S2); in fact, the likelihood of nest defense was lowest at the highest level of relatedness, and subordinates often helped defend unrelated broods (Figure S1).…”

Wandering woodpeckers: foray behavior in a social bird

“…Helper foray behavior is affected by both ecological and social factors. Helper male acorn woodpeckers, as in many other avian cooperative breeders, inherit their natal territories more often and typically disperse closer to their natal territory than females (Koenig et al , Downing et al ). However, we found no difference in foray distance or foray duration between helper males and females, a pattern that may be an artifact of the spatial extent of our receiver array, whose limits are substantially less than many female foray movements (Koenig et al ).…”