Mothers front-load their investment to the egg stage when helped in a wild cooperative bird

Abstract

In many cooperative societies, including our own, helpers assist with the post-natal care of breeders’ young, and may thereby benefit the post-natal development of offspring. Here we present evidence of a novel mechanism by which such post-natal helping could also have hitherto unexplored beneficial effects on pre-natal development: by lightening post-natal maternal workloads, helpers may allow mothers to increase their pre-natal investment per offspring. We present the findings of a decade-long study of cooperatively breeding white-browed sparrow weaver, Plocepasser mahali, societies. Within each social group, reproduction is monopolized by a dominant breeding pair, and non-breeding helpers assist with nestling feeding. Using a within-mother reaction norm approach to formally identify maternal plasticity, we demonstrate that when mothers have more female helpers they decrease their own post-natal investment per offspring (feed their nestlings at lower rates) but increase their pre-natal investment per offspring (lay larger eggs, which yield heavier hatchlings). That these plastic maternal responses are predicted by female helper number, and not male helper number, implicates the availability of post-natal helping per se as the likely driver (rather than correlated effects of group size), because female helpers feed nestlings at substantially higher rates than males. We term this novel maternal strategy “maternal front-loading” and hypothesize that the expected availability of post-natal help allows helped mothers to focus maternal investment on the pre-natal phase, to which helpers cannot contribute directly. Such cryptic maternally mediated helper effects on pre-natal development may markedly complicate attempts to identify and quantify the fitness consequences of helping.

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

“…Thus, to the extent that helping does augment group size in this species, this mechanism is likely to leave females experiencing greater downstream direct fitness costs of helping, rather than benefits. Second, while both of the above mechanisms envisage that helping augments group size, helping may not actually augment group size in sparrow-weavers, as variation in helper numbers (whether female helpers, male helpers or both) does not positively predict either the rate at which dominant females breed (27) or the survival of their offspring to fledging (39, 40). Indeed, it seems unlikely that helping in sparrow-weaver societies yields an appreciable downstream direct benefit of any kind (e.g., via other conceivable mechanisms such as “pay to stay” or signalling “quality” (41–44)), as subordinates only help to rear offspring within their natal groups, where recipients are typically close kin (22, 28).…”

“…However, this mechanism cannot readily account for the female-biased natal cooperation observed here for two reasons. First, recent analyses strongly suggest that subordinate sparrow-weavers suffer markedly lower survival rates in larger social groups (O’Callaghan 2021), which would leave the more philopatric sex suffering a greater downstream cost (rather than benefit) from cooperative group augmentation. Second, helping does not appear to augment group size in sparrow-weavers, as helper numbers (whether female helpers, male helpers or both) do not positively predict either offspring survival to fledging or the breeding rate of dominant females.…”

“…2021), lightening the workloads of related breeders (Capilla-Lasheras et al . 2023; O’Callaghan 2021) and enhancing the quality of their offspring (Wood 2017). Helping could also conceivably yield direct fitness benefits, but as subordinate immigrants rarely help at all (Walker et al .…”