Mothers in a cooperatively breeding bird increase investment per offspring at the pre-natal stage when they will have more help with post-natal care

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 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 either allows or incentivizes helped mothers to focus maternal investment on the pre-natal phase, to which helpers cannot contribute directly. The potential for post-natal helping to promote pre-natal development further complicates attempts to identify and quantify the fitness consequences of helping.

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

“…Here we demonstrate that this marked sex difference in cooperation is evident among subordinates within their natal groups, in the absence of a sex difference in relatedness to recipients (Figure 1). Helper contributions are known to increase the overall rates at which nestlings are fed (Capilla-Lasheras et al 2021) and may yield indirect fitness benefits by reducing environmentally-induced variance in the reproductive success of related breeders (Capilla-Lasheras et al 2021), lightening the workloads of related breeders (Capilla-Lasheras et al 2023;O'Callaghan 2021) and enhancing the quality of their offspring (Wood 2017).…”