Hormonal stress responses in a species with female offspring desertion – testing the brood value hypothesis

Parental investment reflects a trade-off between current reproductive effort and future breeding opportunities. According to the brood value hypothesis, as the ‘cost-to-replace’ a brood increases, hormonal stress responses should be attenuated to ensure the maintenance of care. We examined how stress-induced prolactin (PRL) and corticosterone (CORT) levels – accounting for baseline concentrations – vary in relation to sex, breeding stage, laying date, clutch/brood size, and days relative to hatching in the Whiskered Tern Chlidonias hybrida, a species exhibiting obligate female desertion during the chick-rearing and post-fledging periods. We accounted for individual and seasonal variation by including body condition, laying date, and clutch size as additional covariates. While mean PRL levels did not differ between breeding stages, PRL dynamics diverged significantly following hatching. During incubation, stress-induced PRL remained stable and symmetrical between the sexes. Post-hatching, however, male responsiveness remained constant, whereas females exhibited a rapid down-regulation of the PRL stress response in relation to chick age, independent of baseline variation. This physiological shift likely facilitates offspring desertion – a strategy rendered viable by low predation pressure and effective male compensation, both of which reduce the relative value of the brood for females. Conversely, stress-induced CORT remained stable across both sexes and stages, contradicting the predicted attenuation of the hypothalamic-pituitary-adrenal (HPA) axis. CORT responsiveness was primarily predicted by individual baseline levels, suggesting a consistent hormonal phenotype rather than flexible adjustment. Ultimately, the brood value hypothesis was only partially supported; transitions in parental care appear to be mediated through the sex-specific modulation of prolactin rather than adjustments to the HPA axis. According to the brood value hypothesis, as the ‘cost-to-replace’ a brood increases, hormonal stress responses should be attenuated to ensure the maintenance of care. We tested this in the Whiskered Tern Chlidonias hybrida, a species where females exhibit obligate brood desertion. Our study reveals a critical divergence in hormonal dynamics: while corticosterone responses remained stable across breeding stages and sexes, prolactin (PRL) responsiveness was sex-specifically modulated post-hatching. Specifically, females exhibited a rapid down-regulation of the PRL stress response as chicks aged – a physiological shift that likely facilitates their departure. These findings provide partial support for the brood value hypothesis, suggesting that transitions in parental care and desertion strategies are mediated through prolactin dynamics rather than the HPA axis.