Breeding status affects the hormonal and metabolic response to acute stress in a long-lived seabird, the king penguin

- Glucocorticoid and metabolic responses to acute 30-min capture stresses were measured in king penguins during courtship, incubation and chick-brooding.
- Corticosterone (CORT) and non-esterified fatty acid (NEFA) plasma levels generally increased in response to capture.
- CORT responses increased over 30 min at all breeding stages. NEFA responses increased up to 10 min before being rapidly shunted in incubating and brooding, but not courting birds.
- 30-min CORT increases were lower in chick-brooding birds than other breeding stages.
- Attenuated CORT and NEFA responses with advancing breeding could be a mechanism ensuring continued breeding when reproductive value of the brood is high.

Stress responses are suggested to physiologically underlie parental decisions promoting the redirection of behaviour away from offspring care when survival is jeopardized (e.g., when facing a predator). Besides this classical view, the “brood-value hypothesis” suggests that parents' stress responses may be adaptively attenuated to increase fitness, ensuring continued breeding when the relative value of the brood is high. Here, we test the brood-value hypothesis in breeding king penguins (Aptenodytes patagonicus), long-lived seabirds for which the energy commitment to reproduction is high. We subjected birds at different breeding stages (courtship, incubation and chick brooding) to an acute 30-min capture stress and measured their hormonal (corticosterone, CORT) and metabolic (non-esterified fatty acid, NEFA) responses to stress. We found that CORT responses were markedly attenuated in chick-brooding birds when compared to earlier stages of breeding (courtship and incubation). In addition, NEFA responses appeared to be rapidly attenuated in incubating and brooding birds, but a progressive increase in NEFA plasma levels in courting birds suggested energy mobilization to deal with the threat. Our results support the idea that stress responses may constitute an important life-history mechanism mediating parental reproductive decisions in relation to their expected fitness outcome.