Preparing to fledge: the adrenocortical and metabolic responses to stress in king penguin chicks
Résumé
Summary 1. Modulating the adrenocortical response to stress and the corresponding mobilization of energy substrates allows animals to cope with energy-demanding challenges imposed by ecological constraints. Whether such a modulation occurs during the fledging process of the king penguin chick (Aptenodytes patagonicus) was examined. In this seabird species, fledging is associated with moult completion, prolonged fasting and transition from terrestrial to marine life. 2. Specifically, it was tested whether the adrenocortical and metabolic responses to a capturehandling stress i. depended on the developmental ⁄moulting status of pre-fledging birds, ii. were affected by their nutritional status, and iii. were inter-related. 3. The adrenocortical response was 2–3 times lower at moult end and fledging than at the onset of moult, 1 month earlier. No direct evidence for a link between the adrenocortical response and moult-related changes in circulating thyroid hormones was obtained. 4. At the onset of moult, chicks were in a nutritional status corresponding to phase I of fasting whereas those at moult end and fledging were in phase II of fasting. Thus, the adrenocortical response depended on nutritional status, being blunted during phase II when body proteins were spared and fat stores preferentially mobilized. 5. The metabolic response to acute stress was characterized by an increase in plasma glucose, NEFA and uric acid and by a decrease in plasma b-hydroxybutyrate. Differences in this response between phase I and phase II fasting chicks suggest that the mix of energy substrates used to fuel energy demand during acute stress is the same as that routinely fuelling daily energy expenditure. We obtained no evidence for an interaction between corticosterone and metabolic responses. 6. Overall, the results suggest that, in pre-fledging king penguin chicks, the nutritional rather than the developmental ⁄ moulting status modulates the adrenocortical and metabolic responses to an acute stress. The possible adaptive function of the blunting of these responses by the time of fledging is discussed.