An integrative appraisal of the hormonal and metabolic changes induced by acute stress using king penguins as a model

A large number of studies have focused on the reactivity of the hypothalamic-pituitaryadrenal (HPA) axis and the consequences of glucocorticoids (GC) in mediating life-history trade-offs. Although short-term increases in GCs are viewed as adaptive, mobilizing energy substrates allowing animals to deal with impending threats (e.g. stimulating hepatic gluconeogenesis, stimulating lipolysis, mobilizing amino acids), few studies have actually measured the exact time-course of substrate mobilisation in response to acute stress in natural conditions. We evaluated the hormonal and metabolic components of the stress response to acute stress in 32 free-living king penguins (Aptenodytes patagonicus). We monitored changes in blood GCs (corticosterone, CORT), glucose, lactate, ketone bodies (β-hydroxybutyrate), non-esterified fatty acids, and uric acid in response to a standardized capture-restraint protocol lasting for up to 90min. Furthermore, we tested whether the vigilance status of the animal (alert or asleep) affected its perception of the capture, thereby modulating the hormonal and metabolic stress responses. The time course of energy mobilisation followed the characteristic pattern expected from laboratory and theoretical models, with a rapid depletion of those energy stores linked to rapid adrenergic responses (i.e. glucose and ketone bodies), followed by a mobilisation of energy stores associated with the sustained longer-term GC response (i.e. fats and protein stores). HPA reactivity was generally slower than reported in other birds, and there was high inter-individual variability. Sleeping birds had higher GC and glucose responses to acute stress, suggesting a more rapid mobilization of energy stores. Our results highlight the importance of considering HPA and metabolic responses to acute stress against species-specific life history and ecological relevant backgrounds.