The Adrenocortical Response to Stress in Incubating Magellanic Penguins (Spheniscus magellanicus)

Circulating levels of glucocorticoids increase rapidly in response to capture and handling in many vertebrate species, which is indicative of the sensitivity of the hypothalamo-pituitary-adrenal axis to a variety of acutely stressful events. We measured cir­ culating levels of corticosterone at capture and after 25 min of handling and restraint in free­ living Magellanic Penguins (Spheniscus magellanicus) during their first two long incubation turns. Initial levels of corticosterone were higher for males than for females; however, levels of corticosterone after 25 min of handling and restraint increased throughout incubation and were higher for females than for males. These 25-min levels of corticosterone were negatively correlated with body mass and body condition. Initial levels of circulating corticosterone, however, were not associated with mass and did not change during the fast, except for an increase among three females that had been incubating the longest. The higher 25-min cor­ ticosterone levels for females may be due to their lower body mass and longer fast during incubation. Magellanic Penguins appear to be more responsive to stress as fasting proceeds, suggesting that disturbances should be minimized when penguins have depleted fat stores.