Circulating levels of glucocorticoids increase rapidly in response to
capture and handling in many vertebrate species, which is indicative o
f the sensitivity of the hypothalamo-pituitary-adrenal axis to a varie
ty of acutely stressful events. We measured circulating levels of cort
icosterone at capture and after 25 min of handling and restraint in fr
ee-living Magellanic Penguins (Spheniscus magellanicus) during their f
irst two long incubation turns. Initial levels of corticosterone were
higher for males than for females; however, levels of corticosterone a
fter 25 min of handling and restraint increased throughout incubation
and were higher for females than for males. These 25-min levels of cor
ticosterone were negatively correlated with body mass and body conditi
on. Initial levels of circulating corticosterone, however, were not as
sociated with mass and did not change during the fast, except For an i
ncrease among three females that had been incubating the longest. The
higher 25-min corticosterone levels for females may be due to their lo
wer body mass and longer fast during incubation. Magellanic Penguins a
ppear to be more responsive to stress as fasting proceeds, suggesting
that disturbances should be minimized when penguins have depleted fat
stores.