Annual cycle of sex steroids in the yellow-eyed penguin (Megadyptes antipodes) on south island, New Zealand.

The yellow-eyed penguin (Megadyptes antipodes) is a rare penguin restricted to the South Island and some offshore islands of New Zealand. It is the least colonial of the penguins but shares with many other penguins a distinct breeding season with a short, synchronised period of egg-laying. Blood samples were collected throughout the year from yellow-eyed penguins on the Otago Peninsula, South Island in order to characterise relationships between steroid hormone levels and events of the breeding cycle. Testosterone levels in male penguins were high during the pre-egg phase, dropped dramatically to low levels during incubation, and remained low during the guard and postguard phases of the breeding cycle. Testosterone levels in females showed a similar pattern. Male testosterone levels were significantly higher than female levels during the pre-egg phase. The patterns of changes in estradiol levels were similar to those for testosterone. Estradiol levels were significantly higher in females than males during the pre-egg phase. There was a clear annual cycle of testosterone levels in yellow-eyed penguins, with levels low from January (midsummer) until July. They then rose markedly in August (the time of increased activity around nest sites) and reached a peak in September, followed by a steep decline to low levels again in October. There was also an annual cycle of estradiol levels with a peak that coincided with the testosterone peak. Progesterone levels varied during the year, being elevated in late summer and autumn (February-May) and lower during the remainder of the year.(ABSTRACT TRUNCATED AT 250 WORDS)