REPRODUCTIVE ENDOCRINOLOGY AND WEIGHT CHANGE IN RELATION TO REPRODUCTIVE SUCCESS IN THE MAGELLANIC PENGUIN (SPHENISCUS-MAGELLANICUS)

The Magellanic penguin is a colonial monogamous species that lays only a single clutch of two eggs per year. However, failed breeders remain at the colony and engage in nest building, fights, and copulations wi thout relaying. The seasonal changes in reproductive hormones and body weight through the nesting cycle were studied, with respect to the re productive success or failure of individuals. Body weight changed dram atically in both sexes through the season, in response to fasting duri ng incubation, and high body weight in males at the onset of incubatio n was a strong predictor of eventual reproductive success. Circulating steroid hormones had a biphasic seasonal pattern, with elevated level s during the sexual phase of breeding (prior to egg laying), declining to low, stable levels during the parental phase after eggs were laid. Luteinizing hormone levels were elevated in females, but not in males , prior to egg laying. Both sexes responded to reproductive failure by increasing the secretion of testosterone, and females also increased the secretion of estradiol, a response that would be expected of a spe cies that can renest following failure. However, renesting is extremel y rare, and this hormonal response to failure may instead serve to pro mote maintenance of pair bonds and also territory ownership across yea rs, (C) 1994 Academic Press, Inc.