Reproductive and metabolic endocrinology in the emu (Dromaius novaehollandiae). 2. Reproductive activity, egg incubation, and annual cycles in breeding pairs

The emu is a short-day breeder and only the male exhibits egg incubation, during which there is an obligatory fast, suggesting a conflict between the motivations to reproduce and to feed. We studied reproductive and metabolic endocrinology during the circannual cycles in reproduction and body weight in males and females. Some males were allowed to incubate and, for others, incubation was prevented by egg removal. Females began to lose weight at the start of lay and total loss was related to clutch size. Length of lay and clutch size were strongly influenced by whether the male partner incubated. In males, testosterone concentrations were greatest around the winter solstice, declined immediately when incubation began, and remained low until seven weeks after incubation ended. Prolactin concentrations increased 5-fold greater during incubation, reflecting its role in incubation behaviour. Incubating males fasted, used up their body reserves, and rapidly recovered weight after incubation ended. Changes in body weight were related to changes in the circulating concentrations of thyroid hormones, insulin and glucagon. In conclusion, annual body weight cycles were determined largely by the demands of reproduction and, during the incubation fast, body reserves appear to be utilized in a highly regulated manner.