Reproductive and metabolic endocrinology in the emu (Dromaius novaehollandiae). 3. Egg incubation in the male

In the male emu, the annual breeding cycle includes an obligatory, complete fast during incubation, suggesting physiological mechanisms that coordinate loss of body mass and metabolite supply with reproductive activity. The onset of incubation is quickly followed by changes in the secretion of metabolic hormones and the induction of a physiological state near to torpor in which metabolic fuels are used sparingly. As soon as incubation ends, there are large changes in the secretion of thyroid hormones and insulin, and appetite recovers after only a few days, and body mass is recovered within three weeks. This sequence of hormonal and metabolic changes is similar to that observed in other birds that go through a natural fast, but differs from that in which starvation is imposed by food deprivation. Moreover, the general control of appetite throughout the year is integrated with the control of reproduction so the male emu is either storing fuels in preparation for the next breeding season, or utilising stored fuels during the present breeding season. There seems little doubt that testosterone is important in these processes, a hypothesis to be tested in future studies, along with investigation of the control of appetite suppression and recovery.