Photoperiodic control of seasonal breeding and appetite in the emu.

Seasonal breeding in the emu occurs in winter and is associated with anorexia and a loss of 20-30% body weight. In order to establish whether decreasing day length induces winter breeding and associated anorexia, photoperiodic experiments were carried out on emus held in light-controlled rooms. Transfer from long to short days stimulated reproductive activity and induced anorexia, whereas transfer from short to long days induced the converse responses. Measurements of plasma luteinizing hormone (LH) and prolactin suggested that winter breeding is a consequence of the dissipation of photorefractoriness to the long days of the previous breeding season, resulting in high concentrations plasma gonadotrophins. An associated, but delayed, increase in prolactin secretion may be responsible for terminating breeding by suppressing gonadotrophin secretion. Chronic exposure to long days disrupts this seasonal pattern of changes in LH and prolactin secretion resulting in permanently increased LH secretion. This observation suggests that emus might be induced to breed out-of-season by provision of prolonged extra lighting. The annual cycles of appetite and fat deposition were related to changes in concentrations of blood hormones controlling nutrient partitioning (insulin and glucagon) and metabolic rate (thyroid hormones). Observations on castrated emus showed that the annual cycle of metabolic hormones and associated fattening are dependent on gonadal hormones. It is concluded that the inversely related annual cycles of breeding and fattening in the emu are controlled by day length, and that photoperiod-dependent mechanisms in the hypothalamus, and possibly, changes in the secretion of prolactin play a key role in maintaining this relationship.