Control of seasonality by melatonin

Many mammals display profound changes in their physiology and behaviour coincident with their seasonal environment.These changes in reproductive activity, food intake, body weight and coat condition are physiological adaptations which restrict the birth and rearing of offspring to an optimum time of the year, reduce metabolic demand in the face of restricted food supply and alter body insulation to match the seasonal fluctuations in temperature.Due to the long-term nature of these physiological changes (e.g.re-growth of testes, change in body mass) such adaptations cannot be reactive, but instead must be initiated before the anticipated changes in climatic conditions.Thus, the mechanisms which regulate these seasonal changes demand a temporal component which ensures that physiological changes remain synchronized with the appropriate time of the year.Photoperiod provides a highly predictable environmental time cue which is used by many mammals to co-ordinate their seasonal cycles in physiology (Lincoln & Short, 1980).It is now well documented that within the animal the photoperiodic message is conveyed by the pineal hormone, melatonin (Bartness & Goldman, 1989).The intention of the present paper is to review what is currently known about seasonal physiology, and to discuss this in relation to recent findings about the mode of action of melatonin.