Photoperiod and the regulation of annual and circannual cycles of food intake.

For wild animals, food availability usually exhibits considerable seasonal fluctuations and as a result dramatic seasonal variations in body weight, fatness and food intake are often observed. Birds and mammals living in the temperate zones or higher latitudes, therefore, time the onset of reproductive activity each year with great precision so that young are produced at the most propitious time of year. Numerous studies have now demonstrated that in such animals seasonal physiological changes governing the reactivation of the reproductive axis are tuned by external environmental change, principally photoperiod (for review, see Bronson, 1988; Bronson & Heideman, 1994). The annual daylength cycle, thus, represents the most reliable noise-free proximal factor available to entrain seasonal reproductive cycles. Food intake and body condition are seasonally and photoperiodically regulated in many mammals (Mrosovsky & Sherry, 1980). The phenomenon has been most extensively investigated in ruminants, scuirid rodents and small mammals, including several species of voles and hamsters (Bartness & Wade, 1985; Kay, 1985). The present brief review seeks to highlight aspects of the photoperiodic and seasonal regulation of food intake and metabolic cycles by examining their endogenous nature and considering the manner of their photoperiodic regulation. Two related topics will be considered. First, the role of photoperiod change and the endocrine transduction of the photoperiodic signal will be examined in relation to seasonal changes in metabolism and food intake. Second, the endogenous nature of these processes in many organisms will be emphasized. Since the field has now developed a voluminous literature, reviews and major articles are cited for further reference where appropriate.