Photoperiod and Testicular Function in Phodopus sungorus

Many mammalian species living at medium or higher latitudes show marked annual cycles in various morphological and functional properties. There is a clear cycle of the reproductive activity ranging from a fertile to an infertile state in both the male and female. Such an annual periodicity can be regarded as an adaptation to seasonal changes of environmental conditions such as cli- mate and nutrition, ensuring that birth and development of the litter are re- stricted to a favorable season. These annual cycles consist of cyclic changes of exocrine and endocrine gonadal function, in the hormone-dependent organs (accessory glands, etc.) and in the hormonal hypothalamic-pituitary-gonadal system (for literature, see Hoffmann 1981). Such a seasonal cycle of reproductive activity was found in species from all vertebrate groups (i.e., birds, see Hoffmann 1981; Breucker 1982; reptiles, amphibians, and teleosts, see Hoffmann 1981). In those primate species of the Macaca family which are seasonal breeders (Zamboni et al. 1974), it was demonstrated by Richter et al. (1978) and Wickings and Nieschlag (1980) that these cycles are also evident under constant laboratory conditions, suggest- ing that these cycles are based upon endogenous rhythms which are modulated and synchronized in the natural habitat by exogenous factors.