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...