ABSTRACT
A major task for the comparative physiologist will be to search for common themes in this diversity and to attempt to elucidate patterns of evolution of the mechanisms that are employed in mammalian seasonality. Mammals that inhabit temperate zones frequently develop a winter coat that has greater insulating capacity as compared to the summer pelage. Many mammals exhibit seasonal changes in body mass and lipid stores, and these types of changes are especially pronounced in some species that engage in winter torpor. In mammals that do not exhibit photoperiodism in the traditional sense, it may be that potential photoperiodic responses are blocked at levels downstream from these central neuroendocrine components of the photoperiodic mechanism. Almost all mammals studied show melatonin uptake in the pars tuberalis. Evidence is beginning to accumulate which may eventually provide answers to these questions regarding the mechanisms of evolution of photoperiodic responsiveness in mammals.
