Seasonal physiology: making the future a thing of the past

Abstract

Accurate timing and physiological adaptation to anticipate seasonal changes are an essential requirement for an organism's survival. Animals exhibit a remarkably wide spectrum of seasonal adaptations, which can include annual cycles of growth, immune function, sexual behaviour, fattening/weight loss, hair growth/loss, thermogenesis, sleep, and hibernation. Annual changes in day length (photoperiod) are a highly predictive signal that can be reliably used to activate a seasonal adaptive programme at the correct time of year. These cycles of physiology also persist in constant conditions revealing an innate long-term timer synchronised by photoperiod, which allows reliable anticipation of forthcoming environmental changes (‘circannual clock’). The adaptive value of an innate timer long-term timer is best understood when considering that hibernators and species living at high latitudes, which still have to progress through their seasonal life history despite the absence of a light dark cycle for periods of the year. Calendar time is of vital importance for organisms living in seasonal environments, and, unlike circadian time, the fundamental mechanisms through which organisms maintain an internal sense of calendar time remain unknown. The aim of this review is to present the recent advances in the field, consider the species comparative aspects, discuss non-photoperiodic effects on seasonal rhythms and indicate future directions for the field.