Photoperiodism and Circannual Timing: Introduction and Historical Perspective

Almost all vertebrates display some form of seasonality across their life cycle that requires profound cyclical adaptations in behaviour, physiology and morphology. These are fundamentally controlled by neuroendocrine processes, since it is the brain that senses the environment and co-ordinates seasonality through endocrine and autonomic outputs. This chapter briefly considers how seasonality may impact upon our own species, then provides an introduction to these seasonal processes, considering landmark discoveries and advances in understanding from a historical perspective. It discusses how much of the initial focus was on the direct effects of changing photoperiod on seasonal timing, but how more recent long-term experimental studies have identified innate circannual timers. Seasonal rhythms arise from the interaction of direct photoperiodic effects and circannual rhythms, and timing can also be modified by proximate environmental and social cues; the relative contribution of these processes reflecting the lifespan and environmental ecology of each species.