The first critical feature of circannual timekeeping is its predictive power to
anticipate and prepare for upcoming seasonal changes in the environment. 
In a highly seasonal habitat, where changes in food supply and
other selective pressures can be predicted through photoperiod, the timing
mechanism allows for very precise regulation of the timing of cycles in physiology
and behaviour. The anticipatory mechanism allows timing to be planned
much in advance of the ecological events that ultimately determine survival and
reproductive success.  

Second, circannual timekeeping allows organisms to express robust annual cycles
which at specific phases override the effects of proximate cues, including photoperiod. 
Species differ in their responsiveness to immediate environmental conditions
relative to reliance on internal timekeeping mechanisms. Reliance on endogenous
features safeguards against being misled by untimely cues, such as unseasonal
weather, but this advantage must be carefully balanced against benefits of direct
responses to the environment.

Finally, there is an increasing awareness that circannual biology is of economic
and medical relevance to humans. Increasing our understanding of annual cycles in
farm animals, family pets and wild species is required to help answer many animal
welfare and conservation issues. This is of major current interest because of
concern about the unknown impact of rapid climate change, for example, the
worrying effects of increasing temperature on the ecology of insects 

Barbara Helm and Gerald A. Lincoln, 2017
Circannual Rhythms Anticipate the Earth’s Annual Periodicity

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