Endogenously generated biological rhythm with a period length approximating to 1 year.
The term circannual rhythm is a derivative of the term circadian. Circadian rhythms are endogenously generated biological rhythms with a period length of approximately one day (from the Latin circa diem). Correspondingly then, a circannual rhythm may be defined as annual. There are many different examples of circannual rhythms including the pupation rhythm in carpet beetles , the urge to migrate in birds , hibernation cycles in ground squirrels , and cycles of reproductive activity and moulting in ungulates .
In species living outside of the equatorial zones, the seasonal change in the daily pattern of light/dark exposure, (that is, the photoperiod), is the major synchronising signal for circannual rhythms. Responsiveness to photoperiodic changes ensures that appropriate physiological or behavioral responses occur at the different phases of the sidereal year....