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Photoperiodism and circannual rhythms are intricately linked. While the change in day length may not directly initiate brood rearing in December, brood rearing is timed on an annual cycle, geared to the changes in day length. This cycle is much less pronounced in the tropics, where the day length is constant.
> Circannual rhythms adopt the period of the sidereal year as a result of exposure to environmental cues. Typically, photoperiod acts to entrain the annual cycle of reproduction. Although these rhythms persist in the absence of cues to the 12-month annual cycle, their expression may occur only within a restricted range of constant photoperiods and other environmental conditions. The timing of reproductive activity or inactivity is expected to depend both upon the period of the endogenous oscillator and that of the environmental cue. Thus entrainment is not equivalent to synchronization. The effectiveness or photoperiod-based cues in shifting the phase of the circannual rhythm may vary with the phase of the cycle, although strong resetting curves have been reported. Circannual rhythms occur in large, long-lived mammals such as sciurids and ungulates, although a few examples have been demonstrated in crayfish, cnidarians, and insects. These oscillations have been particularly well studied in birds.
Nelson, R. J., Denlinger, D. L., & Somers, D. E. (Eds.). (2010). Photoperiodism: the biological calendar. Oxford University Press.
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