Abstract
Circannual rhythms have been studied for 25 years. Most information has come from work on hibernators. Like circadian rhythms, circannuals rhythms are endogenous, innate, self sustained, and their spontaneous frequencies (biological period) are always different from astronomical time. They are insensitive to temperature changes. Their mechanisms are not yet known. Circannual rhythms are synchronized by external factors. There is, however, a great variability among these rhythms, some species being better synchronized than others. Differences in species ecology could be involved in this variability. The adaptative value of these rhythms is discussed.
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