Biological Rhythms in Arctic Animals
With increasing latitude the daily light-dark cycle becomes progressively distorted during substantial parts of the year. At high-Arctic latitudes (77–81°N) there is continuous darkness (polar night) between November and February and continuous light (polar day) from April to September. Circadian mechanisms generally rely heavily on the synchronizing or entraining effect of the daily light/dark cycle, and it is therefore important to study animals living under conditions where this zeitgeber is absent. Migratory birds visiting the Arctic in summer to breed apparently perceive sufficient environmental rhythmic information to remain entrained. Humans and resident animals such as ptarmigan and reindeer do not, but whereas humans show persistent circadian freerunning sleep/ wake rhythms, reindeer and ptarmigan become continuously active. This is also revealed by their secretion of melatonin, which is markedly reduced at those times of the year when the light/dark cycle is absent. Presumably, their endogenous biological clocks or circadian machinery is flexible and becomes dampened to such an extent as to allow these animals to exploit their environment maximally at those times of the year when there is no marked differences between day and night.
KeywordsActivity Rhythm Biological Rhythm Melatonin Secretion Polar Night Plasma Melatonin
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