Circadian organization in reindeer

These Arctic animals abandon their daily rhythms when it is dark all day or light all night. Night and day Animals and plants have developed an internal biological clock that uses the light/dark cycle of day and night to synchronize daily rhythms in behaviour and physiological function. Is this circadian rhythm simply an adaptation to make good use of light and dark, or is a daily cycle of activity important to the proper function of an organism? The behaviour of reindeer living at high latitudes above the Arctic Circle provides an answer. Here the Sun neither sets in summer nor rises in winter. In these conditions reindeer lose daily rhythmic activity completely. For herbivores in polar regions at least, there is little selective advantage in maintaining circadian organization. The light/dark cycle of day and night synchronizes an internal ‘biological clock’ that governs daily rhythms in behaviour, but this form of regulation is denied to polar animals for most of the year. Here we demonstrate that the continuous lighting conditions of summer and of winter at high latitudes cause a loss in daily rhythmic activity in reindeer living far above the Arctic Circle. This seasonal absence of circadian rhythmicity may be a ubiquitous trait among resident polar vertebrates.