In the far northern reaches of the Arctic, day versus night is often not a practical concern. During parts of the year, the sun does not set; at other times, it does not rise. A new study by an international team of researchers shows that Arctic reindeer have come up with a surprising adaptation to living under those extreme conditions: They’ve apparently abandoned use of the 24-hour internal clock that typically drives the daily biological rhythms in other organisms. “Our findings imply that evolution has come up with a means of switching off the cellular clockwork,” said Dr. Andrew Loudon of the University of Manchester in England, the senior author of the study. “Such daily clocks may be positively a hindrance in environments where there is no reliable light-dark cycle for much of the year.” Light-dark cycles drive hormone rhythms via a circuit that involves the eye and nervous system projections to structures involved in regulating hormone rhythms, in particular that of melatonin, Dr. Loudon explained. In most mammals, this wiring circuit also involves an internal clock that drives hormone levels in a rhythmic 24-hour fashion, even when there is no light-dark cycle. “In reindeer, it is this clock element that seems to be missing,” Dr. Loudon said. The reindeer show no natural internal rhythm of melatonin secretion at all. Instead, hormone levels rise and fall in direct response to light and dark. The researchers show that melatonin levels remain at or below detectable levels during daylight hours. Those hormone concentrations spike almost as soon as the light goes out, only to plunge again when light returns.
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