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Reindeer stop the clock to cope with polar days and nights

12 Mar 2010

Reindeer have 'switched off' their body clocks in order to survive the extreme Arctic seasons of polar day, when the sun stays up all day, and polar night, when the sun does not rise above the horizon at all, scientists have discovered.

Svalbard reindeer, Spitsbergen Island, at midnight in mid-summer
Svalbard reindeer, Spitsbergen Island, at midnight in mid-summer

The body clock, or circadian clock, is the internal mechanism that drives hormone rhythms - and thus a host other functions - in a rhythmic 24-hour fashion. Light-dark cycles drive hormone rhythms via a circuit that involves the eye and nervous system affecting hormone production, in particular melatonin. In most mammals, this wiring circuit also involves the circadian clock, which is able to drive the hormone rhythms even when there is no light dark cycle.

In Arctic reindeer, however, the internal clock element is missing.

Co-author Professor Andrew Loudon, of The University of Manchester, said: "In order to survive in extreme Arctic conditions, reindeer may have abandoned use of the daily clock that drives biological rhythms in temperate zone organisms.

"In fact such daily clocks may be positively a hindrance in environments where there is no reliable light dark cycle for much of the year. Organisms use their circadian clock to correspond with their living environment; but if their environment has a very different cycle, it may be better to follow that rather than use the internal clock."

He added: "This could be the case for a range of animals living at the poles of the earth or in the depths of the ocean."

Professor Loudon and his colleagues at Manchester's Faculty of Life Sciences and the University of Tromso, in Norway, studied reindeer living in Northern Norway at 70 degN, 500 km above the Arctic circle, where there is 15 weeks of continuous daylight in summer and 8 weeks without sunshine in winter.

They investigated levels of the hormone melatonin in the blood of the reindeer and showed that there was no natural internal rhythm of secretion - rather the hormone simply responded to the prevailing light dark cycle. Melatonin serves as a biological signal for daylength-dependent functions such as sleep, reproduction, behaviour, coat growth and camouflage colouring.

The team, whose findings are published in Current Biology today (11th March 2010), also studied the clock genes by collecting skin cells and showing that the molecular clock in these cells did not oscillate.

They believe that the reindeer have the full range of normal clock genes, but these are regulated in a different way.

Professor Loudon said: "We were very surprised by the clock gene data and that we could not see oscillations in reindeer cells.

"We believe that evolution has come up with a means of switching off the cellular clockwork or perhaps that there is still a circadian rhythm running in some tissues but these are disconnected from the usual outputs - i.e. they have lost the hands but not the gears of the clock. Whether it is a stopped or disconnected clock, the result is a lack of internal daily timekeeping in these animals.

"This study indicates the paramount importance of studying natural diversity in nature before jumping to conclusions about how the clock operates. Fascinatingly, in man there is now strong evidence showing that disruption of the circadian clock can lead to disease. Clearly, in animals such as reindeer these problems have been circumvented."

Notes for editors

The paper 'A circadian clock is not required in an Arctic mammal' (Current Biology) is available.

For a copy of the paper, an image or an interview with Professor Andrew Loudon, contact Media Relations Officer Mikaela Sitford on 0161 275 2111, 07768 980942 or