Perhaps Santa Had to Shout to His Eight Reindeer Because They Fell Asleep

Reindeer can’t fly around the world helping distribute gifts to all the good boys and girls in a single night — at least, as far as science can tell. But they are capable of other impressive feats.

On the Arctic island of Svalbard, Norway, the shaggy white creatures eat almost nonstop to store up fat during the summer months of the midnight sun. Then, during the long winter of total darkness and little food, they live mostly off energy reserves.

“They are normal mammals only about three months a year,” said Walter Arnold, a professor at the University of Veterinary Medicine in Vienna who studies animals’ adaptations to cold.

The rest of the time, extreme conditions demand extreme behaviors. For instance, some research suggested that Svalbard reindeer lacked standard circadian rhythms, leading to speculation that their body clocks had been lost over evolutionary time.

But Dr. Arnold and colleagues recently reported in a small study in Scientific Reports that they do have a circadian cycle. Like most other things about them, it is a bit peculiar. The variations are sometimes so slight that observing them required years of monitoring with sensitive sensors that sit in the reindeers’ stomachs.

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Dr. Arnold and his collaborators used pill-shaped metal capsules containing the sensors. Once swallowed, the capsules stay harmlessly in the animal’s stomach as long as it lives, transmitting information about temperature, movement and heart rate. When the researchers looked at roughly two years of data from four reindeer, they saw interesting patterns.

“From our physiological data, it was clear from the first glance, the first analysis, that in body temperature there was clearly a diurnal rhythm,” said Dr. Arnold. Further analysis showed rhythms in other measurements, too.

In the months leading up to summer, there is sun but not yet much greenery. Then, the cycles of the reindeers’ circadian clock have their highest highs and lowest lows, perhaps driving the animals to forage what is available during the day and prepare for what’s ahead.

When summer arrives, the reindeer’s circadian cycles are weak, and perhaps easier to ignore. During that season, reindeer must stay awake at all hours to eat as much as possible.

“They just don’t listen to the clock anymore. They are shift workers during that time,” said Dr. Arnold.

Staying up late may be key to survival: If they slept more during the precious period when they are actually able to graze, they might not be able to make it through winter.

The intense activity of summer, abetted by their peculiar clocks, is eventually replaced by a winter of striking somnolence. They do not hibernate, but they get very close.

“I would call them walking hibernators,” said Dr. Arnold.

The study’s measurements show that in winter, the reindeers’ metabolic rate is suppressed to just a third of its summer high — not a state that would lend itself to taking off from rooftops.

Humans who ignore their clocks, far from thriving, have higher rates of some cancers, heart disease and other health problems. How do reindeer manage?

“We have no idea how this works,” said Dr. Arnold, noting that there are still many mysteries in the lifestyles of animals in the planet’s most extreme habitats.

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