Reindeer Sleep and Eat Simultaneously, Saving Precious Time in the Short Arctic Summer

While the animals chew their cud, they also enter a state of rest

Eurasian Reindeer
While they chew their cud, reindeer brains shift into a state of non-REM sleep. Leo Rescia

Santa’s reindeer spend the offseason resting up, getting ready for the grueling feat of circumnavigating the globe in a single night. But the demanding Arctic winter means that their non-flying relatives must also prepare diligently for the season. Reindeer graze incessantly during the short northern summer, storing up fat reserves for the cold, dark months when both sunlight and food will be in extremely short supply. Reindeer spend so much of the summer eating that it would appear they have little time for anything else—including sleeping.

Neuroscientist Melanie Furrer of the University of Zurich and colleagues suspected that reindeer might simply sleep less during the short season, and then catch up on their Zs by slumbering away during the winter. “But that’s not what we found,” she says. Instead, reindeer sleep the same amount throughout the whole year, even when they have to focus on eating in warm months. “We found that they have a strategy to deal with it, being able to ruminate and sleep at the same time, to save time in the summer.”

By monitoring the reindeer’s brain activity, Furrer and colleagues found that the animals were able to sleep while stoically chewing their cud. They enjoyed the benefits of sleep at the same time they nourished their bodies, according to a study published Friday in Current Biology. “We need to be open-minded that there is not just one kind of sleep, the way that humans sleep, but there are other ways to sleep,” says Vladyslav Vyazovskiy, a sleep physiologist at the University of Oxford not affiliated with the research. “The traditional way to look at it is that when you’re sleeping, you’re not eating. But in this case there’s no time to sleep. They must find a clever way to combine the two, and this is what they do.”

Reindeer and other ruminants feed on plants made of cellulose that they can’t digest without some work. The deer quickly swallow these foods, then pass them back and forth between their stomachs, where beneficial microorganisms help to break them down, and their mouths, where the cud is laboriously munched. This slow process enables the reindeer to eventually absorb all the beneficial nutrients that their feed contains. And the new study has revealed that chewing the cud has benefits beyond nourishment—the repetitive process also helps the animal reap the mental benefits of sleep.

To see if reindeer really sleep while ruminating, Furrer and colleagues performed non-invasive electroencephalography (EEG) brainwave imaging on four adult, female Eurasian tundra reindeer in a captive herd in Norway. They monitored the animals for four straight days at the summer and winter solstices, as well at the autumn equinox. The researchers found that the reindeer sleep the same amount during 24-hour daylight of the summer, when food is naturally plentiful, as they do during the totally dark winter days when food is scarce, and even during the autumn equinox. “This shows me that somehow their sleep drive, how much they need to sleep, seems to be regulated from something that comes from the inside, not from the outside,” Furrer says.

The study also found that the more time reindeer spent ruminating, the more rested they were. EEG brainwave recordings revealed the reason why: When the reindeer are ruminating, their brain waves resemble those during non-rapid eye movement (REM) sleep. Because they seem to enjoy the benefits of such sleep while serenely chewing their cud, the animals are rested after ruminating.

And when ruminating, the deer displayed behaviors that are more akin to those expected during sleep. For example, they stood or sat quietly and did not react to disturbances, such as neighboring deer moving around, as they would when awake. A neighbor standing up or sitting down would draw a reindeer’s attention 45 percent of the time when awake, the study found, but only 25 percent of the time when ruminating and just 5 percent when the animals were completely asleep.

The reindeer’s behavior made it seem as if they could be asleep, and the EEG recordings confirmed that their brain signals showed the typical characteristics of non-REM sleep. But, the team wondered, were the reindeer really getting the benefits of sleep while ruminating? Were they actually rested?

After researchers kept reindeer awake for two hours, their EEG showed a buildup of slow-wave activity, meaning that the animals were tired and driven to get more and deeper sleep to offset that deprivation. The team then tested to see if ruminating also made the animals crave sleep, as keeping them awake would. Instead, the brain activity showed that it left them rested, as they would be after sleeping. “I think that’s a step further to show, at least indirectly, that sleep during rumination also has a function, and actually that if they sleep there they can save some time and recover brain functions during rumination,” Furrer explains.

Vyazovskiy adds that the brain is surprisingly responsive during sleep and that a rhythmic behavior, like ruminating, may actually be entraining brain activity. “I think that they may even be using the rumination to get the brain into this meditation-like mode to obtain the benefits,” he notes. “I think this is really, really cool.”

During the warm summer months when they need to eat, reindeer have adapted a way to sleep just as much as during the dark winter. Current Biology / Furrer et al.

Reindeer’s amazing ability to eat and sleep simultaneously appears to be an adaptation to their life in the short Arctic summer, enabling them to fatten up without stopping so frequently to slumber. The resting technique is one of many different ways animals adapt their sleeping habits to extreme polar environments. Arctic ground squirrels, for example, hibernate for more than half the year, drastically lowering their metabolic rate and body temperature. Near the other pole, nesting chinstrap penguins sleep more than 10,000 times a day—for just four seconds at a time.

Although reindeer sleep consistently all year long, their activity levels are quite different. The animals appear to be more active in the summer than they are during the dark winter. “In the winter it seems like they are just chilling, but not sleeping more,” Furrer says.

Vyazovskiy notes that scientists are continuing to find that the relationship between activity and sleep is not straightforward. The kind of activity matters. While some behaviors are sure to be tiring, others, like ruminating, may enable species to rest their brains even while their bodies are engaged in some task.

Study co-author Gabriela C. Wagner, a chronobiologist at the Norwegian Institute of Bioeconomy Research, works with some of the world’s top reindeer experts: Sami herders, whose animals are under increasing stress from human activity like mining, traffic and wind farms. “One thing that consistently comes up in discussions is that the Sami say the reindeer need what they call ‘pasture peace,’” she says. “That is something that they can’t easily quantify, it’s a word from traditional knowledge, and I think our study shows that this peace, the time to ruminate, is extremely important physiologically—not just for digestion but for the brain to recover.”

This amazing adaptation reindeer have developed for life in the Arctic also helps explain how they might stay ready to play a unique role when the holidays come around.

“You know what it means for Christmas,” Furrer says. “They don’t need more sleep in winter, so they still have enough time to bring the presents.”

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