A wild boar and its piglet rest in a pile of dirt.

A wild boar and its piglet rest in a pile of dirt.

It’s an awful feeling: Damp with sweat, your skin sticks to the sheets as you lie awake, seemingly for hours, in a bedroom that’s just too hot.

Excessive heat is indeed bad for sleep. It disrupts our body’s natural cool-down process that helps us doze off and stay asleep. But luckily for many of us, we can crank up the AC or turn on a fan.

Wild animals don’t have those luxuries.

A pair of new studies on mammals in Europe shows that extreme heat impairs their sleep, too, often significantly so. Wild boars in the Czech Republic, for example, slept 17 percent less during hot, summer days, compared to colder months, one of the papers found, “potentially leading to sleep deprivation.” The other showed that deer fawns in Ireland similarly had shorter and worse quality sleep on scorching days.

Among the only studies of sleep in wild animals, the research points to yet another way that climate change will likely reshape the natural world. As summers heat up, animals might find it harder to sleep in the habitats they call home, potentially weakening their immune systems and chances of survival. It may also push these creatures to new places, where they might spread disease and disrupt carefully balanced ecosystems.

“These studies point to a novel and potentially ecosphere-spanning way that climate change can impact animals,” Sean O’Donnell, a biology professor at Drexel University, who was not involved in either study, told Vox by email.

What scientists learn when they observe snoozing animals

Euan Mortlock spends a lot of time watching animals sleep. He’s not some sort of animal creep but a postdoctoral researcher at the University of Bristol, where he studies napping in animals, from large mammals to fruit flies.

“People think of sleep as something animals do in between periods of other interesting things, but I think it’s one of the most interesting behaviors to observe,” said Mortlock, lead author of the two new studies, both of which were published this spring.

One thing that makes sleep so fascinating, Mortlock said, is that nearly all animals do it (other than perhaps marine sponges). Seals nap while diving down as deep as 300 meters, one study found. Jellyfish sleep, too, even though they have no brains; research shows they pulse less often when they’re dozing off.

Fruit flies in the lab also nap, Mortlock said. Rather adorably, they tilt their heads slightly down, he said, and drop their little antennae when they’re nodding off. Mortlock is interested in how these tiny insects perceive threats while they’re asleep. His current research aims to figure out how their brains decide whether to wake up or continue sleeping in response to, say, the scent of a predator.

Sleep is incredibly important to human and animal health; it strengthens our immune systems and brains, and provides a range of other benefits. To that end, changes in the environment that impair sleep can have serious consequences for survival, and for ecosystems.

Sleep “is essential for physical recovery and memory consolidation,” Daniel Blumstein, a behavioral ecologist at the University of California Los Angeles, who was not involved in Mortlock’s research, told Vox. “Thus, we should document those things that interfere with it.”

Hogs don’t like heat

Humans have phones and Fitbits and Apple Watches that are constantly tracking sleep. How, though, do you measure this behavior in wild animals?

One way is to strap these sorts of technologies onto them.

For the study on wild boars, Mortlock’s colleagues captured a bunch of pigs in Europe and put collars around their necks fitted with devices called accelerometers. Accelerometers pick up subtle movements. Critically, some of those movements correspond precisely to an animal’s specific posture when it’s asleep. Every mammal species has a specific sleep posture, he said. Boars, for example, will either lie on their stomach with their chin resting on the dirt or on their sides with their heads touching the ground. Accelerometers can pick up these sleep signatures.

a boar sleeping in dead grass.

Beginning in 2019, the researchers monitored the boars for a few years, measuring the duration and quality of their sleep. They then compared those measurements to weather data including temperature and humidity.

Ultimately, they found that sleep is “shorter, more fragmented, and of lower quality at higher temperature,” as they wrote in the study. Snow and rainfall, meanwhile, produced higher-quality sleep, presumably because it cooled the animals down (and didn’t bother them much because boars typically sleep under bushes or trees).

A slightly earlier study — of baby fallow deer in a park near Dublin — found similar results. Also led by Mortlock, that paper, published in April and based on more than 300 days of data, found that total sleep time and quality among fawns declined on hotter days. (The team similarly used accelerometers to study these mammals.)

While Mortlock’s work is among the most comprehensive analyses of sleep in wild animals, a handful of previous studies show how heat impairs sleep. One particularly bleak 2015 article, for example, found that fruit bats in South Africa sleep less on hot days because they spend so much time licking their fur, spreading their wings, and panting to cool off.

Will climate change turn animals into insomniacs?

One clear concern is that extraordinarily hot days are becoming more common. A recent report by the nonprofit Climate Central found that climate change added an average of 26 days of extreme heat globally in the last year.

That could, to an extent, fuel insomnia among some creatures, like these hogs.

“Given the major role sleep plays in overall health, our results signal that global warming, and the associated increase in extreme climatic events are likely to negatively impact sleep, and consequently health, in wildlife,” Isabella Capellini, a co-author on the wild boar paper and researcher at Queen’s University Belfast, said in a press release.

A lack of sleep, in turn, could mean boars are more likely to get sick or spend less time caring for their young, the authors write.

“We know that climate change creates a variety of different stressors on animals, and this study reveals a new axis of stress that animals may experience as a result,” Briana Abrahms, an expert in animal behavior and ecology at the University of Washington, who was not involved in Mortlock’s research, told Vox by email. “Animals (and people) need sleep to recover from other stressors, so this study suggests that the impacts of warmer temperatures on sleep may compound other negative effects of climate change on wildlife.”

It’s also important to keep in mind that many species are highly adaptable, especially wild boars. They won’t just stop sleeping as the planet warms. More likely, they’ll change their behavior — they’ll spend more time bathing to cool off, for example, or migrate to colder regions. That could bring these animals closer to human communities, Mortlock said, where they’re known to root through trash, damage crops, and tear up golf courses.

When wild animals migrate, they can also trigger a cascade of changes in the ecosystem, by adding or subtracting key parts of a region’s food web.

Extreme heat undoubtedly presents all kinds of challenges for wild animals, many of which are already under siege from other threats like deforestation and poaching. It’s wreaking havoc, for example, on coral reefs. Yet the specific problems linked to sleeping under hot conditions are still poorly studied and largely unknown.

“There is an enormous gap in our understanding of sleep in the wild,” Mortlock said. “But with new methods, we can start to peek behind the curtain.”

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