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15 weird sleep habits in the animal world
By Angela Nelson,
Mother Nature Network, 26 July 2016.

Sleeping. Napping. Catching some zzz's. Snoozing. Getting some shuteye. No matter what you call it, (almost) all animals need it. But some creatures do it in interesting and unusual ways. From the ones who sleep nearly 20 hours a day to the ones who slumber with only half their brain at a time, here are some of the more unusual ways members of the animal kingdom doze off.

1. Sperm whales

Photo: YouTube

In 2008, a group of researchers were studying calls and behavior in sperm whales off the coast of Chile when they happened upon an amazing discovery: a pod of sperm whales sleeping so soundly in the water that none of them saw or heard the boat coming, even though whales are unihemispheric sleepers (half of the brain stays awake while the other half sleeps). The whales bobbed vertically in the water - some with their noses above water, some completely underwater - until the small boat accidentally bumped one of them, and then they all swam away, according to an account published in the journal Nature.

Based on this, researchers believe sperm whales do sleep fully while drifting for 10 to 15 minutes at a time, during which they don't breathe. Watch the above Discovery Channel video of sleeping sperm whales.

2. Ducks

Photo: Phillip Roberts/Flickr

Researchers at Indiana State University were filming mallard ducks for a different experiment when they noticed interesting things about how ducks slept. First, ducks almost always sleep in rows or cliques. Second, the ducks at the ends of the row kept the eye facing away from the group open, while ducks in the center of the group closed both eyes. (It seems these feathered friends took the advice Metallica offered in "Enter Sandman;" they really do sleep with one eye open!)

3. Dolphins

Photo: Alexander Vasenin/Wikimedia Commons

Dolphins are similar to whales in that only half the brain sleeps at a time. In this case, it's not just necessary to watch out for predators, but also to know when to rise to the surface for air so they don't suffocate in their sleep.

According to Scientific American, dolphins sleep one of two ways: either resting in the water in a deep form of sleep known as logging (because when they do it, they look like logs floating in the water) or while swimming slowly next to another animal. When they sleep and swim at once, it's a light sleep akin to the way humans nap.

Many new moms wish their new babies would sleep more at night, but imagine being a dolphin mama: Baby dolphins don't sleep at all in their first few months. Mother dolphins must sleep on the move. "In fact, she cannot stop swimming for the first several weeks of a newborn's life. If she does for any length of time, the calf will begin to sink; it is not born with enough body fat or blubber to float easily," Scientific American reports.

4. Walruses

Photo: Smudge 9000/Flickr

You may envy the walrus for its unusual sleep habit: This blubbery guy can sleep anytime, anywhere, whether it's floating in the water, lying on the ground or just leaning on another walrus. Researchers say they've even seen walruses "resting in water while using their tusks to hang from ice floes," NBC News reports. What a visual!

Jerome Siegel, director of the Center for Sleep Research at UCLA, told Discovery News these mammals can hold their breath and sleep underwater for four to five minutes, as they are also unihemispheric sleepers. But on land they settle into a deep sleep that can last for up to 19 hours.

It sounds like they really need that sleep: Walruses can have periods of activity where they stay awake and swim for up to 84 straight hours. "The discovery that walruses remain active for periods lasting up to 84 hours without showing behavioral signs of sleep is unprecedented," sleep specialist Niels Rattenborg told Discovery News.

5. Bats

Photo: rstephemi/Flickr

You probably know bats sleep upside down. They do this because their wings are not strong enough to take off from the ground, so they use gravity by dropping into flight from their perches.

While that's a pretty weird habit on its own, it's only part of their sleep story. Did you know that bats are some of the sleepiest creatures on the planet? The little brown bat, for example, sleeps an average of 19.9 hours each day. (Giant armadillos and opossums are a close second, sleeping about 18 hours a day.)

6. Zebras and horses

Photo: Christina Spicuzza/Flickr

Zebras and horses often sleep standing up (as do elephants and cows) so they can stay on alert for predators. To do this, they use what's called a "stay apparatus," which allows them lock their knees straight in such a way that very little muscle is required to maintain the stance. When they sleep in this position, it's more of a nap than a deep sleep; they do need to lie down once in a while to achieve REM sleep.

7. Sea otters

Photo: Joe Robertson/Wikimedia Commons

The sea otter's special sleep habit might be the cutest one on the list. When these furry little creatures doze off, they float on their backs on the surface of the water. So they don't drift away while they slumber, they've been known to hold hands in pairs or small groups. They also may wrap themselves in a strand of kelp or seaweed growing from the sea floor as an anchor of sorts.

8. Migratory birds

Photo: Rudraksha Chodankar/Wikimedia Commons

Birds like the alpine swift (pictured above) and the albatross spend much of their lives migrating or on the hunt. Because of this, these birds are multitaskers that can sleep and eat while they fly. Researchers have found that alpine swifts can stay in the air for 200 straight days without landing, which begs the question: When do they sleep? Scientists believe that birds, like whales, ducks and walruses, are unihemispheric sleepers that can sleep while they glide and soar (basically, while they're not flapping).

9. Meerkats

Photo: Anguskirk/Flickr

Meerkats are up there with sea otters in terms of cute, cuddly sleeping habits. These cat-like creatures live in underground burrows with as many as 50 meerkats (called a mob or a gang). Their burrows can be six to eight feet deep and contain numerous sleeping chambers, including ones used only when breeding. When they lie down to rest, they do so in snuggly heaps, piled on top of each other for warmth. Though in the summer, they may spread out more and sleep above ground.

10. Sharks

Photo: Jayvee Fernandez/Flickr

Much about sharks and how they get their shuteye is a secret. But here's what we do know: For sharks to breathe, they must pass water over their gills, so most sharks sleep while moving. However, some smaller species of shark, like the nurse shark, can use their spiracles (small holes behind each eye that act like gills) to force water over their gills while they lie still on the ocean floor.

Recently, for the first time, researchers filmed a great white shark sleeping. The footage, which was captured by a robotic submersible near Mexico’s Baja California Peninsula, shows the female shark swimming closer to the shore in shallow waters as night fell. She faces straight into strong currents with her mouth open, likely so water could continue to pass over the gills. The shark's swimming slows, and it's at this point researchers believe she's asleep.

11. Snails

Photo: Rosino/Wikimedia Commons

Have you heard the one about the snail that slept for four years? In the late 19th century, a British museum worker found the shell of an Egyptian land snail, assumed it was empty and attached it to an identification card. Four years later, he noticed traces of slime on the card. He put it in water, and when the shell came off the card, the snail crawled out, according to Natural History magazine.

The common garden snail hibernates from late fall to spring under rocks or dead leaves, or in cracks in rocks or the ground, the magazine reports. Their shells fit snugly to keep out wind and keep them warm. Other snails estivate, which means they spend hot or dry periods in a prolonged dormant state, as was the case with the aforementioned sleepy snail.

12. Frogs

Photo: Vmenkov/Wikimedia Commons

Like snails, frogs use hibernation and estivation as sleep strategies. Frogs that estivate are found mainly in Africa and South America. During dry spells, they burrow into soil and shed several layers of skin to form a cocoon (they leave their nose exposed to breathe). According to Scientific American, when the rain comes again, they shed the cocoon and climb up to the surface.

Some aquatic frogs hibernate underwater on the muddy bottom or partially buried in it, since they need oxygen and would suffocate if they fully buried themselves like turtles. Terrestrial frogs, like the wood frog and American toads, hibernate by burrowing into the soil below the frost line or hiding in cracks in logs or rocks.

Here's the coolest part: Hibernating frogs have a built-in antifreeze system. As ice crystals form in their body (in their bladder or under their skin), a high concentration of glucose in their body keeps their major organs from freezing. The heart may stop beating and the frog may stop breathing, but come spring it will thaw and "come back to life."

13. Bears

Photo: Bob Jagendorf/Flickr

You've heard of walking in your sleep, maybe even eating in your sleep. But how about giving birth in your sleep? Pregnant bears who settle in to hibernate will rouse themselves very briefly to deliver a few cubs, then return to sleep as her cubs nurse and snuggle next to her, PBS reports. It's pretty impressive that those hibernating mama bears, who aren't eating, drinking, urinating or defecating, can still support the life growing inside them.

14. Giraffes

Photo: Marc/Flickr

Giraffes have the honor of being the animal requiring the least amount of sleep. These lumbering giants can go weeks without sleep - though that skill is borne out of necessity. Being large and slow, they're constantly on guard against predators. When they do snooze, it's often standing up - getting those lanky legs off the ground takes time they may not have in the face of an attack. And remarkably, they only sleep for five minutes at a time for a total of around 30 minutes a day.

15. Apes

Photo: Pictureman2014/Pixabay.

The orangutan, gorilla and chimpanzee all like to curl up and sleep just like humans do. Orangutans sleep soundly - they settle down for a long, deep sleep on their fronts or their backs, the BBC says. Meanwhile, chimpanzees are picky sleepers - they basically reconstruct their bed of stems and foliage every single night, according to Smithsonian Magazine. In fact, every population of great ape studied builds platforms to sleep on, the BBC reports.

Top image: Sleeping cat. Credit: awohlfeld/Pixabay.

[Source: Mother Nature Network. Edited. Some images added.]