The 12 Most Bizarre Sleeping Habits in the Animal Kingdom

In the winter at the North Pole, temperatures plummet, making food scarce. As the environment becomes inhospitable and food sources scarce, polar bears enter a state of hibernation. These bears have a thick layer of fat, up to 10 cm, to insulate their bodies, even in temperatures as low as -40°C. Their insulation is so effective that when observed through infrared cameras, we only see their footprints. Therefore, when winter conditions are harsh or when pregnant females need to conserve energy, they retreat into dens, curl up, and enter a state of torpor to avoid the cold and conserve energy.

Their sleep is not very deep. Their heart rate drops from 70 beats per minute to 8 beats per minute, and their body temperature remains unchanged. They can instantly awaken if necessary. While in the den, they do not eat and rely on their body fat reserves; during this time, they do not defecate or urinate.

Despite hibernating for an extended period, polar bears manage to maintain their strength and muscle mass. They will awaken from their three-month hibernation period still retaining over 3/4 of their muscle strength without consuming any food or water. If humans were immobile for a similar period, they would lose approximately 90% of their muscle strength.

Many other animal species also enter hibernation during winter because they cannot flee, forage, or lack thick fur to stay warm. They resort to... sleeping (to conserve energy, evade predators, and avoid the cold), such as marmots or brown bears in the Pyrenees, which hibernate for up to 6 months.

Bats often hang upside down when sleeping. Perhaps bats are the only animals that choose to sleep this way. Being the only mammal capable of flight, bats have short and small hind legs connected to their wing membranes, making it difficult for them to move around. Therefore, when they fall to the ground, they cannot stand up or walk. Hence, bats prefer to climb to high places, hang upside down, and when danger arises, they simply let go and spread their wings to fly away.

Moreover, during winter, while hanging upside down, bats reduce direct contact with the cold cave ceiling, burying their heads and bodies into their wing membranes, which are covered with thick fur, effectively insulating them from the cold outside air.

When sleeping, bats tuck their heads downwards and grip onto something firmly to ensure sufficient blood flow to the brain, aiding in slowing down blood circulation. This sleeping position also enables bats to swiftly escape when necessary. Unlike birds or insects, bats' wing membranes lack the strength to easily lift them from the ground into the air. Therefore, during rest, bats always choose a high perch so that when they need to take flight, they can simply drop and utilize the air resistance to assist in takeoff.

To adapt to their unique resting state and flying mechanism, bats evolved to have a compact size, allowing for easy blood circulation even when upside down. Additionally, to support the large amount of oxygen required during flight, bats have a specially developed heart, three times larger than similarly sized mammals, with increased blood transport capability. Its right atrium is very large; when the right atrium expands, it can pump a large amount of venous blood.

While humans or other animals need to grasp and use muscle energy to hold onto something, bats don't. Their unique claw structure means they don't expend energy to cling to objects. Furthermore, when winter comes, bats also adopt the upside-down position for hibernation, thereby reducing contact with the cold cave ceiling. Some bats may even bury their heads and bodies into their wing membranes, along with their thick, soft fur acting as insulation from the cold outside air.

Bats can hang upside down for extended periods, including during hibernation and even after death. Some bat species can hibernate for six months and survive through a small reserve of body fat. Typically, bats become active around dusk and leave their roosts to hunt for food.

Hibernation isn't just about cold temperatures, sometimes it's about coping with seasonal food scarcity. Like the Madagascar flying fox (even though they live in tropical regions where winter temperatures can reach 30°C), they still hibernate because they can't find their preferred food. While hibernating, their bodies rely on the fat accumulated in their tails (which accounts for up to 40% of the body's fat reserves). During the 7-month hibernation period, their body temperature fluctuates from a few degrees to tens of degrees Celsius, depending on the external environment. By regulating their body temperature according to the ambient temperature, flying foxes can reduce metabolic rate and conserve energy, a strategy similar to that of lizards and other reptiles.

In addition, if animals were nominated for a Grammy award, the Madagascar flying foxes would win with their adept singing ability, perfectly in rhythm. Singing and rhythm in other animal species have fascinated scientists for decades, partly because they can provide insights into the evolutionary process of humans.

According to a recent study published in the prestigious scientific journal Current Biology, the Indri indri, a species of flying fox in Madagascar, is one of the few animal species that produce rhythmic sounds. Chiara De Gregorio, a researcher at the University of Turin, Italy, and co-author of the study, said the Indri indri's sounds fluctuate between low and high through the air. 'The rhythm created by the flying fox includes sound and silence,' Chiara De Gregorio said.

Collecting 'singing' recordings of Madagascar flying foxes is not an easy task. These flying foxes live in trees deep within the tropical rainforests of Madagascar. Researchers have spent years tracking them in the forest to try and hear them sing. However, there are days when the flying foxes don't sing at all.

Have you ever wondered how those giraffes with their 'long and long' legs and long necks sleep? When in a 'light sleep', giraffes only sleep part of their brain, and their neck remains raised. Only when in deep sleep do giraffes lay their heads on their tails, though not for more than 20 minutes. As giraffes are often attacked by lions suddenly, they use the 'sleep and stay awake' tactic, combined with 'short deep sleep' to guard against enemies. By doing so, they achieve both safety and proper rest.

Through studying animal sleep, researchers have found that when animals sleep, their brains can emit electrical waves similar to those of human brains, so animals can also dream. Some species dream a lot, for long periods, while others dream less, for shorter periods. For example, squirrels and bats often dream, while bird species dream relatively little, and reptiles almost never dream. This may be related to the fact that they must remain vigilant against enemies at all times, to be able to flee in time.

In addition, giraffes are the mammal species that sleep the least, averaging only half an hour per day. Each nap lasts only a few minutes. During that time, they fall into a state of half-sleep, half-awake to guard against natural dangers. In captivity, giraffes can sleep for 4-5 hours a day. Giraffes are mammals belonging to the deer and cattle genus, but are classified into a different family from other species, that is the giraffe family, which includes giraffes and their closest relatives, the okapis. The range of giraffe habitats stretches from Chad to South Africa.

Giraffes can live in grasslands, savannas, or mountain forests. However, when food becomes scarce, they will aggressively enter denser forest areas. They often prefer to reside in areas with many acacia trees. They usually drink a large amount of water at once and can retain water in their bodies for a long time, so they can live in dry areas for extended periods.

Giraffes know how to run fast and in emergencies can reach speeds of up to 55 km/h, meaning that over short distances they can keep up with racehorses.

The Swainson's Thrush can sleep standing on branches without falling to the ground. The secret lies in the gripping system on the bird's feet, which holds the feet firmly onto the tree branch. Additionally, because the bird's brain is more developed than that of reptiles, although the cerebral hemisphere lacks convolutions, the size has increased significantly. The cerebellum is the smallest, the visual cortex is very large, allowing them to maintain balance very well even while sleeping.

For migratory thrush species, they fly long distances continuously. Their flights often occur at night. Swainson's Thrushes have to fly about 3,000 miles from their breeding grounds to warm, sunny areas in South America, Canada, and Alaska. When spring arrives, they continue their journey back from South America to their homeland. When do they find time to sleep?

Scientists have discovered that they stay awake all night and rest during the day. However, instead of long sleeps, they will nap multiple times, with each nap lasting an average of 9 seconds. This bird species also sleeps in various ways. Sometimes they close one eye, while the other eye and half of the brain remain active, helping them avoid impending dangers. Sometimes they close both eyes but only sleep lightly.

Its original distribution area is Europe, Asia, and North Africa; due to human activities, the thrush has expanded its distribution range to Australia and New Zealand. This species has several subspecies across its extensive distribution range; some of the Asian subspecies are sometimes considered full species. Depending on the latitude, this bird species may be common as a resident species, partial migrant, or complete migrant.

The male of this designated species, found mostly in Europe, has an all-black plumage except for a ring around the eye and a yellow bill with a rich melodic song; adult females and juveniles have predominantly dark brown plumage. This bird species breeds in forests and gardens, building cup-shaped nests lined with neat mud. It is an omnivorous species, feeding on a variety of insects, earthworms, berries, and fruits.

Both males and females occupy separate territories in the breeding area, and display clear threat behaviors to intruders, but they are more sociable during migration and in wintering areas. Pairs of birds stay in their territories year-round in areas with a mild climate. This common and easily spotted bird species has contributed to an increase in literary and cultural references, often related to its melodious singing.

The seahorse primarily inhabits the Pacific coast of North America. This adorable creature not only has a charming appearance but also melts our hearts with its habit of sleeping 'hand in hand.' This romantic behavior stems from the seahorse's tendency to sleep on the water's surface in a supine position, making them susceptible to being swept away by waves. Therefore, to avoid 'waking up in a distant place,' dozens of seahorses will hold hands while sleeping, forming a sturdy 'raft' firmly anchored on the water surface. When sleeping alone, seahorses will use seaweed wrapped around their bodies to avoid being washed away by the waves.

The seahorse has a thick inner layer of fur (1,000 hairs/mm²) protected by an outer layer of fur, keeping them dry underwater and retaining a layer of air to keep warm. All seahorse species have elongated, slender, and flexible bodies; short legs with webbed feet. Most have sharp claws for capturing prey, but the short-snouted seahorse from South Asia has only vestigial claws, and two African seahorse species have a close relationship and no claws: these species live in the muddy rivers of Africa and Asia and locate prey by touch.

Seahorses not only rely on their special fur to survive in cold water where many species live: they also have a very high metabolic rate and consume energy at a wasteful pace: for example, European-Asian seahorses must eat an amount of food equal to 15% of their body weight each day; seahorses from 20 to 25%, depending on the temperature. In 10°C water, a seahorse needs to catch at least 100g of fish every hour, if less it will not survive. Most of them hunt for prey for 3-5 hours a day, if it is a nursing mother, they need to hunt for prey for 8 hours a day.

Most seahorses eat fish as their primary food source, in addition to supplementing with tadpoles, shrimp, and crabs; some specialize in eating clams while others eat small mammals or birds.

The wood frog species inhabits the North American range up to the Arctic Circle and also exhibits hibernation habits. However, their hibernation habits differ greatly from other creatures, and can even be considered 'uniquely unparalleled' in the natural world. Specifically, as they begin to sense the cold of winter, wood frogs enter a special hibernation state by ceasing all living functions, from heartbeats to brain activity, as if they are freezing their entire bodies. During this process, ice crystals may even form from the frog itself.

As the weather gradually warms up, the wood frog's body will thaw, all living functions will be restored, and they can return to normal life. It is noted that wood frogs can survive in a frozen state for up to 8 months and come back to life without any damage. To explain this special ability, scientists point out that wood frogs have a high concentration of cold-preserving substances in their bodies. Additionally, their livers can convert glycogen into glucose (sugar) and pump it directly into the bloodstream. The sugar in the blood stabilizes the cells and prevents dehydration, helping them survive through the freezing cold period.

Alaskan wood frogs can freeze for nearly 7 months at an average temperature ranging from -14.6 to -18 degrees Celsius, then come back to life in the summer. They are adept at cold tolerance, as they can freeze for nearly 7 months at an average temperature of -14.6 degrees Celsius, even -18 degrees Celsius. In spring, they come back to life. During hibernation, over 60% of the wood frog's body freezes, the animal stops breathing, and the heart stops beating. Physical appearance as well as metabolic and excretory activities in the body temporarily cease, much like a state of death.

In fact, wood frogs can undergo 10-15 freeze/thaw cycles throughout a single winter. Temperature changes cause the bodies of wood frogs to expand and freeze every time night falls. At the same time, molecular mechanisms allow the wood frog's body to efficiently undergo freezing. In the tissue of wood frogs, there is a high concentration of cold-preserving substances. These are solutes, including glucose and urea. Their livers have the ability to convert glycogen into glucose (sugar) and pump it directly into the bloodstream. Sugar in the blood stabilizes cells and prevents dehydration, helping it survive the cold period.

Meerkats always stack on top of each other to sleep, with their ears always perked up to listen for strange noises. Researchers further discovered that the sleep of each animal is influenced by its survival abilities in the environment. Animals more likely to become prey tend to sleep less than others at the top of the food chain.

For example, meerkats have the habit of stacking on top of each other in burrows to sleep, with one ear always perked up to listen for danger. In this arrangement, the dominant female in the group will lie in the middle and have the deepest sleep. Sentry meerkats will lie asleep on the outskirts and always be the first to wake up if there's any noise.

Meerkats are small burrowing animals, living in extensive underground burrows with many entrances and usually leaving during the day, except when they need to avoid the afternoon sun. They live in highly organized groups, averaging 20 to 30 members. Meerkats within the same group regularly groom each other to strengthen social bonds. The dominant pair often emits a characteristic scent - a sign of their power within the group. This behavior is also often performed when group members reunite after a short period apart. Especially when all meerkats are siblings or offspring of the dominant pair.

Meerkats show mutual care within the group, with one or more meerkats standing guard while others search for food or play, to warn of approaching danger. When a carnivore is detected, the sentinel meerkat calls out a warning, and other group members will run and hide in one of the small burrows, spreading the message throughout their territory. Sentry meerkats are the first to reappear from the burrows and search for carnivores, continuously calling out to inform the others below. If there is no threat, the sentry meerkats cease signaling, and the others feel safe to emerge.

Dolphins have a highly peculiar sleeping habit, they only sleep with half of their brain and one eye. This is called 'slow-wave sleep with half a brain.' Specifically, when sleeping, half of a dolphin's brain will shut down, and they will close one eye opposite to the inactive hemisphere. For example, if the left hemisphere is inactive, the dolphin will close its right eye. The eye and the remaining hemisphere continue to function normally.

There are three reasons that scientists have put forward to explain the dolphin's unique sleeping ability. First, unlike fish with gills, dolphins are mammals and need to regularly surface to breathe. If dolphins slept and entered an unconscious state like humans, they would be at risk of drowning. Therefore, letting half of the brain rest while the other half remains active allows dolphins to surface to exchange air through the blowhole located on the back of their heads.

Secondly, 'slow-wave sleep with half a brain' helps dolphins detect other dangers in the ocean. Thirdly, this sleeping pattern helps dolphins maintain physiological activities such as muscle movement to regulate body temperature in the cold ocean.

A recent study has revealed the astonishing ability of dolphins: They can stay awake for many consecutive days or weeks without sleeping. How do they manage to survive without sleep? Dolphins possess an incredibly unique sleep mechanism, they can let half of their brain rest at a certain time while the other half remains awake - a process called 'Unihemispheric sleep.' This unique sleep mechanism not only prevents dolphins from drowning but also allows them to remain vigilant against any dangers and even promotes brain development.

In addition, dolphins use the toxin of prey as a 'drug.' We know that pufferfish have a strong toxin. Clearly, dolphins are also aware of this but they use it for 'recreational' purposes. Typically, the toxin of a pufferfish is lethal. However, in small doses, it can act as an addictive substance. The BBC has filmed dolphins gently playing with a pufferfish for 20 to 30 minutes, then lingering around and exhibiting 'strange' behaviors afterward.

Golden Mantled Ground Squirrels have a unique talent for balancing on a tree branch while sleeping, and any slightest movement immediately awakens them.

Golden Mantled Ground Squirrels or Redwood chipmunks are commonly found in California. Their habitat is a small area along the northern coastline of California. Extending no more than 40km from the coast. Their range starts from Sonoma County and extends north to the Eel River in Humboldt County. The entire range is less than 20,000 square kilometers.

These squirrels dig burrows to store food and avoid predators. These burrows are extensive and are made in dense thickets and lush foliage. This animal relies on coastal redwood forests and mixed forests of Douglas fir or Pacific madrone to create a living environment. Although they are rarely found in the canopy. A study showed that the squirrel density of this group is significantly higher in old-growth forests than in secondary forests.

These squirrels, when mature, reach sizes of: Length from 233 to 297 mm and weight: 78 - 118 grams. Golden Mantled Ground Squirrels have cheek pouches like other squirrels. With these pouches, they can store large amounts of food. Their cheeks are large and swollen with a characteristic golden color. Small triangular ears sit atop their heads. Their eyes are almond-shaped with deep black tones and white eye rims.

They have black and light brown stripes alternating on their backs. The underside is covered with a soft, white fur. Sometimes the belly is covered with light brown or light yellow fur. Their winter fur is longer, softer, and thicker. Golden Mantled Ground Squirrels exhibit slight sexual dimorphism. Females are on average about 5% larger than males.

Flamingos have a sleeping style of one eye closed, one eye open to always be on guard against nocturnal predators.

Flamingos often gather in large flocks to forage, living in shallow waters, with the color of their feathers depending on their daily diet. Flamingos are considered one of the most loyal animal species. Birds in the same flock will move together when faced with threats. When a research plane approaches the living area of a flock of flamingos in Sisal, Mexico, the birds move together to a safer place without dispersing individually.

When newly born, flamingos have white feathers. The color of flamingo feathers is later formed by the food source where they live, which can vary from light pink, salmon pink to deep red. Pigments found in the shrimp species in Yucatan, where these flamingos live, give their feathers colors like coral reefs.

A pair of flamingos using their bills to feed a chick. This is also where the parent birds nest to welcome the soon-to-hatch eggs. When the eggs hatch, the chicks are fed a fatty and protein-rich milk produced from the parents' intestines.

When the chicks are slightly older, the parent flamingos will send them to other adult birds for care and foraging, then return to feed them in the evening. Although watched over by larger birds, young flamingos are still vulnerable to other species.

Flamingos wake up before dawn, forming into large flocks to forage. Flamingo flocks can travel hundreds of kilometers to find food.

Koala, also known as koala bear or scientifically Phascolarctos cinereus. This animal spends about 20 to 22 hours a day sleeping, with only a short amount of time for movement. They are herbivorous marsupials and primarily inhabit Australia. The only surviving species in the family Phascolarctidae, with its closest living relative being the wombat.

This sleep-loving animal is distributed in areas along the eastern and southern coastlines, specifically in Queensland, New South Wales, Victoria, and South Australia. Their bodies measure about 60–85 cm in length and weigh around 4–15 kg. Their fur ranges from silver-gray to chocolate-brown. Koala populations in the north tend to be smaller and lighter than those in the south.

Koalas are medium-sized animals with flattened noses, small eyes, seemingly short-sighted. The nose is also somewhat flattened, but it's quite large and fleshy. Their teeth are designed to process tough leaves, with sharp edges distinguishing the incisors. The ears are round, widely spaced, relatively large, with long hair at the edges.

All limbs are elongated, as they constantly move through trees. Hence, the brush of the top foot includes several parts – for easy movement. The feet are lower, and they are not too strong. The claws of a koala have some advantages: they are long and curved in an arc shape. Thanks to such qualities, it's convenient to be on branches, and you can hold a weight of up to 15 kg.

What surprised scientists at the time was the animal's fingerprint. It turned out that in all aspects, it corresponds to a human fingerprint. There's another unusual fact, and it relates to the structure of the reproductive organs. Females have two vaginas and two uteri. Accordingly, males have two penises.

But the koala's brain is remarkably small compared to the total mass – only 0.02%. Perhaps, the ancestors of this species were smarter, had a larger brain mass, but because the koala's lifestyle doesn't imply much brain activity, similar evolutionary changes have occurred. However, this sympathy for him does not diminish. The animal's fur is short and dense, very comfortable to touch. Incidentally, this quality of hers has led to the animal being heavily destroyed by humans for the allure of fur. The color of the coat is usually smoke gray, and lighter on the belly, a brown color can also occur.