Are you one of those people who can only sleep with the sound of the TV on in the background? Or maybe you need the soft, steady patter of raindrops hitting the roof to drift off. Humans have their own unique bedtime rituals, which can range from drawing the blackout curtains closed to cuddling up with their favorite stuffed toy, and beyond. These might sound like odd sleeping habits to some, but they’re nothing compared to how some animals catch their Z’s in the wild.

Dolphins

How do marine mammals sleep without breathing? Well, they don’t. Aquatic mammals like dolphins still come up for air even if they’re mid-sleep. How? By keeping half of their brain awake at all times. This is an evolutionary trick known as unihemispheric sleep, which allows an animal to shut down one half of its brain for rest, while keeping the other half awake, alert, and able to execute important functions. Dolphins aren’t the only animals that rely on this trick to survive while they sleep. Whales, manatees, and Northern fur seals also sleep this way so that they can keep breathing and remain alert for predators. Common swifts can stay in the air for up to 10 months at a time, and they never have to touch ground thanks to unihemispheric low-wave sleep adaptations. 

Ducks

Ducks also utilize unihemispheric sleep to get their much-needed rest, but they don’t always have to. When ducks sleep alone, they use the half-awake/half-asleep brain technique, but when they’re resting with pals, they rely on the safety of the group to protect them while they get a full night’s rest. When ducks sleep in a flock, whether on land or on the water, the ducks on the outside of the flock sleep with one eye open in an effort to protect and alert the pack should danger arise. The lucky ducks in the middle of the flock are able to indulge in a two-eyed sleep. These birds also rotate who gets guard duty, allowing everyone to have their turn to snooze soundly in the middle. 

Bats

It’s common knowledge that bats sleep upside down, but why do they choose to rest this way rather than curl up on the ground or in a nest? They do it to stay safe when they’re in their most vulnerable state. Bat legs are relatively small, thin, and weak compared to their wings. This makes it hard for them to push off of the ground when it’s time to take flight, which might need to occur in a split second when predators approach. Hanging from their feet allows them to simply release their grip and fly elsewhere to safety, and it doesn’t even cost them energy to do it. The tendons found in a bat’s legs connect to their toes and automatically lock them onto the surface they’re touching, keeping them secure for their slumber.

Giraffes

Think you could get by on just 20 minutes of sleep a night? That’s exactly what many giraffes do. Despite their massive size, giraffes are actually quite vulnerable when they’re not standing tall. This is why they mostly sleep standing upright, and in short bursts of about 20-30 minutes at a time. They may do this several times throughout the day to accumulate more sleep, but rarely venture beyond about six hours all-told. Giraffes can and do sleep on the ground, which is how they get that restorative REM sleep. Lying down and getting up does force them to let their guards down, but when they do it, they protect themselves by wrapping their necks around their bodies and resting their heads right on their rumps, like a built-in pillow.

Koalas

On the other hand, some animals need extra beauty rest in order to function at all, and the koala takes the cake for the longest dozer on land. Racking up an impressive 22 hours per day, koalas sleep so much to preserve energy. Their diets are made almost entirely of eucalyptus leaves, which are not only low in nutritional value, but also high in toxins that are indigestible by most animals. The fibrous plant requires a lot of energy to digest, and offers little energy to burn in return, hence the lengthy slumber. 

Desert Snails

When people think of hibernation, they probably picture Grizzly bears and groundhogs first. But did you know there’s a snail that can out-hibernate even the longest napper, and by several years? Desert snails regularly sleep for around 15 hours per night, which is impressive on its own but isn’t even close to the longest stretch they can go for. These animals don’t hibernate necessarily, they aestivate, or go dormant, for up to three years by essentially encasing themselves in their shells with a layer of mucus. They do so to survive the hot, arid climates of the desert. Aestivation is one of the purest forms of metabolic preservation, and according to one study, can allow an animal’s essential organs to keep working despite losing up to 30% of the water in its body.

Chinstrap Penguins

Chinstrap penguins take their breeding season seriously. So seriously, in fact, that they completely change their entire sleeping routine. When at sea, penguins float and use unihemispheric sleep to survive. During the breeding and egg-rearing season, they assemble in colonies on land, where they bring a new meaning to the term “micronap.” Studies have shown chinstrap penguins can sleep up to 600 times per hour, or over 10,000 times in a single day, but for only about four seconds at a time. They’re able to accumulate around 11 hours of rest this way, and they do it so they can keep a very close eye on their precious eggs, which are prime pickings for avian predators like Skuas and Sheathbills. 

To most creatures, blood may seem like an unlikely food source on its own. It’s not exactly easy to get at, hidden beneath skin and often protected by the claws, teeth, or swatting hands of the animal it belongs to. Yet throughout the animal kingdom, a surprising number of species — upwards of 30,000 — have evolved to feed on blood.

Several animals, from mosquitoes to ticks and, yes, even vampire bats, all rely on blood to survive. This is known as hematophagy, or blood feeding, and it has evolved independently many times in different groups of animals. So why does nature keep nurturing such a strange strategy?

Blood Feeding Is Ancient

Mosquitoes are among the most familiar blood feeders today. The earliest mosquitoes likely appeared in the Jurassic over 130 million years ago, and even some of the oldest-known mosquito fossils already showed evidence of blood-feeding adaptations.

Today, only fertilized female mosquitoes feed on blood, using it as a source of protein and iron to develop their eggs. Males, by contrast, feed entirely on plant nectar. This split diet suggests that blood feeding didn’t replace plant feeding, but was instead adapted as a specialized reproductive strategy. Blood feeding, then, likely emerged as an added tool — a way to unlock a valuable resource when the opportunity arose.

Over time, different mosquito lineages refined this strategy in different ways, shaped by local ecology and available hosts. In some cases, that included the rise of a preference for human blood.

Why Blood Is Worth the Risk

Despite being relatively difficult to access, some animals continue to seek it out for its many beneficial nutrients. Blood is rich in proteins, lipids, salts, and iron — important parts of growth and reproduction. For a small animal, a single feeding of blood can provide far more nutrition than many plant-based meals combined. 

Blood is also relatively predictable. Unlike food sources that fluctuate seasonally, vertebrate animals are widespread and available year round. Although hematophagy remains rare overall, the high payoff and steady availability of blood helps explain why it has evolved repeatedly.

There is another advantage: Unlike leaves, wood, or even many seeds, blood contains nutrients in a liquid form that can be absorbed more quickly. For small animals with limited feeding opportunities, that efficiency matters. A mosquito, for example, may only have a moment to feed before being smacked or swatted away, making a concentrated meal especially desirable.

The Blood Feeder’s Dilemma

Of course, blood is found inside living, reactive hosts that will actively try to prevent being fed on, so getting to it is not always straightforward. Blood-feeding animals have evolved specialized tools to attempt to solve that problem.

Mosquitoes, for example, use six needle-like mouthparts to pierce skin and locate blood vessels, while their saliva contains a chemical concoction that prevents clotting and reduces pain. Ticks anchor themselves with barbed mouthparts that can remain embedded for days. Leeches secrete a powerful natural anticoagulant called hirudin that keeps blood flowing freely. And vampire bats use their razor-sharp teeth to create small incisions while their saliva prevents clotting. They have also evolved unique heat-sensitive receptors that help them locate the best feasting areas on their hosts.

Feeding on blood also presents other challenges. Blood is very high in iron and proteins but relatively low in some vitamins, and consuming large amounts can expose animals to potentially harmful levels of iron. Many blood-feeding species have evolved specialized digestive systems or excretory processes that help them process excess nutrients and avoid toxicity.

A Pattern That Keeps Repeating

Blood feeding has evolved independently in different groups of animals: Mosquitoes are insects, ticks are classified as arachnids, leeches are segmented worms, oxpeckers are birds, and vampire bats are mammals. These groups are separated by hundreds of millions of years of evolution, yet they were all drawn in by the same opportunity: a resource that is valuable, abundant, and accessible with the right techniques. Blood-feeding animals may seem strange from a human perspective, and for animals, hematophagy can definitely be difficult and risky. But the misunderstood practice has proved valuable enough to evolve repeatedly throughout the animal kingdom.