Why do bats sleep upside down?

Bats: the flying mammals

Bats are a very ancient animal, with the earliest fossil specimens of bats being 51 million years old. Although bats are somewhat bird-like in appearance, when they do not lay eggs or have feathers, they are unique mammals (Adopter). They are the only mammals in the world that can fly on wings, can echo-locate, have a superb immune system and excellent DNA damage repair, carry a large number of viruses without getting sick themselves, and have survived a long history of living for over 30 to 40 years, which is a high life expectancy in the small mammal world!

There are thousands of species of bats, and they are widely distributed, except for the poles.

Bats like to lie around at night, and they have a very peculiar way of resting. Most bats, when not flying, usually hang upside down on the inside of caves, bridges, tree holes, holes in buildings, and other hidden and suitable places for gripping. They can quickly flip over and hang themselves upside down, head first, in just a few seconds after flying to a corner.

They can hang upside down to eat, hang upside down to feed their young, hang upside down to urinate and defecate, hang upside down to sleep, and can keep sleeping in this position for a whole day, and even hibernate for a whole winter. What is even more incredible is that they will continue to hang upside down even after death.

If we humans are upside down for a few minutes, too much blood will quickly gather in the brain, which will lead to brain congestion, lack of oxygen, and unbearable headaches. So, why do bats prefer to choose this upside-down resting method? Hanging upside down for a long time, won't the bat's head be congested with blood? Not afraid to fall asleep? They will continue to hang after death, and what is going on?

Why do bats sleep upside down?

The bat's habit of hanging upside down has evolved along with flight, which is closely related to its body structure, activity pattern, and habits.

It is well known that wings are the organs of flight for birds. Bats do not have the muscular and feathered wings of birds, but they have a unique organ of flight, the featherless pterodactyl. The pterodactyl hand evolved from the forelimbs, and the finger bones and metacarpals of the forelimbs are greatly extended, and a thin and soft skin membrane is formed between the side of the body, the hind limbs, and the tail, called the "pterodactyl hand". During the flight, became convert their membrane-like "wings" into various shapes, which helps them to quickly change the angle and direction of flight, enabling them to dodge and swoop flexibly during flight.

Flying is an extremely physically demanding way to travel, consuming much more physical energy than running or swimming. To conserve energy while flying, flying animals find ways to make reduce their weight. One way that bats have evolved to reduce their weight is to make their hind leg bones shorter and thinner. Such hind legs have little strength and cannot support their weight, so bats cannot stand, walk, and jump on the ground like other animals, and can only crawl on their hands and knees on the ground with difficulty.

And "hanging upside down" has become a better way for them to rest, living in high places not only to avoid predators but also to help flight. Because of the "short hind legs", bats can't take off like birds or insects, they can't run first or jump up high to generate upward force and then spread their wings to fly. In other words, bats directly from the ground aren't able to fly. But hanging upside down from a high place, they can fall from the trees at any time and open their "winged hands" to glide for a distance, and then they can fly.

If a bat sleeps on the ground, it will not be able to fly off the ground as quickly as a bird at any time and will be easily "killed" by predators. In high places, the view is wide, once a wind blows, but also can quickly slide down and take off to escape, then on the ground much safer. And in high places easier to observe suitable prey, and easier to hunt.

In addition, hanging upside down has other benefits, such as freeing the bats' "hands" so that mothers can use their wings to hold their young in their arms, making it easier to nurse them; when hibernating, the upside down sleeping position can reduce the contact surface between their bodies and the cold top wall to keep their bodies warm.

How do bats hang upside down from trees? Why don't they fall?

It is important to know that for our hands and feet to grasp things, it takes a coordinated effort between the ex tensor muscles (the muscles that support the maintenance of straightness) and the flex or muscles (the muscles that help the joints flex). The muscles are connected to the bones through the tendon membrane and pull the bones into motion. When we grasp something with both hands, the ex tensor muscles stretch, the flex or muscles contract, and the joint flexes; when we let go, the opposite happens: the ex tensor muscles contract, the flex or muscles stretch, and the joint straightens.

For an animal to hold onto a branch and stand firmly, the floors in the paws must contract very tightly. However, the hind legs of bats are very underdeveloped, and the floors on the paws are degenerated, with the corresponding tendon membrane of the foot attached to the toe bone at one end and directly connected to its upper body at the other. When the bat hangs upside down, its gravity replaces the pull of the muscles, and the tendons are pulled tight by gravity alone, without the need to exert themselves, and gravity makes the claw joints normally just clenched in a natural grip.

That is to say, bats hanging upside down, can grasp objects, not because of the claw muscle contraction, but the result of the role of the Earth's gravity, so bats hanging upside down when the muscles are all in a relaxed state, do not need to bother a little effort, their claws can remain closed, off tightly locked in the tree, so there is no fear of falling, even if they are asleep will continue to hang upside down. Sleeping upside down is not uncomfortable for bats at all. If a bat dies during sleep, it will also continue to hang upside down until an outside force (a person or something) takes them down or shakes them loose.

It's no exaggeration to say that bats spend their entire lives nursing, feeding, sleeping, defecating, and dying upside down!

Bats don't fill their heads with blood while hanging upside down?

Bats hibernate all winter long and nothing happens to them! So, how do bats hang upside down all the time, and their heads do not fill with blood?

For humans, if you are upside down for a long time, it will cause a lot of blood to flow to the head and stagnate in the head, but not back to the heart, resulting in congestion in the brain, dizziness, and uncomfortable.

When the human body stands, or normal upright walking, blood does not stay in the lower extremities, because in the veins of our extremities (blood vessels that carry blood back to the heart) there is something called a "venous valve". Due to the existence of the venous valve in the blood vessels, it will limit the passive flow of blood only from the bottom to the top, thus preventing the backlog of blood, while promoting the return of blood to the heart to complete the blood circulation.

The bat does not have a structure similar to a venous valve as a barrier in its blood vessels, and its venous wall muscles are strong enough to rhythmically and actively constrict the blood vessels during the backward flow, promoting the rapid flow of blood back to the heart.

In addition, bats are very small and do not have much total blood volume. The smallest bat species is only about 2 grams (concave-faced bat), and the largest bat species only weigh about 1.6 kg (Malayan big fox bat), and with this weight, the gravitational force when hanging upside down is not enough to affect the blood flow.

At the same time, because of the large amount of oxygen consumed in flight, bats have a particularly well-developed heart (three times larger than other mammals of the same size) and a strong ability to transport blood, which can control the blood flow back when hanging upside down, so that not too much blood is left in the head when it is upside down.

When bats are hanging upside down, their strong heart and vein walls can overcome the effects of gravity so that their blood can still flow normally and how it should flow, so they will not suffer from brain congestion even after hanging upside down for a long time.

Summary

For bats, hanging upside down is an efficient way of resting that is well suited to their physical conditions, as they can easily hang upside down from high places effortlessly, relying only on gravity. It is with this unique way of survival that bats have successfully found their ecological niche and lived for tens of millions of years.