Bats are truly unique creatures. Apart from being the only mammals capable of powered flight, meaning they are able to gain altitude through their own effort, they also hang upside down to sleep. There’s a reason for this distinct resting posture, though — and it has to do with their ability to fly.
Hanging Upside Down Aids in Flight
Bats are different even than other vertebrates who evolved for flight, such a birds. Bats are mammals and their morphology differs from that of birds — while there are similarities, there are several, notable differences. Achieving flight for these mammals means they needed to evolve differently.
Where birds have strong but lightweight skeletal systems and stiff feather shafts, which allow them to take off from the ground, bats do not. Bats have heavier bodies, more dense bones, and very flexible wings. Bats also have short and weak hind legs, which makes it difficult to launch their bodies into the air when combined with the rest of their anatomy.
Compared with other flying vertebrates, it would seem they simply aren’t built for flight. But as bats evolved, they have developed other strategies for achieving flight — and hanging from an elevated position is one of them. By taking advantage of gravity, they simply let go and flap their wings to achieve lift.
Once they are in the air, bats are able to fly in a similar way to birds with a few differences. While bird wings are basically an elongated arm with a single “finger,” bats have an elongated arm with three fingers. These additional fingers allow for better maneuverability in the air and don’t require the wing to be extended as far during flight. Little hairs on their arms and flaps also help them detect small changes in air currents during flight.
Since bats spend most of their day sleeping, hanging upside down ensures that they are always ready to fly — to hunt and feed at dusk and dawn, or to escape from predators. But there are other benefits to this odd habit.
Other Benefits of Inverted Resting
For bats, sleeping upside down also offers protection from dangerous situations. When bats sleep, they enter a state of torpor, or a state of mental and physical activity. To conserve energy, their heart rates slow down and their body temperatures lower. As a result, they are very vulnerable when they sleep.
Additionally, bats are nocturnal and are asleep when most other animals are awake, which makes them vulnerable to attacks from birds of prey, humans, and other daytime predators. Because of this, bats choose dark, secluding places to roost, like caves, hollow trees, bridges, or secluded urban structures such as chimneys and attics. All of these places are out of the way and inaccessible to most other animals, who lack the ability to get to the same places bats can.
Hanging upside down gives bats more perching options that they don’t have to compete with other animals for. They also wrap their wings around their bodies while they sleep, which makes them even less visible in their dark, secluded roosts.
How do Bats Hang Upside Down?
Bats spend most of their lives upside down. When not actively hunting for food, they are hanging inside their roost. They spend about 15 to 20 hours each day in this position. Apart from sleeping, they also feed, digest, hibernate, socialize, mate, and raise their young. They are exceptionally good at hanging upside down.
Bats have a tendon locking mechanism that evolved over time. When they go to sleep, the tendons in the feet lock and their body weight and gravity holds them in place while requiring no effort or energy on the part of the bat.
Before taking off in flight, the bat relaxes the tendon, which releases their grip. This locking mechanism works so flawlessly that if a bat dies while perched, its body will continue to hang upside down until something shakes it loose.
Bats can also hang upside down without getting dizzy. Bats are small and light enough that gravity doesn’t interfere with their blood circulation. And the same system that keeps blood from pooling in our legs and feet works in reverse for bats.
What if they Fall?
Because of the way they’re built, bats don’t walk around in the same way other mammals do. And while they spend most of their lives in the air, either flying or hanging, they can still get around if they find themselves on the ground.
Unlike other mammals, bats’ knees face backward which prevents them from standing and walking upright — they’re built to support the wings and hold the bat in place while resting rather than propel the bat forward. But they can crawl and climb.
If they fall from their perch, bats can climb back up using their hind legs and the claws on their wing tips. It’s clumsy and expends a lot of energy, but a fallen bat is not necessarily a stranded bat.
There are a few species of bat that are adapted to walking. For example, the New Zealand lesser short-tailed bat is a strong and agile crawler and climber. These bats have strong hind legs and sharp claws that allow them to maneuver well on their feet.
Vampire bats are also good crawlers and can actually take off from the ground. They have strong legs and even stronger wings that enable them to walk, run, and even jump. These blood-sucking bats have retained strength in their forelimbs which allows them to propel themselves forward, an adaptation having to do with their diet. Because they can ingest almost 60% of their body weight in blood, vampire bats need to be able to achieve lift even with the added weight.
Bats have evolved to hang upside down because it allows them to efficiently take off, sleep and avoid predators. Because bats aren’t typically built to launch themselves into flight, they hang from an elevated location which allows them to fall into flight. Sleeping upside down also allows them to roost in locations that are inaccessible to most other animals and keeps them away from daytime predators.
The only mammal capable of powered flight, bat wings are built for maneuverability. Their backwards hind limbs are built for holding them in place while they roost upside down, and can be used for crawling and climbing when necessary. These small creatures have evolved in a way that is different from most mammals but which aids them in their own survival.