Whether in dark caves, under bridges, or in inaccessible crevices, bats intrigue us with one unique habit: they sleep hanging upside down. This behavior, as curious as it is effective, is the result of millions of years of evolution. But why do these flying mammals adopt such an unusual position to rest?
An Adaptation Linked to the Evolution of Flight
The origins of bat flight can be traced back to their terrestrial ancestors, who likely began by gliding between trees, explains Tara Hohoff, a bat biologist and coordinator of the Illinois Bat Conservation Program. “As they evolved from land mammals to fliers, bats started by gliding, much like flying squirrels,” she told Live Science.
Alexander Lewis, a researcher at California State Polytechnic University, adds to Live Science that these ancestors would climb trees to launch themselves into the air. These efforts promoted the development of strong limbs, which over time transformed into wings.
Unlike birds, whose hollow bones lighten their skeletons and facilitate lift during flight, bats have solid bones—a characteristic of land mammals. This bone density makes them heavier and limits their ability to generate enough thrust to take off from the ground.
To compensate, they developed a unique strategy: hanging upside down and using gravity to drop into flight with minimal effort.
A Body Designed for Energy Efficiency
Hanging upside down, bats gain a significant energy advantage thanks to their anatomy. “When a bat finds a perch, it contracts the muscles connected to its claws to open them, so that it can grip the surface,” explains Daniel Pavuk, a zoologist and head of the Biology Department at Bowling Green State University, speaking to Live Science. Once in place, the weight of its body relaxes its muscles and tendons, locking its claws in a closed position. In other words, “its body relaxes, and gravity does the rest,” continues Pavuk.
This ability allows them to remain suspended effortlessly, while their small size facilitates blood circulation even when upside down.
Unlike humans, who would suffer from blood pooling in the head, bats are unaffected, according to the Iowa Department of Natural Resources.
This mode of resting also offers survival advantages. By hanging high up, they escape most predators, such as snakes or birds of prey. Moreover, their legs are poorly adapted to support their weight, making vertical resting impractical. “Bat leg bones do not bear their weight well over long periods,” Daniel Pavuk notes.
However, while most bats sleep hanging upside down, not all do. For example, “Thyroptera tricolor bats from Central and South America have special suction cups on their thumbs, which they use to adhere beneath leaves at all sorts of angles,” recalled Alexander Lewis.
“Further research on the evolution of bat flight and on how most of them manage to hang upside down would be truly interesting, as they are the only flying mammals,” declared Tara Hohoff. She concluded, “Some species seem to have an easier time taking off from the ground, so it would be useful to study what is morphologically different.”
