The secret is in their bones
08.21.23
By Kristen Pope
If you’ve ever been stuck upside down on a rollercoaster–or seen those who have been in this unfortunate position–you probably wonder why any creature would voluntarily hang from its feet. But for bats, being upside down is a convenient and useful way to roost.
While some other mammals—like flying squirrels and sugar gliders—can soar across the sky, bats are the only mammals that can truly fly. When you’re a bat that might want to fly at any minute—whether to snatch a tasty insect out of the sky, grab a drink of water on the wing, or escape a predator—hanging by your feet makes it easy to take flight. Simply let go, and you’re instantly airborne. Hanging upside down in hard-to-reach locations is also a handy way to dodge predators when you’re dangling far out of their reach.
For bats, hanging is also easy. When humans hang from, say, a chin-up bar, it takes a great amount of effort to clutch the bar and dangle, whether upside down or right-side up. People quickly tire from a hanging position. Bats don’t have to expend nearly as much effort to hang since the tendons and ligaments in their legs and feet help aid the process.
Bats’ ancestors were mammals that glided through the air, but modern day bats have evolved to be able to fly. However, unlike birds that have hollow bones—including some that are “pneumatized” with air pockets to make them even lighter—bats do not have hollow bones. This means that bats need to reduce weight in other ways to be able to take flight. Their long bones, like femurs, are light so they can fly, and because of this, they can’t withstand the compression standing up. In 1977, D.J. Howell and Joe Pylka published a paper in the Journal of Theoretical Biology delving into this issue. They found that most bats’ leg bones “cannot withstand compression stress,” so “hanging is advantageous.” Bats also have skin membranes that spread out between their light, elongated fingers and ankles, which is great for flying, but makes it difficult to sit or stand.
While many bats can still take flight from the ground, already being in the air—dangling from their toes while grasping their perch with their claws—makes going airborne effortless.
