Bats and Sloths Don't Get Dizzy Hanging Upside Down—Here's Why

There’s a reason gravity boots never caught on: Being upside down can get pretty uncomfortable after a while.

Noting the headaches that come with being inverted, National Geographic writer and editor Jane J. Lee asked Saturday’s Weird Animal Question of the Week: "Why don’t bats, and other animals that hang upside down, suffer the same fate?"

The average adult human carries about 2 gallons (7.5 liters) of blood, according to the American Red Cross. That’s a lot of liquid suddenly rushing to your head if you were to hang upside down—hence the discomfort.

By comparison, bats are lightweights. The tiniest bat in the world, Kitti’s hog-nosed bat, also known as the bumblebee bat, weighs in at 0.07 ounces. Even the two largest known bat species, Australia’s black flying fox and the Philippines' golden-crowned flying fox, weigh only up to 2.5 pounds (1.1 kilograms). (Watch a video of black flying foxes in action.)

As a result, bats don't "weigh enough for gravity to affect their blood flow," says Rob Mies, director of the Michigan-based Organization for Bat Conservation via email.

There's another benefit to hanging upside down—it's takes less effort. Specialized tendons in bat feet enable them to hang while being perfectly relaxed. If they were sitting right side up, they'd have to contract a muscle—and thus expend energy—to let go and begin flying.

Though they're among the biggest of the bats, black flying foxes are still lightweight enough that the blood rushing to their heads doesn't make them uncomfortable.