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.

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.

View Images 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. Photograph by Joel Sartore, Nat Geo Image Collection

The tendon is so effective that even a dead bat will continue to hang. (See "6 Bat Myths Busted: Are They Really Blind?")

It doesn’t get more relaxed than that.

Life in the Slow Lane

Another animal living on the flip side is the sloth.

Native to forests of Central and South America, sloths don’t spend as much time upside down as we might think, says Don Moore, associate director of Smithsonian's National Zoo in Washington, D.C.

When the gangly animals do move upside down through the trees, they travel so slowly that the fluid in their middle ear is always stable, which prevents them from getting dizzy.

Two-toed sloths stay on track by keeping their head in one position—nose down along the tree branch—as they move, Moore says.

But it's three-toed sloths that have the best parlor trick. "This is astounding, but they flip their heads 180 degrees when they’re moving horizontally" to see the branches in front of them, he says.

"They’re fabulous at moving very, very slowly up in the treetops and looking like a clump of algae," he adds. Not only does algae grow in sloth fur but, as this 2014 study discovered, certain moth species "are known to colonize moth fur exclusively."

Sloths leave the trees once a week to poop, and female moths lay eggs in the sloth’s dung. Once hatched, those moths can fly up to mate in the sloths' fur, according to the research. (Read more about why sloths leave the safety of trees to use the bathroom.)

And that’s the story of the sloth and the moth. Anyone else feel a children’s book coming on?