How Sloths Breathe Upside-down

Sloths spend a lot of time hanging from tree branches, often upside-down. In other animals, this would cause the liver and intestines to press down on the lungs and make breathing difficult. But sloths have developed their own solution. Rebecca Cliffe from Swansea University studies sloths at Costa Rica’s Sloth Sanctuary, and has done many dissections on dead sloths. She found the internal organs were held in place by a strange fibrous adhesions she at first thought was scar tissue. But all sloths have it, and the natural “adhesive tape” eases their breathing between 7 and 13%.

“For many mammals, an energy saving of 7 to 13 percent wouldn’t be particularly game-changing,” says Cliffe. In fact, some mammals like howler monkeys do regularly hang inverted without any organ-anchors. The difference is that “sloths have virtually no flexibility in their energy budget. They generate just about enough energy from their diet to move when and where required, but there is not much left in the tank afterwards. To a sloth, an energy saving of 7 to 13 percent is quite a big deal.”

They also take a very long time to digest their food, and they only urinate and defecate once a week. Most of the time, a sloth is carrying a third of their body weight in waste matter, which means that its stomach and bowels are very heavy. “It would be energetically very expensive, if not completely impossible, for a sloth to lift this extra weight with each breath were it not for the adhesions,” says Cliffe.

Ed Yong has more about the recently-released study of sloth anatomy, which is different from all other animals because sloths are weird.

(Image credit: Flickr user Cliff)


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Proof that evolution isn't real. There's no way a species' survival would depend on it being really lazy, barely eating enough food to survive, hanging upside-down all the time, and carrying a week's worth of poop around.
Also, proof that God has an odd sense of humor.
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