The secret to living upside down, according to sloths

Hanging upside down all day would wreck your inner organs — so how do sloths make do?

(Image credit: (Michael & Patricia Fogden/CORBIS))

Sloths spend most of their time eating and hanging around in trees. Nice as that kind of lifestyle sounds, it's easy to see how it might get a little uncomfortable after a while.

With a full stomach, a sloth's abdominal organs can account for a quarter to more than a third of its total body weight. They can store about an equal amount of weight in feces and urine before relieving themselves, which they only do about once a week.

Now remember that sloths hang from the bottom of tree branches, gripping them with their arms and legs while either partially or completely upside-down. Those inverted positions mean that the weight isn't just pulling on their limbs, but also potentially pushing down on their lungs, making it a chore to inflate the lungs with each breath.

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And yet, sloths don't seem to have a hard time breathing. That's because all their guts, food, and waste aren't actually resting on the lungs when they're upside-down, says zoologist Rebecca Cliffe. Instead, the abdominal organs are held by bands of fibrous tissue that keep them from sliding around too much and pressing into the chest cavity.

When Cliffe and colleagues dissected a pair of three-toed sloths that had died of natural causes at the Sloth Sanctuary of Costa Rica, they found that these tissue bands connected the animals' livers and stomachs to their lower ribs, and anchored their kidneys to their pelvises. Other bands divided the abdomen from the chest. Tissues like these, technically called adhesions, had been found in other sloths before, but no one had figured out what their function was. Cliffe and her colleagues now think that because the bands hold organs against bone, they help bear the weight of the abdomen.

To see if the adhesions had any effect on a sloth's breathing, the researchers compared the lung capacity and breathing rates of regular sloths and a model of one without the bands. Their number crunching suggested that a fully upside-down sloth would use up to 13 percent more energy than normal to breath if the adhesions weren't there.

Given sloths' low-energy diets and slow metabolisms, every little bit of energy counts, and these organ anchors are probably key to balancing the animals' energy budget. Cliffe and her team also think that not having their organs jostle around could also help sloths avoid energy costs while hanging in odd positions to reach out-of-the-way food.

They can also see a potential downside to the adhesions, though. The extra tissue could cost sloths' flexibility in the abdomen, making their slow, plodding movements not just a consequence of their metabolism, but also of their anatomy.

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