Sloths starve even with full bellies

A three-toed sloth takes up to 30 days to digest a single leaf. Its stomach is a massive, four-chambered fermentation vat that makes up a third of its total body weight, filled with specialized bacteria that slowly grind down tough cellulose.

This process is so sluggish that if a sloth gets too cold, the bacteria in its gut stop working entirely. The sloth can have a belly full of leaves and still starve to death because it can no longer extract nutrients from the food sitting inside it. It effectively freezes its own fuel line.

Because digestion is so energy-intensive, the sloth lives on a razor-thin metabolic budget. It only leaves the safety of the canopy once a week to use the bathroom. This trip is the most dangerous moment of its life, as it loses up to one-third of its body weight in a single sitting on the forest floor.

Even their muscles are built differently. Sloths have only about 25% of the muscle mass found in other mammals of their size. They rely on a unique arrangement of tendons that lock their claws in place, allowing them to hang upside down and even sleep or die without using a single ounce of energy to stay gripped to the branch.

In a strange twist of biology, the sloth’s fur grows in the opposite direction of most mammals—away from its extremities—to allow rainwater to run off while it hangs upside down.