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In the animal world, speed is king. Fast animals have a leg up in outrunning other animals, which puts them high on the food chain. It would seem that all animals would go for speed, but then there’s the sloth (树懒). While a lion can go from 0 to 60 miles an hour in only five seconds, it takes a sloth all day to cover no more than 50 meters.

Sloths live entirely in trees on a diet of leaves. And for this, they are extremely rare. While most of the land world is covered in trees, there are very few vertebrates (脊椎动物) that call the tree home. The aim of a 2016 study, says Jonathan Pauli, a University of Alabama professor of forest and wildlife ecology, was to help uncover why sloths are indeed so unique. “Among vertebrates, this is the rarest of lifestyles”, says Pauli. “When you picture animals that live off plant leaves, they are almost all big-things like deer. What’s super interesting about sloths is that they can’t be big.”

For their research, Pauli and his Wisconsin team studied wild sloths at a field site. When the researchers measured the energy use of sloths, they found a wildly low burning of as little as 110 calories of energy a day. And for this, they take the cake: it is the lowest measured energetic output for any mammal (哺乳动物).

“The measurement was intended to find out what it cost sloths to live over a day,”says Pauli, who adds that a diet of little but leaves lacks nutritional value and the animal’s small size doesn’t allow for overeating-so sloths need to find ways to make the most of their diets, which means using tiny amounts of energy, dramatic control of body temperature and living at an extremely slow pace.

Their reward? A wonderfully widespread ecological system to call their own, one slow inch at a time.