Why Are Sloths Sooo Slow?

In the animal world, speed is king. Fast animals have a leg up in outrunning both predators and prey, which puts them high on the food chain. It would seem that all animals would strive for speed ... but then there’s the sloth. While a cheetah can go from 0 to 60 miles an hour in only three seconds, it takes a sloth all day to cover 41 yards.

Such a distinct lack of alacrity would seem to be a strange way to evolve, but according to a 2016 study, the lethargic lifestyle of tree sloths is the direct result of the animal's adaption to its arboreal niche.

Sloths live entirely in trees on a diet of leaves (making them folivores). And for this, they are extremely rare. While most of the terrestrial world is covered in trees, there are very few vertebrates that call the canopy home. The aim of the 2016 study, says Jonathan Pauli, a University of Wisconsin-Madison professor of forest and wildlife ecology, was to help explain why arboreal folivores are indeed so rare and why more animals have not evolved to take advantage of a widespread ecological niche.

"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 moose, elk and deer. What's super interesting about arboreal folivores is that they can't be big."

For their research, Pauli and his Wisconsin team studied wild two- and three-toed sloths at a field site in northeastern Costa Rica.

"Most of the world is forested, but the energetic constraints of a leafy diet seem to prevent adaptive radiation," Pauli notes. As organisms evolve, they "radiate" out from their ancestral group, and in doing so take on various traits and forms to allow them to live more specialized lives. For the sloth, this means “specialized limb adaption, reduced body mass, a slow metabolic rate and claws that act like fulcrums—hooks to accommodate the animals' need to hang in and traverse the treetops.”

"This study explains why eating leaves in the canopies of trees leads to life in the slow lane, why fast-moving animals like birds tend not to eat leaves, and why animals like deer that eat a lot of leaves tend to be big and live on the ground," says Doug Levey, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research.

When the researchers measured the energy use of three-toed sloths, they found a wildly low expenditure of as little as 460 kilojoules of energy a day, the equivalent of burning 110 calories. And for this, they take the cake: It is the lowest measured energetic output for any mammal.

"The measurement was intended to find out what it cost the sloth to live over a day," says Pauli, who says that a diet of little but leaves lacks nutritional value and the animals' small size doesn’t allow for gorging—so sloths need to find ways to maximize their meager diets. Which means using tiny amounts of energy through a reduced metabolic rate, dramatic regulation of body temperature, and living life at an exceedingly languid pace.

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