Three-toed sloths are even more slothful than two-toed sloths

There are degrees of slothfulness, it turns out, even when it comes to sloths. And three-toed sloths may be the most slothful of them all: A species of the animal has a field metabolic rate that is the lowest ever recorded for any mammal in the world.

Jonathan Pauli, an ecologist at the University of Wisconsin-Madison, got interested in sloths not because they’re adorable but because “other things eat them,” he says. And he stayed interested in the animals because they are “biologically fascinating.”

Sloths are a type of arboreal folivore, a group that includes all animals that live in trees and eat only leaves. What most people lump into the category of “sloth” are really six species in two families (two-toed and three-toed) separated by millions of years of evolution. Both families live in trees in Central and South America and eat leaves, but three-toed sloths tend to have smaller ranges and more constricted diets, eating from only a few species of trees and only a limited number of them. Studies have also shown that these sloths have a very slow metabolic rate.

But how slow? To find out, Pauli and his colleagues captured 10 brown-throated three-toed sloths (Bradypus variegatus) and 12 Hoffmann’s two-toed sloths (Choloepus hoffmanni) from a study site in northeastern Costa Rica. There, the sloths live among a variety of habitat types, ranging from pristine forest to cacao agroforest to monocultures of banana and pineapple. “It’s really a quilt of different habitat types,” Pauli says, and one that allows the researchers to not only study many habitats at once but also more easily capture and track sloths than if they were in dense jungle.

The researchers injected the sloths with water labeled with isotopes of oxygen and hydrogen and released the animals, tracking them with radiotelemetry. After a week to a week and a half, the scientists again captured the sloths and took blood samples. By seeing how much of the oxygen and hydrogen isotopes remained, the scientists could calculate the sloths’ field metabolic rate — the energy that an organism uses throughout the day.

The field metabolic rate for the three-toed sloths was 31 percent lower than that of two-toed sloths and lower than that found in any mammal outside of hibernation, the researchers report May 25 in the American Naturalist.

“There seems to be kind of a cool combination of behavior and physiological characteristics that lead to these tremendous cost savings for three-toed sloths,” Pauli says. Three-toed sloths spend a lot of time in the canopy eating and sleeping, he notes. “They don’t do a lot of movement, whereas two-toed sloths are much more mobile. They’re moving around a lot more.”

But it’s more than just that. “Three-toed sloths have the capacity to fluctuate their body temperature,” he says. Unlike humans, who need to keep their temperature within a few degrees to function properly, the sloths can let theirs rise and fall with the ambient temperature, a bit like how a lizard or snake might regulate its body temperature. “Those are big cost savings to let your body change with your surroundings.”

The results of the study help explain why there aren’t more kinds of sloths and other arboreal folivores, Pauli and his colleagues argue. “Being an arboreal folivore is really tough living,” Pauli says. Leaf eaters tend to be big because they need to accommodate a large digestive system capable of processing all the leaf matter they need to survive. But to live in the trees, an animal can’t be too big. And this could be why arboreal folivory is one of the world’s rarest lifestyles. The need for all the various adaptations for that lifestyle could prevent the rapid diversification seen among other groups, such as Darwin’s finches.