To quantify specialization in sloth species and explore how it influences population viability, we characterized the resource use for 68 adult and 12 subadult sloths across 3 y. Both species inhabit shade-grown agro-ecosystems but, at least at one study site, only the two-toed sloth was viable. The two-toed sloth is considered a generalist while the three-toed sloth is more specialized. The two-toed sloth ( Choloepus hoffmanni) and the three-toed sloth ( Bradypus variegatus) are arboreal herbivores distributed across the Neotropics. In general, specialists are more prone to extinction than generalists and, thus, are often the first species to be lost when habitats are modified. The need for all the various adaptations for that lifestyle could prevent the rapid diversification seen among other groups, such as Darwin’s finches.Although resource specialization occurs along a continuum, species are often defined as either specialists or generalists. And this could be why arboreal folivory is one of the world’s rarest lifestyles. But to live in the trees, an animal can’t be too big. 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. “Being an arboreal folivore is really tough living,” Pauli says. The results of the study help explain why there aren’t more kinds of sloths and other arboreal folivores, Pauli and his colleagues argue. “Those are big cost savings to let your body change with your surroundings.” 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. “Three-toed sloths have the capacity to fluctuate their body temperature,” he says.
They’re moving around a lot more.”īut it’s more than just that. “They don’t do a lot of movement, whereas two-toed sloths are much more mobile. Three-toed sloths spend a lot of time in the canopy eating and sleeping, he notes. “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. 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. 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. After a week to a week and a half, the scientists again captured the sloths and took blood samples. The researchers injected the sloths with water labeled with isotopes of oxygen and hydrogen and released the animals, tracking them with radiotelemetry. “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. There, the sloths live among a variety of habitat types, ranging from pristine forest to cacao agroforest to monocultures of banana and pineapple. Studies have also shown that these sloths have a very slow metabolic rate.īut 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. 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. 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. Sloths are a type of arboreal folivore, a group that includes all animals that live in trees and eat only leaves. And he stayed interested in the animals because they are “biologically fascinating.” 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 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. There are degrees of slothfulness, it turns out, even when it comes to sloths.