Researchers studied two species of sloth in Costa Rica. They measured the rate at which these animals' bodies operate, converting food to fuel and growth. And this metabolic rate in one species of three-toed sloth was the lowest ever recorded — not just for a sloth, but for any mammal.
Studies had shown that three-toed sloths have a very slow metabolic rate. But how slow? To find out, researchers captured 10 brown-throated sloths. They are three-toed species. The scientists also collected 12 Hoffmann's sloths, which are a two-toed type. All came from a study site in northeastern Costa Rica. Here, the sloths live among a variety of habitats. These range from the pristine forest and cacao agroforest to fields of banana and pineapple. And it is one that allowed the researchers not only to study many habitats at once but also to more easily capture and track sloths than if they were in dense jungle.
The researchers injected the sloths with water labeled with specific isotopes of oxygen and hydrogen, then released the animals back to the wild. After 7 to 10 days, the scientists again captured the sloths and sampled their blood. By seeing how much of the isotope labels remained, they could calculate the sloths' field metabolic rate. This is the energy that an organism uses throughout the day.
The field metabolic rate for the three-toed sloths was 31 percent lower than that for two-toed sloths. It also was lower than that found in any mammal that was not hibernating.