Five things everyone should know about… Sloths

1) A sloth is not a sloth. There are two types of tree sloths that diverged roughly 20 million years ago—two-toed and three-toed sloths, so named for the number of digits on their forelimbs. They differ greatly in what they eat, when they’re active, the trees they use, how far they move and even their mating systems. In a nutshell, two-toed sloths are generalists, using a wide variety of habitat types and resources, while the three-toed sloth is much more specialized, eating leaves from just a few species of trees, and even spending the majority of their lives in just a few individual trees.

2) They are extremely low-energy. Both types of sloths have slow metabolisms, but the three-toed sloth has the lowest energetic needs of any mammal ever recorded. Sloths achieve this by not moving very much, and also by letting their body temperatures fluctuate with outdoor temperatures. In terms of calories, a single potato is all a three-toed sloth would need each day to survive (if sloths actually ate potatoes).

3) Constipation is a way of life. Sloths consume plenty of fiber in the form of leaves (three-toed sloths) and a variety of leaves and fruits (two-toed sloths). Yet these foods are digested so slowly that sloths need to pass feces and urine only about once a week. Three-toed sloths climb down to defecate at the base of their host trees—practically the only time they leave the canopy.

4) The sloth is a miniature ecosystem. And understanding that ecosystem helps clarify sloths’ odd bathroom behavior. Sloths host a dedicated species of algae in their fur as well as scores of flightless “sloth moths” that depend on the sloth’s defecation descent for reproduction. The moth lays eggs in the sloth’s dung and then returns to the sloth’s fur. After the eggs hatch, the caterpillars feed on the dung, become moths, and the moths find—during the only brief moment in their lifetime that they can fly—another sloth to live on. When the moths die, their bodies are decomposed by fungi and bacteria in the sloth’s fur. The products of this decay, nitrogen in particular, provide fertilizer for the algae, which the sloths eat—thus adding nutrients to their diet.

5) Made for the shade. As tropical forests in Central and South America are cleared for agriculture and other uses, sloths (like many other species) need to find or adapt to new habitats in order to survive. Our team studied sloth populations at a large shade-grown cacao plantation in Costa Rica. With its diverse overstory of native trees, the plantation provides suitable habitat for sloths—especially two-toed sloths—and seems to point the way to at least one kind of farming that can benefit sloths and other native tropical animals.

Jonathan Pauli and Zach Peery, professors of forest and wildlife ecology, have studied sloths in Costa Rica since 2009.

PHOTO: Jonathan Pauli watches after releasing a two-toed sloth in Costa Rica
Photo courtesy of Jonathan Pauli