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 forestry 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

Sloths Thrive at Chocolate Source

Like many and much more nimble Neotropical fauna, sloths are running out of room to maneuver.

As forests in South America and Central America are cleared for agriculture and other human uses, populations of these arboreal leaf eaters, which depend on large trees for both food and refuge, can become isolated and at risk. But one type of sustainable agriculture, shade-grown cacao plantations, could become critical refuges and bridges between intact forests for the iconic animals.

In Costa Rica, CALS forestry and wildlife ecology professors Jonathan Pauli and Zach Peery are using a complex of intact tropical forest, pasture, and banana and pineapple plantations—all connected by a large, shade-grown cacao farm—as a field laboratory to explore the ecology of two species of sloths in a rapidly changing environment.

“We know a lot about sloth physiology,” says Pauli. “But when it comes to sloth ecology and behavior, we know almost nothing. It’s a giant black box.”

But some of that mystery is being peeled away as studies of both the brown-throated three-toed sloth and Hoffmann’s two-toed sloth, two common species, are yielding new insights into their mating habits and how the animals navigate the landscape.

The fact that sloths require forested habitat and are sedentary makes them vulnerable to deforestation, says Peery. “Once a tract of tropical forest has been cleared, sloths have relatively little capacity to seek out new habitats.”

But the shade-grown cacao plantation, with its tall trees and network of cables for moving the pods that ultimately become chocolate, seems to be a de facto refuge and transit hub.

“Because of the diverse overstory of native trees, the cacao farm appears to provide excellent habitat for both species of sloths,” explains Peery. “We want to compare sloth populations in cacao to populations in intact tropical forests to see if cacao provides habitat that is of as high a quality as their natural forests.”

Fleshing out those ecological parameters, however, requires a better basic understanding of sloth behavior, knowledge the CALS researchers are now beginning to accumulate.

For example, in a study recently published in Animal Behavior, Pauli and Peery described the mating system of Hoffmann’s two-toed sloths and showed that, unlike many other animals, the females tend to disperse from their home range and that the breeding territories of males can slightly overlap, with males tolerating competitors on the fringes but excluding them, sometimes violently, from the core. And Hoffmann’s two-toed sloths of both sexes seem to have multiple partners as well. “They’re more promiscuous than previously thought,” says Pauli. “We see a much more flexible system of multiple matings.”

That’s not so for the three-toed sloth. In another study, published in PLoS ONE in December, they found that three-toed sloths are strongly polygynous—males exclude other male competitors and mate with many females.

In addition to contributing to basic sloth knowledge, these findings should help wildlife and land managers in the Neotropics make sound decisions to better balance development and conservation.

“Understanding how shade-grown agriculture can benefit sensitive tropical animals such as sloths is highly relevant, considering the ongoing and rapid loss of biodiversity in the Neotropics,” notes Pauli. “What kinds of ecological services can these already altered landscapes provide? Can we mitigate future biodiversity loss with a greater emphasis on shade-grown agricultural systems than crops grown in monocultures? That’s the future we’re facing.”

Because of their sedentary nature and dependence on forest, sloths can be viewed as an “umbrella species,” says Peery. “Protecting sloths could indirectly protect many other animal species in tropical forests that are harder to measure and study.”