Written by Veronique Greenwood
Look closely up in the trees of a shade-grown cacao plantation in eastern Costa Rica, and you’ll see an array of small furry faces peering back at you. Those are three-toed sloths that make their homes there, clambering ever so slowly into the upper branches to bask in the morning sun. You might also spot them munching on leaves from the guarumo tree, which shades the cacao plants.
Scientists have long known that this tree is important to the diets of sloths. Its foliage is highly nutritious, available all year and easy for the creatures to digest. But in a new study published Tuesday in Proceedings of the Royal Academy B, researchers report that a population of sloths with more guarumo trees in their cacao plantation habitat had more babies and were more likely to survive.
Their findings suggest that the tree’s presence can help ensure the health of sloth populations even in environments that have already been disturbed by humans, like farms. It also shows how animals that have a specialized ecological niche, while traditionally thought of as vulnerable, can persist in changed circumstances as long as the resource that they depend on is available.
For almost 10 years, Jonathan Pauli and M. Zachariah Peery, professors at the University of Wisconsin, and their colleagues have tracked a group of sloths in Costa Rica. The animals are equipped with radio collars that transmit their location five or six times a month, so the team knows where each sloth’s usual territory is. The team has also taken DNA samples and figured out the sloths’ family tree, so they can tell which individuals are having the most babies.
When Mario Garcés-Restrepo, the paper’s lead author, grew interested in whether what the animals were eating could be linked to their reproductive success, the groundwork to answer the question was already in place.
Three-toed sloths are not known for their dietary flexibility — they tend to rely on a small handful of plants. But while they usually live in tropical rain forests, sloths can manage surprisingly well in other environments as long as the right variety of trees and nearby streams are available.
Garces-Restrepo mapped the numbers of various trees in about 40 sloths’ favored territories and checked to see whether there was a correlation between the density of a given tree species and an animal’s number of offspring and survival over the course of the study.
While the survival of juvenile sloths was not linked to guarumo, the trees appeared to be important for the group’s adults. The five adults who died over the course of the study had markedly lower numbers of guarumo in their area than those that survived. At the same time, both male and female adults with more guarumo available to them had more offspring, the males in particular. That may be not only because of the nutrition the tree offers, but the visibility, as lanky guarumo trees have open structures with plenty of places to see and be seen.
“Sloths are often seen sunbathing in the mornings,” Pauli explained, and if they are attracting mates by calling to them or making themselves visible, he said, “being in open trees might actually enhance those reproductive opportunities.”
This improved understanding of what the sloths need to thrive could be important in this area and others where they live. In northeastern Costa Rica where the study was completed, the area around the cacao plantation is shifting. “The landscape is changing,” said Pauli, with more fields of pineapple being planted, which grows only in open, unforested spaces. There may be fewer places for sloths there in the future.