New Study Showcases Three-Toed Sloth’s Unsung Adaptability

A new analysis of three-toed sloths’ survival in the forests of Costa Rica offers two-fold insights for scientists hoping to aid the tree-dwelling critters.

As Veronique Greenwood explains for The New York Times, researchers from the University of Wisconsin-Madison juxtaposed data on the animals’ reproductive success and longevity with the presence of leafy Cecropia, or guarumo, trees, which are known for providing shade to cacao plants and serving as sloths’ favorite treat. The team found that adult sloths living in guarumo-heavy habitats enjoyed higher survival and birthing rates, suggesting the tree’s prevalence can have a powerful effect on sloth populations’ survival.

At the same time, biologist Jan Hoole of England’s Keele University writes for the Conversation, the scientists also discovered that the density of guarumo trees in a certain area had no correlation with the survival of juvenile sloths. Instead, the researchers report in Proceedings of the Royal Society B, young sloths often abandoned the nutrient-rich tree in favor of lower quality saplings, trading in a rich diet for the protection afforded by such trees’ denser leafy coverings.

Combined, these two lines of inquiry paint a complex yet promising vision of sloths’ future in the wild. According to Greenwood, adult sloths’ reliance on guarumo trees indicates that the tree’s presence can foster the growth of populations living in human-disturbed habitats, even as evolving circumstances threaten to otherwise upset the ecosystem’s balance. And juveniles’ embrace of other tree species further speaks to the three-toed sloth’s perseverance: As Hoole points out, the fact that the creatures can branch out from their preferred diet means they are more adaptable than previously believed.

To gauge connections between sloths’ eating habits and biological success, study authors Mario Garcés-Restrepo, Jonathan Pauli and M. Zachariah Peery turned to a group of Costa Rican sloths equipped with location-transmitting radio collars. Lead author Garcés-Restrepo mapped the density of various trees across 40 areas of the animals’ habitat, then waited to see if the presence of a certain plant species correlated with sloths’ number of offspring and longevity.

All five adult sloths who died over the course of the study frequented areas with significantly fewer guarumo trees, Greenwood writes for The New York Times. Adults living in areas with more trees also went on to procreate more than their guarumo-bereft counterparts. Male sloths were particularly attuned to the presence of the plant species. It’s possible that this boom in reproductive activity stems not only from the nutritive value offered by the tree, but the visibility afforded by the open nature of its canopic covering. According to the Conversation’s Hoole, sloths have very poor vision, meaning they move to areas with higher visibility—and subsequently stronger chances of successfully spotting and wooing a fellow tree-dweller—during mating season.

Ironically, juvenile sloths’ avoidance of guarumo trees springs from their need for camouflage, as they are poorly equipped to fight off predators such as jaguars and eagles. The same logic applies to mother sloths, with new parents sometimes settling in trees with thicker canopies until their babies mature.

The implications of the new study essentially come down to habitat preservation and conservation efforts. Incorporating guarumo trees into so-called “agro-forestry” efforts in Brazil and Costa Rica could help scientists better sustain these regions’ sloth populations. Hoole explains that this deliberate agricultural method is used to grow cocoa trees, which are typically planted underneath a layer of native forest trees.

But guarumo trees aren’t sloths’ only path toward long-term survival: As the juveniles’ reliance on other tree varieties shows, these “specialized” herbivores are capable of surviving—and even thriving—on a more diverse diet than simply guarumo leaves.

Meilan Solly | | READ MORE

Meilan Solly is Smithsonian magazine's associate digital editor, history.