Like many Neotropical fauna, sloths are running out of room to maneuver.
As forests in South 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, a source of chocolate, could become critical refuges and bridges between intact forests for the iconic animals.
In an ongoing study in Costa Rica, wildlife biologists from the University of Wisconsin-Madison are using a complex of intact tropical forest, pasture, 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 Jonathan Pauli, a UW-Madison assistant professor of wildlife ecology who, with colleague Zach Peery, has established a sloth study on a private cacao farm in rural Costa Rica. "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 now being peeled away as studies by the Wisconsin team of both the brown-throated three-toed sloth and Hoffmann's two-toed sloth, two fairly common species, are yielding new insights into their mating habits and how the animals transit the landscape.
The fact that sloths require forested habitat and are sedentary makes them vulnerable to the deforestation common to many parts of Central and South America, notes Peery, also a UW-Madison assistant professor of wildlife ecology. "Once a tract of tropical forest has been cleared, sloths have relatively little capacity to seek out new habitats."
The setting Pauli and Peery are using to study sloths is increasingly representative of the Central American landscape. It is a mix of tropical forest, pasture, banana and pineapple plantations with a large organic cacao operation as a hub. As far as sloths go, the fields where bananas and pineapples are grown may as well be deserts, Pauli says: "Sloths don't go there. They just don't move through it."
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 for the two- and three-toed sloths.
"Because of the diverse overstory of native trees, the cacao farm appears to provide excellent habitat for both species of sloths," explains Peery.
Sloths also turn up in the few relic trees in pastures adjacent to the cacao farm, and the Wisconsin researchers hope to find out if the animals are using that habitat as spillover. "And then, of course, 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 of a quality as their natural forests," Peery says.
While Pauli and Peery are conducting some of the baseline research needed to understand the behavioral and ecological proclivities of the two sloth species, they also seek to inform how land use in the Neotropics can affect animals like sloths that depend on trees for their survival.
The questions we hope to address, says Pauli, relate to the landscape -- the forests, pastures and cacao farms and how the animals exploit the available habitat. "How do they compete in these environments? What are the limits to the resources? How do they partition into these different kinds of habitats?"
To flesh out sloth ecological parameters, however, requires a better basic understanding of sloth behavior, knowledge the Wisconsin group is now beginning to accumulate.
For example, in a study to be published in the journal Animal Behavior in September, Peery and Pauli describe the mating system of Hoffmann's two-toed sloth and show 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," notes Pauli. "We see a much more flexible system of multiple matings."
In addition to contributing to the store of basic sloth knowledge, the work of the Wisconsin researchers should help wildlife and land managers in the Neotropics make sound decisions to better balance development and conservation.
"Beyond the basic science, 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 their 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."
The curious thing about sloths, which are widespread in both Central and South America, is that they are highly successful in intact forests despite their iconic lackadaisical mode of locomotion.
"It hardly seems like a recipe for success and, in fact one 18th century biologist wrote that sloths were 'one defect away' from disappearing from the Earth altogether," Peery relates. "Clearly that biologist was wrong as sloths are an extremely successful taxon. Studying the reason why is fascinating."
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