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Sloth's arboreal niche explains its slow-paced lifestyle

The three-toed sloth burns just 110 calories each day.

By Brooks Hays
Sloth's arboreal niche explains its slow-paced lifestyle
Researchers say the sloth's sluggishness is the unavoidable result of adapting to a life in the trees and a leaf-only diet. Photo by Zach Peery/University of Wisconsin

MADISON, Wis., July 20 (UPI) -- Sloths are one of only a few mammal types to carve out an exclusively arboreal niche. And new research suggests it's the sloth's tree-based lifestyle that explains its sluggish disposition.

"Among vertebrates, this is the rarest of lifestyles," Jonathan Pauli, a professor of forest and wildlife ecology at the University of Wisconsin, explained in a news release. "When you picture animals that live off plant leaves, they are almost all big -- things like moose, elk and deer. What's super interesting about arboreal folivores is that they can't be big."

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Recently, Puali and a trio of Wisconsin researchers, M. Zachariah Peery, Emily Fountain and William Karasov, ventured into the rainforests of northeastern Costa Rica to measure the energy expenditures of wild two- and three-toed sloths.

Their aim wasn't to explain the slowness of the sloth, specifically, but to explain the scarcity of arboreal folivores in the animal kingdom. Trees are everywhere, after all.

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Their findings confirm what previous studies have found -- that sloths have no choice but to slow way down. Leaves are a poor source of energy. Yet, sloths eat them exclusively. For most species, it isn't worth it to spend evolutionary capital on such a crummy food source.

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"The food sucks," Pauli explained. "It's only plant leaves. You have to exploit a very constrained niche."

The paucity of calories in a tree-leaf-only diet demands extreme sluggishness. Researchers found that the three-toed sloth burns just 110 calories each day. It can't afford to use any more.

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When compared with the results of similar studies, the new findings -- published this week in the journal The American Naturalist -- show that the most specialized tree-living, leaf-eating species expend the least energy.

In other words, the more specialized the arboreal folivore, the slower the arboreal folivore.

The findings reinforce the stark reality of a tree-based lifestyle -- it's really hard.

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"The findings reinforce the concept that arboreal folivores are tightly constrained by nutritional energetics," Pauli concluded. "It takes a suite of extraordinary adaptations to survive in forest canopies, and this may help explain the lack of species diversification among arboreal folivores."

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