“Arboreal folivores limit their energetic output, all the way to slothfulness”

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Jonathan N. Pauli, M. Zachariah Peery, Emily D. Fountain, and William H. Karasov

To navigate a life in the trees, sloths (and other arboreal folivores) possess extraordinarily low metabolic rates

The brown-throated three-toed sloth (Bradypus variegatus) has the lowest field metabolic rate recorded for any mammal.
(© 2011 Jeff Whitlock, The Online Zoo, used with permission)

Even though two-thirds of terrestrial earth is forested, very few vertebrates live up in the canopy and subsist solely on tree leaves. The rarity of this lifestyle and lack of species diversification into it has been hypothesized to be a consequence of energetic constraints. Tree sloths – two- and three-toed sloths – typify arboreal folivores, having characteristically slow life histories, unique anatomical and physiological adaptations for a nutritionally poor diet, and reduced metabolic rates.

In “Arboreal folivores limit their energetic output, all the way to slothfulness,” ecologists from University of Wisconsin-Madison quantified the daily energy expenditure (via the use of isotopically labeled water) at a field site in northeastern Costa Rica for two species of cooccurring tree sloths and monitored their daily movement, habitat use, and body temperature. They found that, regardless of habitat, the more specialized three-toed sloth moved less, exhibited greater variation in body temperature, and had lower daily energy expenditure. While both species of sloth expended little energy, the three-toed sloths had the lowest energy expenditure recorded for any mammal. The authors then compared previous estimates of energetic expenditure recorded for different species of free-living arboreal folivores around the world, and found that increasing specialization led to lower daily energy expenditure. Reduced energy demands appeared to be the result of behavioral strategies and savings from relaxing controls on body temperature.

Altogether, their findings reinforce the concept that arboreal folivores are tightly constrained by nutritional energetics; these findings also reveal strategies used to minimize energetics costs and support the notion that the suite of extraordinary adaptations needed to survive in a stark energetic landscape of forest canopies may explain the lack of species diversification among arboreal folivores. Read the Article