Arboreal Folivores Limit Their Energetic Output, All the Way to Slothfulness

By exploiting unutilized resources, organisms expand into novel niches, which can lead to adaptive radiation. However, some groups fail to diversify despite the apparent opportunity to do so. Although arising multiple times, arboreal folivores are rare and have not radiated, presumably because of energetic constraints. To explore this hypothesis, we quantified the field metabolic rate (FMR), movement, and body temperature for syntopic two- and three-toed sloths, extreme arboreal folivores that differ in their degree of specialization. Both species expended little energy, but three-toed sloths (162 kJ/day*kg0.734) possessed the lowest FMR recorded for any mammal. Three-toed sloths were more heterothermic and moved less than two-toed sloths. We then compared FMRs and basal metabolic rates (BMRs) for 19 species of arboreal folivores along a spectrum of specialization. Overall, arboreal folivores had lower BMRs and FMRs than other mammals, and increasing specialization led to lower FMRs but not BMRs. Thus, reduced energetic expenditure in specialized species was the result of thermoregulatory and behavioral strategies, rather than simply a proportionate reduction in BMR. Altogether, our findings support the concept that arboreal folivores are tightly constrained by nutritional energetics and help to explain the lack of radiation among species exploiting a lifestyle in the trees.