Forests cover more than one-third of the land on Earth, yet few vertebrates make the canopy their home, and even fewer subsist solely on a diet of tree leaves.
In a new study in American Naturalist, researchers from the University of Wisconsin – Madison explain why this lifestyle is so rare and why animals that live in trees and eat leaves tend to live life at a slower pace.
Those species that do take advantage of this niche do not often radiate afterwards; that is, they don’t diversify and take on a variety of specialized forms. The energetic constraints of a leafy diet are thought to prevent such adaptive radiation.
Leaves are an energetically and nutritionally poor food source. Most animals that live off plant leaves tend to be large, such as moose, elk, and deer.
“Leaves are everywhere, but you need pretty complex gut machinery to be able to extract energy and nutrients from them,” says Jonathan Pauli, one of the study’s authors. “Most herbivores are big-bodied and they carry around big guts to break down and detoxify plant leaves.”
But animals that live in the treetops cannot be too big, or else the branches won’t support their body weight. So how do they make it on a nutritionally challenging diet?
Pauli and Zachariah Peery, along with co-authors Emily Fountain and William Karasov, set out to answer this question by measuring the daily energy expenditure of both two-toed and three-toed sloths in Costa Rica.
Both species of sloth are at the extreme end of specialization for a tree-dwelling, leaf-eating, lifestyle. Pauli, Peery, and colleagues found that both sloth species expended very little energy, but three-toed sloths were especially slothful. Three-toed sloths expended as little as 460 kilojoules of energy a day, the equivalent of burning only 110 calories. It is the lowest measured energetic output for any mammal.
Three-toed sloths use both behavioral and thermal strategies to limit their energy output. “They really are a slothful bunch,” says Pauli. “While two-toed sloths have bigger home ranges and move around quite a bit, three-toed sloths have very small home ranges and spend most of their time in just one or a few individual trees. To limit energy costs, three-toed sloths find a good tree and camp out for a while and eat from it.”
Pauli, Peery, and colleagues concluded that much of the difference in metabolic rate between two-toed and three-toed sloths is due to regulation of body temperature. Three-toed sloths relax control over their body temperatures, letting them fluctuate quite a bit for a mammal. They often ascend to the top of the canopy in the morning, presumably to warm in the sun, and descend into the shade as daytime temperatures increase.
The researchers then compared their sloth data to similar studies of other tree-dwelling, leaf-eating species from around the world. Overall, the more specialized for the niche an animal was, the lower its daily energy expenditure. While these species had lower metabolic rates than most mammals in general, they also relied heavily on thermoregulation and behavioral strategies to reduce their energetic expenditure.
The findings support the idea that tree-dwelling, leaf-eating mammals are tightly constrained by the poor nutritional quality of their diet, and thus, exhibit extremely low energetic output.
“Arboreal folivores have all these oddities – anatomical, behavioral, and physiological – that enable them to exploit this lifestyle,” says Pauli. “One of the ways they are able to survive in this energetically stark niche is they have evolved a whole suite of adaptations. They require all these unique adaptations to live in the trees and survive solely on leaves.”
The researchers believe this impedes the opportunity for organisms to rapidly radiate into this niche. For tree-dwelling, leaf-eating animals, there is a whole series of key innovations that are needed before they can crack into that open niche.
Sloths are the poster children for making a living in the treetops by saving energy. For them, slothfulness is a necessary virtue, not a deadly sin.