Although most of the terrestrial world is covered in trees, there are precious few vertebrates that make the canopy their home and subsist solely on a diet of leaves.Tree sloths are among the most emblematic tree-dwelling mammals. However, they are best known for their pokey demeanor rather than the fact that they spend the majority of their lives in trees munching leaves. But the slow motion lifestyle of tree sloths, according to a new study, is the direct result of the animal’s adaption to its arboreal niche.“Among vertebrates, this is the rarest of lifestyles,” says Jonathan Pauli, a University of Wisconsin—Madison professor of forest and wildlife ecology and the senior author of a report to appear in the August 2016 edition of the American Naturalist. “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.”