The relationship between sloths and moths is known as mutualism, where two species mutually and closely benefit from each others' lifestyles. Other examples include cleaner fish that eat parasites off of reef fish, and ants that defend acacia trees.
In this case, the moth presence could be a lifesaver for sloths.
Adriano Chiarello, a conservation biologist at the University of São Paulo, found that sloths are strict arboreal herbivores -- meaning plant-eating tree dwellers -- that primarily eat leaves, "survive on an energy-poor diet," and have "a very long passage time of digesta." In other words, their metabolism is so slow they can wait a week before pooping.
Pauli said the sloths groom each other while in the trees. While doing so, they ingest the algae, which provides the much-needed nutritional boost.
It's a tightly structured system without much flexibility.
Pauli said less than .2 percent of all animals are arboreal herbivores. Another well-known example is the koala.
"Herbivores (like cows) tend to be very big because they often carry within themselves a ton of machinery to hold and emulsify their food," he said. "On the other hand, life as a prey animal in trees necessitates a smaller body size, so arboreal herbivores are caught between these two extremes and have a constrained lifestyle as a result."