There are two groups of sloth: two- and three-toed. Whilst two-toed sloths roam widely, foraging in the South American jungle with ranges that can stretch up to 140 hectares, their three-toed counterparts range on average just 5.4 ha. Three-toed sloths are fussier eaters and spend most of their time resting high in the jungle canopy eating foliage from a limited selection of tree species. The three-toed sloth emerges rarely, descending its tree only once a week in a risky journey to defecate at its base.

Three-toed sloths are one of just 10 mammal groups that live in trees and dine exclusively on foliage. Their leafy diet provides few digestible nutrients so sloths have a very slow rate of digestion and a very low metabolic rate. Climbing down their tree can cost 8% of their meagre energy budget and also puts sloths at heighted risk from predators like the jaguar. Scientists questioned why the sloths make this perilous descent rather than defecating from the canopy which the two-toed sloth has been known to do. The researchers behind the study said the costly and risky behaviour was ‘inexplicable’ so set about solving the mystery of the sloths’ dangerous endeavour.

Sloth fur harbours a diverse range of arthropods and algae. Some moth species, including Cryptoses Choloepi Dyar, are known to colonize sloth fur exclusively. When a sloth climbs down their tree female moths lay their eggs in the fresh sloth dung. From this nursery adult moths emerge and fly to the canopy to mate in the sloths fur. The scientists hypothesized that the moth, whose lifecycle is entirely dependent on the sloth descending from its tree, must offer an important nutritional benefit to the sloth in exchange for its baffling risk taking.

The researchers captured two- and three-toed sloths and compared the number of moths inhabiting their fur. They also measured the biomass of the algae growing on the sloth and the amount of inorganic nitrogen in the fur- an important nutrient for algae growth. The team found that sloths with more moths colonising their fur also had more nutrient-rich fur and more algae growth. The researchers found that three-toed sloths had significantly more moths, nutrients and algae growth in their fur than two-toed sloths harboured in theirs.

The researchers investigated if the algae growth was the missing nutritional supplement in sloths’ diets. Testing the algae they found that it was not only digestible but also lipid-rich and so would be an advantageous addition to a sloth’s leafy diet. The scientists suggest that sloths cultivate the algae in their fur by aiding the moths’ colonization. The moths act as ‘portals’ for nutrients say the scientists, increasing the nitrogen content of the fur to help grow nutritional algae.

The team, led by Dr Jonathan Pauli, suggest the complex relationship between moths and sloths could explain how sloths have become successful arboreal herbivores eating only nutritionally poor foliage. The mutually beneficial relationship has ‘locked’ the three-toed sloth into an ‘evolutionary trade-off that requires it to face increased predation risk in order to preserve linked mutualisms,’ say the researchers whose work solves the mystery of why sloths defecate in the most dangerous part of the forest.