Three-toed sloths descend at intervals of about eight days to defecate at the foot of a tree, scraping a hollow in the soil and covering the dung. On the ground a sloth is slow and poorly defended, which is why the trip has long been treated as a costly habit that must buy something in return.
Several explanations have been offered: fertilising the tree, covering the smell, chemical signalling between individuals, picking up trace nutrients on the claws, and supporting the moths that live in sloth fur. The moth version is the best known. Sloth moths lay eggs in the dung, moth-rich coats carry more nitrogen and more algae, and the sloth eats some of that algae.
None of this is settled. A more recent argument holds that predation during defecation is in fact rare, and that ground defecation simply persists from ancestral sloths for want of pressure to abandon it. Whether fur algae contribute meaningful nutrition is also disputed. Two-toed sloths, which host fewer moths, descend less consistently.