The Sloth Is an Ecosystem: Algae, Moths and a Fur Coat That Grows Backwards

Look closely at a sloth's outer hair and you will find it cracked with fine longitudinal grooves. Those grooves trap rainwater and hold it, and in a humid canopy that means every sloth is carrying a set of tiny reservoirs. Green algae colonise them. The animal turns faintly green, which is a useful thing to be when your main defence against eagles is being mistaken for a clump of leaves.

The algae attract sloth moths, several species of which live nowhere else. When the sloth makes its weekly descent to defecate, female moths drop off and lay eggs in the dung. Their larvae develop there, and the emerging adults fly up into the canopy to find a sloth of their own. Moths dying in the fur are thought to add nitrogen, which fertilises the algae, which the sloth has been observed grooming and eating — a small nutritional bonus from a garden it grows on itself.

The fur runs the other way to almost every other mammal's: from the belly towards the spine. That is exactly right for an animal that spends its life hanging upside down, because rain then runs off the back rather than pooling along it.

Add specialised beetles and mites and the total is startling. Individual three-toed sloths have been recorded carrying nine hundred or more moths and beetles at once. The animal is not merely slow. It is inhabited.