Brown-throated sloth

This sloth was spotted in the Peruvian Amazon.

Sloths are famous for being slow. They aren’t very active, and only bother to climb down from their tree once a week (to make some fertiliser). Sloths move slowly because they have to conserve energy; the plant leaves they eat are low in calories. They also use the sun’s energy to warm up their blood – this is why they sunbathe often.

The sloth has been criticised for moving so slowly that algae grows on it. This isn’t quite true. Algae does grow on the sloth, but it’s almost certainly meant to be there (as much as anything is meant to be in nature).

Sloths participate in a three-way mutualistic relationship with algae and moths. To begin with, sloth fur is coarse with lots of cracks, which encourages the growth of microorganisms like algae. Each week when the sloth climbs down and defecates, the sloth moths lay their eggs in the dung. When the larvae mature they fly back to the sloth, where their predecessors are. These moths help to fertilise the algae that grows in the sloth’s fur (possibly by bringing back a little bit of dung with them).

The sloth gains a food supplement (they eat the algae), and some camouflage when there’s enough of the algae to tint the fur green. The moths gain a home – sloth moths only ever live on sloths – and possibly food from the algae. And the algae gains a place to live, and continuous nourishment. Everyone wins.

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