The two-toed sloth is a walking hotel. The animal is so inactive that its fur acts as an ecosystem in its own right, hosting a wide variety of algae and insects. But the sloth has another surprise passenger hitching a ride inside its body, one that has stayed with it for up to 55 million years – a virus.

In the Cretaceous period, the genes of the sloth’s ancestor were infiltrated by a “foamy virus“, one of a family that still infects humans, chimps and other mammals today. They are examples of retroviruses, which reproduce by converting an RNA genome into a DNA version and inserting that into the genome of whatever animal they’re infecting. If these hitchhikers