A sloth can’t vomit. It has a one-way throat, handy for eating while dangling upside down by the toes. But the animal has to be careful not to poison itself by nibbling too many toxic leaves that it can’t easily purge.
There are upsides and downsides to evolving a body that can hang toes-up for some four to six hours a day, as the three-fingered sloth does, says sloth biologist Rebecca Cliffe of the Sloth Sanctuary of Costa Rica near Limón.
She calls her sloths “three-fingered” instead of the more common “three-toed” because all sloths, even so-called two-toed species, actually have three toes on their hind legs. It’s the digits of the front limbs, the fingers, that number either two or three.
One reason both two- and three-fingered sloths have ended up up-ended is that it helps them carry out their fastidious grooming for several hours a day. “They like to scratch with both hands,” Cliffe says.