“Three-toed sloths can TURN THEIR HEADS almost ALL THE WAY AROUND. Thanks to an extra NECK VERTEBRA.”

National Geographic Kids, 5,000 Awesome Facts, sloth #19

Most mammals have 7 vertebrae, such as humans, whereas three-toed sloths have eight or nine located at the base of their neck next to their rib cage. Those extra vertebra allow the three-toed sloth to turn it’s head almost all the way around, at 270 degrees in either direction. Basically three quarters of motion.

Two-toed sloths have between 5-7 vertebrae, limiting their range of neck motion.

Owls also have extra vertebrae in their neck, twice the amount of humans at a total of 14. Both the owl and the three-toed sloth can turn their head at 270 degrees, making them look as if they can follow you with their eyes. 

sources: https://asknature.org, Wikipedia