Black-handed spider monkeys (Ateles geoffroyi) don’t have thumbs, but what they do have is the most mobile and dextrous of all primate tails! The underside of the tail is covered with ridges that help them grip branches as they swing.

[Photo by Grace Davis and caption by Maggie Creamer]

Reference

Mittermeier, R. A. (1978). Locomotion and posture in Ateles geoffroyi and Ateles paniscus. Folia Primatologica, 30(3), 161-193.