Squirrel monkeys are seasonally sexually dimorphic (with two distinct genders). Males get larger during the breeding season. The upper parts of the body are a grayish olive color, the lower parts are white, and there is a wash of yellow or orange on the limbs. The tail is black-tipped. They have black on the muzzle and head with white around the eyes, creating a mask. This facial mask is either characterized as having a "Roman" or "gothic" arch shape. Their short, well-developed thumbs are slightly opposable and their tail is slightly prehensile (able to grasp) and can be slightly used for support.