Sakis are known for their long, coarse hair. It has the ability to piloerect—that is, tiny muscles attached to the hair follicles can raise to make them appear bigger and even more fluffy. This is the same mechanism that causes dogs’ hackles to rise, porcupines to raise their quills, and humans to develop goosebumps. 

They are quite obviously sexually dimorphic, which means that males and females look different. Males are inky black over most of their body—sometimes they even have a purplish hue in the right light. They are named for the males’ prominent white head, which sticks out like a sore thumb against their dark bodies. Towards their face, the white hair darkens to a rusty orange.

Females, on the other hand, are gray over their entire body, with highlights of brown and orange. Females also have a pair of white or light tan stripes on either side of their nose and mouth, resembling a pair of parentheses around their muzzle. The white hair on a male’s head is parted in the middle, whereas females have more of a bowl-cut “hairstyle”. 

Both sexes have a triangle of bare black skin in the middle of their face. Their noses are rather unique in appearance, with their nostrils pointing out to the sides. At some angles, they almost appear noseless. Saki’s hands are rather unique in appearance. Their fingers are long with swollen alien-like finger pads. All five of their fingers point straight out, their thumb appearing just like their other fingers, and there is a small gap between their index and middle fingers. This is because sakis actually grip between their index and middle fingers, not between their thumb and index finger like we might expect. Their long tails are not prehensile, which means that they cannot grasp. Instead, their tail is used as a counterbalance as they move about in the trees.