The Humboldt's woolly monkey has a prehensile tail, which is lacking hair on the ventral side. This species has relatively enlarged lower incisors (Eaglen, 1984; cited in Peres, 1994) and bunodont molar crowns (Rosenberger and Strier, 1989; cited in Peres, 1994). The forms of sexual dimorphism for this species are the canines of the males being longer than that of the females, the body and head length of the male is longer than that of the female, and the mid-ventral fringe is longer, thicker, and darker in males as compared to females (Ramirez, 1988). The tail of the female, however, is usually longer than the tail of the male (Ramirez, 1988). Adult males also have well-developed bumps on both sides of the head (Nishimura, 1994). The females' clitoris is as long or longer than the males' penis, which makes identification in the wild difficult. The average body mass for this species is 7.5 kilograms (Peres, 1990). Humboldt's woolly monkey has a pelage color that can be dark brown, pale smoky brown, dark smoky gray, pale gray, red-brown, or olivaceous on the dorsal side and dark on the ventral side (Emmons, 1997). The limbs, tail, and/or head is usually darker than the back (Emmons, 1997). The face of this species is black (Emmons, 1997). The infant pelage color is paler than that of the adult's and the palms of the hands and feet are a deep pink color (Ramirez, 1988).