Spider Monkey

(Ateles fusciceps)

I. DESCRIPTION: Head and body length 15 to 25 inches, tail 20 to 35 inches. Weight 12 to16 lbs. (6 to 8 kilograms). Hair generally coarse and stringy. Lacks underfur. Colors: golden, red, buff, brown or black, with hands and feet generally black. Hands are like hooks with long, narrow palms, long curved fingers, and no thumb. Exceptionally long limbs and tail in relation to body. Tail is prehensile. Head is small, muzzle prominent (but brain capacity, 107 grams, is twice that of same-sized howler monkeys). Clitoris of the female is greatly elongated.

II. GEOGRAPHICAL RANGE AND HABITAT: Tropical forests from southern Mexico, south to Bolivia and the Matto Grosso in Brazil. Range from sea level to higher ground.

III. DIET: Fruits, 72%; young leaves (carefully selected), 22%; flowers, 6%. Characterized by rapid passage of food through the digestive system (4 hours).

IV. LIFE CYCLE/SOCIAL STRUCTURE: Menstrual cycle: 24-27 days. Gestation: 139 days. Young carried on mother's abdomen for about four months, then carried on the back. Infants use prehensile tail to hold onto mother's tail. Infant is black for six months, during which time it is entirely dependent upon its mother. Adult coloration completely acquired by 10 months. Life expectancy is 25 years. Associate in groups of 2 to 8 individuals; groups are units of a main band which usually consists of 15 to 25 animals. Each band inhabits an area of forest centering around food supplies and sleeping trees. When approached in the wild, spider monkeys scatter. Most frequently heard call resembles whinnying of a horse-probably a "greeting" call. Female groups with offspring are the most cohesive units in an otherwise loose fusion/fission group. High ranking individuals groom more than they are groomed.

V. SPECIAL ADAPTATIONS: Diurnal. Only gibbons exceed spider monkeys in agility in the trees. Normal movement through trees is along upper surfaces of limbs with tail arched over the back. Tail used as fifth limb. Able to grasp objects with single hand, foot, or tail alone. Basically quadrupedal, but capable of rapid progression with legs extended by swinging below branches using arms and tail. Rarely descend to the ground, but capable of bipedalism on the ground with tail held in a curve parallel to the back.

VI. INTERPRETIVE INFORMATION: End of tail is extremely flexible, forming a "hand" (with skin and grooves). Thumbs on feet only. There are no ground-dwelling primates in the "New World." Elongated clitoris is sometimes mistaken for a penis; its function is unknown.

VII. STATUS IN WILD: Some populations threatened by habitat destruction. Two subspecies of spider monkeys in Costa Rica an Nicaragua are listed as endangered by the U.S. Fish & Wildlife Service.

VIII. EVOLUTION: Part of the "radiation" of New World monkeys about 33 mya in late Oligocene. Milton (1) feels large brain selected because of maintaining a diet centered on ripe fruit. This would also explain fusion/fission social organization.