Spider monkeys are New World monkeys of the family Atelidae, subfamily Atelinae. Found in tropical forests from southern Mexico to Brazil, spider monkeys belong to the genus Ateles; the closely related woolly spider monkeys, are in the genus Brachyteles.
As they require large tracts of undisturbed forest and specialize on ripe fruits, spider monkeys may be considered an indicator species; the monkeys are threatened by habitat destruction through continued growth in South American agriculture.
A recent comparative intelligence study gives spider monkeys a value a little above gorillas, so it is reasonable to believe that spider monkeys are among the most intelligent New World monkeys.
Disproportionately long, spindly limbs inspired the spider monkey's common name. Their deftly prehensile tails, which may be up to 89 cm (35 inches) long, have very flexible, hairless tips and skin grooves similar to fingerprints. This adaptation to the spider monkey's strictly arboreal lifestyle serves as a fifth hand. Adults reach an average body length of 50 cm (20 inches) and a weight of 6.4 kilograms (14 pounds).
The arms are very thin and very long, while the legs are shorter. When the monkey walks, its arms practically drag on the ground. Unlike many monkeys, they don't use their arms for balance when walking, instead relying on their tail. The hands are long, narrow and hook-like, and have no thumbs. The fingers are elongated and recurved.
The hair is coarse, ranging in colour from ruddy gold to brown and black; the hands and feet are usually black. Heads are small with hairless faces. The nostrils are very far apart, which is a distinguishing feature of spider monkeys. The unusually long labia in females may be mistaken for a penis; its function is unclear.
Spider monkeys are highly agile; they are said to be second only to the gibbons in this respect.
Spider monkeys form loose groups of 15-25 individuals. During the day, groups break up into subgroups of 2-8 individuals. This social structure ('fission-fusion') is found in only one other primate species, the Chimpanzee. The size of subgroups and the degree to which they avoid each other during the day depends on food competition and the risk of predation. Also less common in primates, females rather than males disperse at puberty to join new groups. Males tend to stick together for their whole life. Hence males in a group are more likely to be related and have closer bonds than females. The strongest social bonds are formed between females and young offspring.
Spider monkeys communicate their intentions and observations using postures and stances, such as postures of sexual receptivity and of attack. When a spider monkey sees a human approaching, it barks loudly similar to a small dog. When a monkey is approached, it climbs to the end of the branch it is on and shakes it vigorously to scare away the possible threat. It shakes the branches with its feet, hands, or a combination while hanging from its tail. It may also scratch its limbs or body with various parts of its hands and feet. Seated monkeys may sway and make noise. Males and occasionally adult females growl menacingly at the approach of a human. If the pursuer continues to advance, the monkeys often break off live or dead tree limb weighing up to 10 pounds and drop them towards the intruder. They do not actually throw the branches but they twist so the branch causing it to fall closer to the threat. The natives of the area know very well of this risk. The monkeys also defecate and urinate toward the intruder.
Spider monkeys are diurnal and spend the night sleeping in carefully selected trees. Groups are thought to be directed by a lead female who is responsible for planning an efficient feeding route each day. Grooming is not as important to social interaction, due perhaps to a lack of thumbs.
At 107 grams, the spider monkey brain is twice the size of a howler monkey's of equivalent body size; this is thought to be a result of the spider monkeys' complex social system and their frugivorous diet, which consists primarily of ripe fruit from a wide variety (over 150 species) of plants and this requires the monkeys to remember when and where fruit can be found. The slow development may also play a role: the monkeys may live 20 years or more, and females give birth once every 3-4 years.
Spider monkeys have a unique way of getting food: a lead female is responsible for feeding. If she cannot find enough food for the group, it splits into smaller groups to find food easier. The traveling groups have four and nine individuals. Each group is closely associated with its territory. If the group is big, it spreads out while eating.
Till age six to ten months, infants rely completely on their mother. Males are not involved in raising the offspring.
A mother carries her infant around her belly for the first month after birth. After this she carries it on her lower back. The infant wraps its tail around its mother’s and tightly grabs her midsection. Mothers are very protective of their young and are generally good mothers. They have been seen grabbing their young and putting them on their backs for protection and to help them navigate from tree to tree. They help the more independent young to cross by pulling branches closer together. Mothers also groom their young.