Spider monkeys are among the largest New World monkeys; black-headed spider monkeys, the largest spider monkey, have an average weight of 11 kilograms (24 lb) for males and 9.66 kg (21.3 lb) for females. Disproportionately long, spindly limbs inspired the spider monkey’s common name. Their deftly prehensile tails, which may be up to 89 cm (35 in) long, have very flexible, hairless tips and skin grooves similar to fingerprints. This adaptation to their strictly arboreal lifestyle serves as a fifth hand. When the monkey walks, its arms practically drag on the ground. Unlike many monkeys, they do not use their arms for balance when walking, instead relying on their tails. The hands are long, narrow and hook-like, and have reduced thumbs. The fingers are elongated and recurved.
Spider monkeys form loose groups of 15 to 25 individuals, but can have even 30 to 40. During the day, groups break up into subgroups of two to eight animals. This social structure (fission-fusion) is found in only two other types of primates: chimpanzees and Homo sapiens. 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. The average subgroup size is between 2 and 8 but can sometimes be up to 17 animals. Also less common in primates, females rather than males disperse at puberty to join new groups. Males tend to stick together for their whole lives. 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 their young offspring.
The diets of spider monkeys consist of about 71.4 to 84 percent of fruits and nuts. They can live for long periods on only one or two kinds of fruits and nuts. They eat the fruits of many big forest trees, and because they swallow fruits whole, the seeds are eventually excreted and fertilized by the feces. Studies show the diet of spider monkeys changes their reproductive, social, and physical behavioral patterns. Most feeding happens from dawn to 10 am. Afterward, the adults rest while the young play. Through the rest of the day, they may feed infrequently until around 10 pm.
If food is scarce, they may eat insects, leaves, bird eggs, bark and honey.