Behavior
Spider monkeys form loose groups of 15 to 25 animals. 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 less than four 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.
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 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 limbs weighing up to 4 kg and drop them towards the intruder. They do not actually throw the branches, but twist to cause the branch 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, which is responsible for planning an efficient feeding route each day. Grooming is not as important to social interaction, owing perhaps to a lack of thumbs.
Spider monkeys have been observed avoiding the upper canopy of the trees for locomotion. One researcher speculated this was because the thin branches at the tops of trees do not support the monkeys as well.
At 107 grams, the spider monkey brain is twice the size of a howler monkey brain of equivalent body size; this is thought to be a result of the spider monkeys' complex social system and their frugivorous diets, which consist primarily of ripe fruit from a wide variety (over 150 species) of plants. 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 three to four years.
Read more about this topic: Spider Monkey
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