The Peruvian spider monkey (Ateles chamek) also known as the Black-faced black spider monkey, is a species of spider monkey that lives not only in Peru, but also in Brazil and Bolivia. At two feet (0.6 m) long, they are relatively large among species of monkey, and their strong, prehensile tails can be up to three feet (1 m) long. Unlike many species of monkey, they have only a vestigial thumb, an adaptation which enables them to travel using brachiation. Peruvian spider monkeys live in groups of 20-30 individuals, but these groups are rarely all together simultaneously. The size and dynamics of the resulting subgroups vary with food availability and sociobehavioral activity. They prefer to eat fleshy fruit, but will change their diet in response to scarcity of ripe fruit. Individuals of t