woolly spider monkey, (genus Brachyteles), extremely rare primate that lives only in the remaining Atlantic forests of southeastern Brazil. The woolly spider monkey is the largest monkey in South America and is intermediate in structure and appearance between the woolly monkeys (genus Lagothrix) and the spider monkeys (genus Ateles). Its thick fur, heavy body, and protruding abdomen are similar to those of woolly monkeys, while its reduced thumbs and long limbs resemble those of spider monkeys.
The woolly spider monkey is 45–63 cm (17–25 inches) long, excluding the 65–80-cm prehensile tail; females average 8 kg (17.6 pounds), males 9.6 kg. Its body is yellowish or brown, and the face is hairless. Recent study has shown that there are two species. The southern muriqui (B. arachnoides), from the states of Rio de Janeiro and São Paulo, has a black face and no thumb at all, and the male’s canines are much longer than the female’s. In the northern muriqui (B. hypoxanthus), from Bahia, Minas Gerais, and Espiritu Santo, the face is mottled pink and black, there is a rudimentary thumb, and the two sexes have canines of the same size.
Both species are active by day and move among the treetops, gathering their food of leaves, seeds, and fruit and frequently hanging by their tails. Field studies report small, flexible social groups. Some behavioral variations observed between species, such as the degree of competition between the males, may be correlated with the difference in canine teeth.
Woolly spider monkeys are more closely related to woolly monkeys than to spider monkeys, but all belong to the family Atelidae of New World monkeys. They are considered critically endangered, as their forest habitats are extremely fragmented.