New World Monkeys

The douroucouli (Aotus trivirgatus), or night monkey, is the only nocturnal monkey. It has a round, furfringed face, no obvious external ears, and big eyes surmounted by white triangles of face fur. The fur is brown-gray with an orangish tone on the chest and abdomen, but the color can vary considerably.

The night monkey measures about 14 in (36 cm) long, plus a 12 in (30 cm) tail and weighs slightly more than 2 lb (1 kg). Night monkeys are found in forested regions from Panama to Argentina. These monkeys sleep in holes in hollow trees and eat fruits, leaves, and some insects. Unlike most monkeys, it does not use its fingers to remove foreign bodies from the fur of a monkey it is grooming; instead, it sorts through the fur with fingers and then removes the offending bits with its teeth. The eyes of the night monkey lack the reflective layer behind the retina that most nocturnal mammals have.

Male night monkeys are very aggressive, and family groups of parents with one or two young avoid contact with other groups. The males scare away other night monkeys by arching their backs, spitting, leaving scent on branches, and growling with a variety of sounds that are made more resonant by an inflatable sac on the throat. After a 133-day gestation period, the female produces usually one young, which is then carried by the father. The young become sexually mature at about two years.