What is the only monkey species that is nocturnal?

The only truly nocturnal monkey is the night monkey, also called the owl monkey, of the genus Aotus.

Night monkeys are New World monkeys found in forests from Panama through much of South America. Their large eyes help them see in dim light, while their mostly hidden ears inspired the genus name Aotus, meaning “earless.” They rest in sheltered tree sites during daylight and become active after dark.

The wording is often simplified: “night monkey” refers to a whole genus containing multiple species, rather than one single species. The group is nevertheless the standard quiz answer because Aotus is the only living monkey lineage described as truly nocturnal. One subspecies of Azara’s night monkey is an important qualification because it can be cathemeral, meaning active during both day and night.

Night monkeys typically live in small family groups made up of a mated pair and their immature offspring. Unlike many primates, they have monochromatic vision, an adaptation suited to their low-light lifestyle.