The night monkeys, also known as the owl monkeys or douroucoulis, are the members of the genus Aotus of New World monkeys (monotypic in family Aotidae). The only nocturnal monkeys, they are native to Panama and much of tropical South America. Night monkeys have large brown eyes; the size improves their nocturnal vision, thus increasing their ability to be active at night. Their ears are rather difficult to see; this is why their genus name, Aotus (meaning "earless") was chosen. From the figures that have been collected, it appears that males and females are similar in weight; the heaviest species is Azara's night monkey at around 1,254 grams (2.765 lb), and the lightest is Brumback's night monkey, which weighs between 455 and 875 grams (1.003 and 1.929 lb). The male is slightly taller than the female, measuring 346 and 341 millimetres (13.6 and 13.4 in), respectively.