Physical DescriptionNight monkeys are small monkeys, from 455 to 1254 grams in weight and from 240 to 370 mm in head and body length. The tail is not prehensile and is from 316 to 400 mm. Males and females are similar in size and appearance. Pelage color and pattern is somewhat similar across species, with short, dense, woolly fur that is silvery gray dorsally and yellow or buff to orange-brown ventrally. The face is round and usually marked with 3 dark brown or black lines; one line on either side of the eyes and one marking the middle of the forehead. They have white or light gray areas of fur above and below their exceptionally large eyes and on the chin. In some species these markings are indistinct. Gray-necked species have grayish fur on the sides of their necks, red-necked species have red fur on the sides of their necks. The ears are short and rounded, sometimes completely covered by the thick fur. Night monkeys have a sac under their chin that can be inflated during vocalizations. (Cawthon Lang and Fernandez-Duque, 2005; Nowak, 1991)