Night monkeys typically occur in primary and secondary forest (including disturbed forest and selectively logged forest), seasonally flooded and terra firma, lowland forest, and submontane and montane (cloud forests) in Colombia and the Andes to 3,200 m above sea level (the specifically montane forms are Aotus lemurinus and Aotus miconax) (Hernández Camacho and Cooper 1976, Aquino and Encarnación 1994a, Defler 2004). Aquino and Encarnación (1994b) reviewed the habitat and forest preferences of the genus. A research in San Martin area, in 100 forest fragments, found A. brumbacki groups in fragments of gallery forest, lowland rainforest and Mauritia flexuosa swamps (Carretero-Pìnzon and Defler in prep.)

Night monkeys are nocturnal: they are most active at dawn and dusk. The only exception is Aotus azarae azarae of the Chaco of southern Bolivia, Paraguay and Argentina, which is cathemeral (active during night and day). They are frugivorous; their diet includes fruit, nectar and flower (seasonally important for the A. a. azarae in the Chaco), leaves, and small animals prey such as insects (Wright 1989; Fernandez-Duque 2007). Aotus azarae has been observed to include a significant portion of leaves in its diet (Ganzhorn and Wright 1994, Wright 1985, Arditi and Placci 1990, Giménez and Fernandez-Duque 2003), as has Aotus zonalis on Barro Colorado Island in an early study by Hladik and Hladik (1969).

They are socially monogamous, living in small groups of an adult pair and offspring of different ages (infant, one or two juveniles and sometimes a subadult. In A. a. azarae, a significant number of adults range alone. They may be subadults that have left their natal groups or older adults which have been evicted from their groups by competitors (Fernandez-Duque and Huntington 2002, Fernandez-Duque 2004). Both sexes disperse. Males care for the infants (carry them) (Rotundo et al. 2002, 2005). Lone adults were observed by Villavicencio Galindo (2003) in northern Colombia. Night monkeys are territorial—groups occupy overlapping territories of 5-18 ha (depending on the species and location) (Wright 1978, Fernandez-Duque 2007). Solano (1995) estimated a home range of 17.5 ha for A. brumbacki on the Río Duda in Tinigua Natural National Park, Colombia. Wright (1994) and Fernandez-Duque (2007) review the behaviour and ecology of the genus.

Captive male A. lemurinus reach sexual maturity when two years old, and captive female A. vociferans and A. nancymaae first breed when 3-4 years old (Dixson 1983; Fernandez-Duque 2007). In the wild, male A. azarae reach adult weight only when about 4 years old, and age at first reproduction is about 5 years of age (Juárez et al. 2003, Fernandez-Duque 2004). A female A. azaraewas found to breed for the first time at 58 months of age (Fernandez-Duque et al. 2002). Single offspring are the rule. Wright (1985) recorded births between August and February for A. nigriceps in Peru (Manu National Park), and Aquino et al. (1990) indicated a birth season between December and March) for A. nancymaae in north-eastern Peru. In the Argentinean Chaco, A. azarae shows a peak of births between March and June (Fernandez-Duque 2007).

Size: Hernández-Camacho and Defler (1985) recorded a weight of 875 g for one individual (sex unknown), and 455 g for a female, probably not fully grown.

Aotus brumbacki is a lowland night monkey, that extends East from the Cordillera Oriental in Colombia between the Ríos Arauca and Guaviare (Hershkovitz 1983; Defler 2003, 2004). The form occurring along the middle and lower reaches of the Meta, Tomo and Vichada and Guaviare has not been ascertained. Based on a specimen collected at Maipures, near the mouth of the Río Tomo on the Río Orinoco, Defler and Bueno (2007) indicated that the night monkeys of eastern central Colombia North of the Río Guaviare may be A. brumbacki, extending the range of this species to the Río Orinoco. Neither Bodini and Pérez-Hernández (1987) nor Linares (1998) give any indication of the occurrence of A. brumbacki in Venezuela, north of the lower Río Arauca.

This species is confirmed, or may occur, in the following protected areas:

• El Cocuy Natural National Park (306,000 ha) (possibly in range, Defler 2003, 2004)
• Serranía de la Macarena Natural National Park (630,000 ha) (in range, Defler 2003, 2004)
• El Tuparro Natural National Park (548,000 ha) (possibly in range, Defler 2003, 2004)
• Tinigua Natural National Park (201,875 ha) (Solano 1985, 1986).

This species is listed on Appendix II of CITES (as Aotus lemurinus brumbacki).