Owl monkey
Aotus sp.

TAXONOMY

Suborder: Haplorrhini
Infraorder: Simiiformes
Family: Aotidae
Genus: Aotus
Species: azarae, hershkovitzi, lemurinus, miconax, nancymaae, nigriceps, trivirgatus, vociferans
Subspecies: A. a. azarae, A. a. boliviensis, A. a. infulatus, A. l. brumbacki, A. l. griseimembra, A. l. lemurinus, A. l. zonalis

Other names: douroucouli or night monkey; doeroecoeli (Dutch); douroucouli or singe de nuit (French); macaco da noite (Portuguese); cuatro ojos, marta, marteja, mono de noche, mono nocturno, or tutamono (Spanish); mirikina, nattapa, or negronattapa (Swedish); A. azarae: A. azarai; Azara's night monkey; A. lemurinus: Colombian night monkey, gray-bellied night monkey, or lemurine night monkey; A. l. brumbacki: Brumback's night monkey; A. l. griseimembra: grey-legged night monkey; A. hershkovitzi: Hershkovitz's night monkey; A. miconax: Andean night monkey or Peruvian night monkey; A. nancymaae: A. nancymai; Nancy Ma's night monkey or Peruvian red-necked owl monkey; A. nigriceps: black-headed night monkey or Peruvian night monkey; A. trivirgatus: northern night monkey, northern owl monkey, or three-striped night monkey; A. vociferans: noisy night monkey, Spix's night monkey, or Spix's owl monkey

Aotus nancymaae

The Genus Aotus used to contain only one species (Aotus trivirgatus) with ten subspecies, but in 1983, all ten subspecies were elevated to species level and subsequent genetic work has refined the taxonomy even more (Groves 2001). Within the genus, there are two groups of owl monkeys, the gray-necked (lemurinus and subspecies, hershkovitzi, trivirgatus, and vociferans), found north of the Rio Amazonas (Amazon River) and the red-necked (miconax, nancymaae, nigriceps, azarae and subspecies), found south of the Rio Amazonas (Groves 2001). Because this change in taxonomy was introduced fairly recently and is disputed, some published research continues to refer to the entire group as A. trivirgatus rather than distinguishing among species and subspecies. It may be necessary to generalize about some of the characteristics of owl monkeys because of the confusion of taxonomy in the literature in addition to the limited published information about this taxonomic group (Wright 1994; Sussman 2000).

MORPHOLOGY

Separated into two groups based on their coloration, karyotype, and geographic distribution, owl monkeys have gray-tan to brown bodies and either gray or red fur on the sides of their necks (Ford 1994). They have pale yellow to orange fur on their stomachs, underarms, and inner legs, light gray to white markings above and below their eyes, and three conspicuous, black stripes from the top of their head to either side of each eye and straight down the forehead between the eyes to the bridge of the nose (Ford 1994; Rowe 1996; Groves 2001). Their coats range in thickness and length depending on the altitudes at which they are found, with species living at higher elevations having thicker, shaggier coats than those living at sea level (Wright 1981; Groves 2001). They are unusual in their appearance compared to other primates because of their disproportionately large, brown eyes which have evolved as an adaptation to their nocturnal lifestyles. All owl monkeys are at least somewhat nocturnal and they are the only New World monkeys active at night (Wright 1994). While a nocturnal lifestyle is associated with primitive primates like prosimians, owl monkeys are not primitive but rather they re-evolved nocturnality from diurnal ancestors (Wright 1994; Rowe 1996). Changes in the morphology of the eye and brain of owl monkeys reveal how specialized they have become to keep this nocturnal lifestyle but also reveal structural commonalities between other diurnal primates (Rowe 1996). To see objects in low light levels, owl monkeys have evolved large eyeballs, their lenses have a more spherical shape than is seen in diurnal primates, and there are also more rods and fewer cones (Noback 1975). Though they do see in color, owl monkeys have less acute color vision than other primates. This is not a disadvantage as they are faster at locating and following moving objects at low light levels and have better spatial resolution at low light levels than other primates which helps them capture insects and move through arboreal habitat (Wright 1994).

There is very limited data on body size and weight for Aotus, most measurements from wild animals are extrapolated from only a few samples. Owl monkeys are not sexually dimorphic so males and females are about the same size and weight (Wright 1994). Average male A. a. azarae weigh about 1254 g (2.76 lb) while females average 1246 g (2.75 lb). Wild A. a. boliviensis males weigh, on average 1180 g (2.60 lb) while females are slightly heavier, having an average weight of 1230 (2.71 lb) (Fernandez-Duque in press). The weights from one wild A. a. infulatus male and one female are 1190 g (2.62 lb) and 1240 g (2.73 lb), respectively, and data from one male and one female A. brumbacki reveal weights of 875 g (1.93 lb) and 455 g (1.00 lb) (Fernandez-Duque in press). Data from wild A. lemurinus reveal an average male weight of 920.7 g (2.03 lb) and average female weight of 859 g (1.89 lb). The weight of one male A. nigriceps was 875 g and the average weight of two females was 1040 g (2.29 lb). There is a fair amount of information available on the body weights of A. nancymaae and A. trivirgatus. A. nancymaae males weigh, on average, 794 g (1.75 lb) and females weigh 780 g (1.72 lb) while wild A. trivirgatus males weigh, on average 813 g (1.79 lb) and females average 736 g (1.62 lb) (Fernandez-Duque in press). The weights of wild A. vociferans and A. zonalis are also based on small sample sizes. Male A. vociferans weigh 697.5 g (1.54 lb) and there are no data for wild females while A. zonalis males have an average weight of 889 g (1.96 lb) and females weight 916 g (2.02 lb), on average (Fernandez-Duque in press). There are no data available for A. miconax.

Owl monkeys move quadrupedally on branches and vines in the forests in which they live and are skilled leapers, able to cross gaps in the canopy up to four meters wide (Wright 1984). The maximum recorded longevity for a captive owl monkey is 20 years (Rowe 1996). The only data on their longevity in the wild comes from an A. azarae population where it is known that a male has lived at least 11 years (Fernandez-Duque pers. comm.).

RANGE

Owl monkeys are widely distributed across southern Central America and northern South America (Groves 2001). A. lemurinus subspecies are found in Panama, northern Colombia, and northwestern Venezuela while A. hershkovitzi ranges only in Colombia. The other gray-necked species include A. trivirgatus, which occupies a range extending from eastern Colombia and southern Venezuela into part of northern Brazil and A. vociferans, which is widely distributed across Colombia, northern Brazil, northern Peru, and eastern Ecuador (Groves 2001). All of the gray-necked species live north of the Amazon River while the red-necked owl monkeys are found south of the Amazon. A. miconax is limited to a very small region in northwestern Peru; A. nancymaae occupies a small region straddling the Peru-Brazil border and is sympatric with both red-necked (A. nigriceps) and gray-necked species (A. vociferans); A. nigriceps is found in Brazil and Peru, and A. azarae subspecies are found in Paraguay, Argentina, southern Bolivia, and Brazil (Groves 2001; Fernandez-Duque et al. 2001).

Aotus sp. range (in red)

Little research has been conducted on Aotus in the wild probably primarily due to the difficulty of studying nocturnal primates (Sussman 2000). Research on owl monkeys requires not only specialized forest skills, including the ability to maneuver through the forest at night and good auditory and nocturnal visual skills, but radio-tracking equipment as well (Sussman 2000; Aquino & Encarnación 1994). Some of the intrepid owl monkey researchers and contributors to the knowledge of wild Aotus include Patricia Wright (Peru and Paraguay), Rolando Aquino and Filomeno Encarnación (Peru), and Eduardo Fernandez-Duque (Argentina). Captive studies of Aotus have been conducted at the New England Primate Research Center.

HABITAT

Aotus species widely inhabit primary, secondary, and remnant tropical forests as well as seasonally deciduous scrub forest, subtropical dry forest, and gallery forest from Panama to Argentina and Paraguay at a wide variety of elevations from sea level to 3200 m (10,498 ft) (Wright 1981; Aquino & Encarnación 1994; Fernandez-Duque et al. 2001). Temperatures at higher elevations can be as low as 5° C (41° F) and in the Chaco of Argentina, night temperatures as low as -5° C (23° F) are tolerated by Aotus (Fernandez-Duque 2003). They tend to inhabit areas of high plant species diversity with a relatively dense canopy and will forage at all canopy levels (Kinzey 1997). In Peru, they specifically prefer two types of habitats: lowland forest that is flooded seasonally or highland forest that is never flooded. In the flooded forest, where water can be as deep as seven meters (23.0 ft) during the rainy season, there is a predominance of trees with thickened trunks and branches that provide hollows used as sleeping sites, as well as pervasive growth of lianas and epiphytes, important plants used as sleeping sites and refuges (Aquino & Encarnación 1994). In Colombia, Aotus species are found in high, primary forests and in remnant and older secondary forests, but not in young secondary forests with low species diversity. In Argentina, Aotus is found in both high and low forests as well as gallery and island forests with dense canopies (Wright 1981; Fernandez-Duque et al. 2001). In Brazil, A. infulatus uses a variety of habitats including coastal forests with palm trees, mangroves, and gallery forest (de Sousa e Silva & Fernandes 1999). All of the habitats in which they are found exhibit seasonal patterns of rainfall and temperature changes, with the rainy season generally lasting from about September to about May and the dry season occurring during June through August (Wright 1981).

ECOLOGY

Aotus is the only nocturnal monkey. Adopting a system of nocturnal behavior seems disadvantageous to animals like primates that have a high dependence on vision: food, including insects, animal prey, and fruit, is more difficult to find at night, locomotion through the forest canopy is significantly more dangerous in dim light, and a different set of environmental conditions such as cooler temperatures, differences in humidity, competitors, and predators pose threats (Wright 1989). Despite these obstacles, the fossil record proves that owl monkeys assumed this nocturnal role from diurnal ancestors and, based on their wide geographical distribution, have been successful in exploiting this niche. Many of the ecological and behavioral characteristics of Aotus can be explained or at least linked to their unique pattern of life. Though most species of owl monkeys are nocturnal, A. a. azarae shows the particularly rare pattern of cathemeral activity in which it is regularly active during both daylight and nighttime hours (Fernandez-Duque 2003). Because of its range in the very seasonal Chaco of Argentina and Paraguay, the ability to forage during daylight hours is probably an adaptation to avoid activity on extremely cold, moonless nights (Fernandez-Duque 2003).

Members of the genus generally leave their sleeping site about 15 minutes after sunset and return before sunrise each day (Wright 1986; Garcia & Braza 1993). After leaving the sleeping site, groups travel and feed until about midnight, when they stop to rest for 90 to 120 minutes. Owl monkeys are frugivores and supplement their diet with flowers, insects, nectar, and leaves (Wright 1989; 1994). They prefer small, ripe fruit when available and in order to find these, they forage in large-crown trees (larger than ten meters [32.8 ft]) (Wright 1986). Seasonal availability of fruit varies across environments. Aotus species in tropical forests eat more fruit throughout the year because it is more readily available compared to the dry forests where fruit is limited in the dry season and owl monkeys are more dependent on leaves (Wright 1994). It is difficult to quantify leaf and insect eating during the night, though, and researchers sometimes have to rely on fecal sample composition to project quantity and therefore importance to owl monkeys. Insect foraging occurs at dawn and dusk. Rather than seeking out insects in holes and crevices, owl monkeys are adept at grabbing flying insects out of the air or snatching them off of a branch (Wright 1989). They most often eat large orthopterans, moths, beetles, and spiders and probably specialize in these because of the insects' rhythms. Moths are active at night, orthopterans call loudly at night, exposing their location, and beetles are active at night are easier making them easier to see (Wright 1989). After spending the night traveling, foraging, resting, and socializing, as dawn approaches owl monkeys increase their activity levels, and forage as they travel to either the same sleeping site from the previous day or another one within their home range (Wright 1981). Sleeping sites are either tree holes or thickets of dense foliage made up of lianas or epiphytes and, if they are big enough, may be shared with other nocturnal animals like bats (Aquino & Encarnación 1986; Garcia & Braza 1993). Owl monkeys spend most of their time foraging and sleeping above in the high canopy, above ten meters (32.8 ft) (Wright 1994).

Density of owl monkeys ranges from two to 16 groups per km² (.772 to 6.18 groups per square mile) (Fernandez-Duque et al. 2001). Home ranges of owl monkeys are smallranging between .031 and .175 km² (.012 and .068 mi²), depending on the species (Fernandez-Duque in press). The average nightly range is about of 708 m per night (.441 mi), but they use a larger area during the wet season, 829 m (.515 mi) per night and a smaller area during the dry season, 252 m (.157 mi) (Wright 1989). This use of space indicates that they spend more time resting and rely on fewer resources during the dry season, but resource availability is not the only factor influencing nightly travel distance. Nightly path length is also correlated with available light; the distance traveled is directly linked to the amount of moonlight on a given night. Owl monkeys travel twice as far on bright nights with full moons than on dark nights, and they concentrate their movement during the times of night with the most moonlight (Wright 1989; Fernandez-Duque 2003). Even though they do not travel as far, they still need to move on dark nights to find food and one adaptation to this need to forage is to travel the same routes repeatedly. They probably memorize routes and use chemical cues to help them find their way between food sites and sleeping sites (Wright 1989).

Aotus azarae azarae

Though owl monkeys are sympatric with other primates that use similar resources, they do not compete for access to resources because of their alternative schedules. Furthermore, non-primate nocturnal competitors are usually small-bodied (like bats) or forage alone or in pairs, not posing a significant source of competition (Wright 1989). Nocturnal predators do not usually eat monkeys, but some potential threats include owls, snakes, and felids. Diurnal raptors may pose a threat to owl monkeys if they are not well hidden in their sleeping sites (Wright 1989; 1994).

SPECIAL NOTES

At any given time, there are about 150 to 200 million people worldwide infected with malaria and over a million children annually under the age of five die from this disease spread by infected mosquitoes (Dixson 1994). Owl monkeys have been successfully employed in biomedical research on malaria since the 1960s because of their natural resistance to the parasites that cause the disease (Ford 1994). Individuals of some species are intermittently removed from the wild to bolster captive colonies for research, breeding programs, and zoos, but effects on the total population are not significant and the population can recover after these "cropping" events (Aquino & Encarnación 1994).

Content last modified: July 18, 2005

Written by Kristina Cawthon Lang. Reviewed by Eduardo Fernandez-Duque.

Cite this page as:
Cawthon Lang KA. 2005 July 18. Primate Factsheets: Owl monkey (Aotus) Taxonomy, Morphology, & Ecology. <http://pin.primate.wisc.edu/factsheets/entry/owl_monkey>. Accessed 2008 May 14.