SPIX'S NIGHT MONKEY
Geographic Distribution and Habitat
First described in 1823 by German naturalist Johann Baptist von Spix, Spix’s night monkey (Aotus vociferans)—also known as the Colombian gray night monkey, noisy night monkey, or Spix’s owl monkey—is found north of the Amazon River in Brazil, Colombia, Ecuador, and Peru. Its range is limited by geographical barriers: the Guabiare River to the north, the Andean Ridge to the west, and by the Rio Negro to the east.
An adaptable species, Spix’s night monkeys are one of the relatively few species that survives well even in disturbed habitat, such as fragmented or selectively logged forests. Their preferred habitat is seasonally flooded and terra firme forests and lowland and sub-montane forests.
The taxonomy of the genus Aotus, whose members are collectively referred to as the night or owl monkeys, is a matter of debate. Prior to 1983, the genus contained just one species (Aotus trivirgatus), with its ten subspecies being elevated to species level after genetic analysis. Today, there are two groups of night monkeys: the gray-necked owl monkeys, which include Spix’s night monkey and live north of the Amazon River, and the red-necked night monkeys, which live south of the Amazon River. The Spix’s night monkey is believed to be one of the oldest of the night monkey species.
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The average weight of a Spix’s night monkey is 1.5 lbs (0.7 kg), with males being only slightly larger than females. The head and body measure 9–15 inches (24–37 cm) in length, with the tail adding an additional 13–16 inches (32–40 cm). The maximum recorded lifespan in captivity for a night monkey is 20 years.
Spix’s night monkeys have a gray pelage over most of their body, changing to a light tan on their bellies and dark gray on their tails and hands. Their faces are white, and framed by three vertical black stripes extending towards the crown of their head. Their eyes, as with many nocturnal animals, are very proportionately large, and their fingers are very long. Males and females are not sexually dimorphic, that is the look alike.
As frugivores, Spix’s night monkeys eat mainly fruit supplemented with nectar, flowers, and leaves, as well as small animals such as insects. Their preferred fruit is ripe and small, and they prefer to look for food in large trees. Night monkeys hunt for insects by snatching them out of the air or grabbing them from a branch, rather than the more typical primate approach of seeking insects out in holes and crevices. The most common insects eaten are grasshoppers, cockroaches, moths, beetles, and spiders, all of which are active at night, making them easier for the nocturnal night monkeys to find.
Behavior and Lifestyle
Night monkeys are some of the most completely quadrupedal monkeys—even when resting, they almost always support some of their weight with one or both hands. They are adept leapers, able to cross a 13 foot (4 m) gap in a single jump. The power from their leaping comes from their hind legs, with their forelegs used to absorb the shock of their landing.
As their name suggests, Spix’s night monkeys are nocturnal. While nocturnality is often associated with prosimians, night monkeys evolved from diurnal ancestors and re-evolved nocturnality. Pressures such as diurnal predators and competition for resources with other diurnal mammals may have favored this re-evolution of nocturnality.
Spix’s night monkeys typically wake up about 15 minutes after sunset, and return to their sleeping site before sunrise. They are most active around dusk, after they wake up, and at dawn, as they return to their sleeping sites. During the day, individuals sleep in groups in tree holes 33–66 feet (10–20 m) off the ground or in thickets of dense foliage. Groups usually sleep in one of several preferred trees, which they come back to night after night throughout the year. This is at odds with some other species of night monkeys, who choose their sleeping trees seemingly at random. They sometimes share sleeping quarters with other mammals, such as bats.
Spix’s night monkeys are one of the few monkey species that are monochromats. Unlike some forms of color blindness in which the affected individual cannot distinguish between certain colors, night monkeys do not perceive color at all, seeing the world in black, white, and shades of gray.
Spix’s night monkeys live in small family groups of between 3 and 5 individuals, with an average size of 3.3 individuals. Groups are composed of a mated pair and their offspring. Both sexes disperse to form their own groups upon reaching maturity.
Spix’s night monkeys are among the most aggressive of the New World monkeys. Adult individuals of the same sex may fight savagely, and adults of the opposite sex, while rarely overtly fighting, still sometimes display hostile body language and vocalizations, even if they are a mated pair.
Night monkeys are very territorial, so groups rarely come in contact with one another. Territories range in area from 12 to 44 acres (5–18 ha), and are patrolled mostly at night. Spix’s night monkeys range an average of 2,720 feet (829 m) per night during the wet season and 827 feet (252 m) per night during the dry season. This discrepancy is likely explained by resource availability: during the dry season, when resources are more scarce, night monkeys rest more to conserve energy, and thus require fewer resources. During the wet season, when food is more abundant, the night monkeys can spare the energy to patrol a larger area.
Nightly travel is also directly correlated with available light. During a full moon, night monkeys travel twice the distance they do during a new moon, and nightly travel is concentrated during times of the night with the most available moonlight. On dark nights, night monkeys travel along memorized routes, likely using olfactory cues to help them navigate.
Spix’s night monkeys communicate through vocalizations, body language, and olfactory cues. They make a wide array of vocalizations, such as grunts, trills, screams, and moans, and use scent marking and calls to defend their territory. Unlike many New World monkeys, allogrooming (mutual social grooming) is rare for Spix’s night monkeys, and only performed in sexual contexts.
Spix’s night monkeys are socially monogamous, with a male/female couple forming a long-term bond and rearing young together. Social monogamy is a living arrangement between an adult male and an adult female that does not necessarily describe the sexual interactions or reproduction between monogamous pairs; rather it refers to their living conditions. The arrangement consists of, but is not limited to: sharing territory; obtaining food resources; and raising offspring.
Gestation lasts between 122 to 153 days, and only one offspring is born at a time. Males help with raising offspring, such as by carrying infants. Sexual maturity is reached at about two years of age, and age of first breeding (based on captive individuals) is 3–4 years.
As nocturnal (night-dwelling) monkeys, Spix’s night monkeys and their Aotus relatives are specialized animals that occupy a specific ecological niche. They have a sympatric relationship with other primates that use similar resources, largely because they utilize those resources during different times of the day. Non-primate nocturnal mammals, such as bats, are usually either much smaller than night monkeys and/or they forage alone, and so do not pose much of a competitive threat. Spix’s night monkeys are not typically hunted at night, although owls, snakes, and large cats may predate them occasionally. During the day, they may be hunted by raptors if their sleeping site is not well hidden. As frugivores (fruit-eaters), Spix’s night monkeys likely contribute to seed dispersal, a crucial ecological function.
The Spix’s night monkey is classified as Least Concern by the International Union for Conservation of Nature (IUCN, 2015), as it has a wide range and presumed large population size. However, its current population trend is unknown. The most recent estimate for population density is 64.5 individuals/mi² (24.9 individuals/km²) in the Amacayacu National Park in Colombia.
While adaptable to many changes, Spix’s night monkeys are not exempt from the impacts of anthropogenic disturbance. It is believed that the population has been negatively impacted by rampant deforestation in Ecuador, and specifically deforestation along rivers and from oil exploration and exploitation in Peru.
The main threat against the species along the Brazil-Colombia-Peru border is the systemic capture of individuals for biomedical research. Night monkeys are one of the few non-human primates that are susceptible to malaria, a deadly disease afflicting 150 to 200 million people at any given time, and boast an unusual resistance to the parasites that cause it. The species has been collected for study from the north bank of the Amazon River in Colombia since the 1980s, and it has not been observed there since 2013. Research also suggests that the post-experimental release of Nancy Ma’s night monkeys in Colombia may have displaced Spix’s night monkey populations. In 2015, the environmental authority Coproamazonia authorized the taking of 1,463 Spix’s night monkeys over the course of 566 days for use in malaria research. However, because population data on local populations of Spix’s night monkeys is nonexistent, this authorization was made without regard to the local population.
The Spix’s night monkey is protected under Appendix II of the Convention on International Trade in Endangered Species (CITES), and it occurs in about 19 protected areas throughout its range. The most pressing need for this species is more field research. Effective policies and protections cannot be enacted without knowing basic population information on a species. Specifically, research is needed to determine the population stability of Spix’s night monkey, especially in areas where the species is the most vulnerable, the effects of collection and post-experimental release for individuals used in biomedical research, and the species’ resilience to human impact.
- Fernandez-Duque, E., A. Di Fiore, G. A. Carrilla-Bilbao. 2008. Behavior, Ecology, and Demography of Aotus vociferans in Yasuní National Park, Ecuador. International Journal of Primatology 29(2):421-31.
- Jacobs, G. H., et al. 1993. Photopigments and color vision in the nocturnal monkey, Aotus. Vision Research 33(13):1773-83.
- Moynihan, M. H. 1964. Some behavior patterns of platyrrhine monkeys: I. The Night monkey (Aotus trivirgatus). Smithsonian Miscellaneous Collections 146(5).
Written by K. Clare Quinlan, March 2020