Geographic Distribution and Habitat
Northern muriquis (Brachyteles hypoxanthus), or northern woolly spider monkeys, are endemic to the country of Brazil. They occupy the states of Minas Gerais, Espírito Santo, and Bahia—and a recently confirmed single population resides in Rio de Janeiro within Itatiaia National Park, on the border of Minas Gerais in the Serra da Mantiqueira mountain range.
They reside in tropical semi-deciduous, late successional forests; that is, forests that remain unchanged in composition so long as they remain undisturbed. Here, large trees with an average height of 44 ft (13.4 m) provide a dense canopy that allows the monkeys to easily travel from one treetop to another. The monkeys are also found in patches of primary forest, secondary forest, highly disturbed areas of secondary vegetation, and scrub forest. Their range excludes the lowland forests in the extreme south of Bahia and northern Espírito Santo.
Human development, a lack of contiguous forest, and natural barriers that include the Rio Grande, Rio Paraiba do Sul, and the Serra da Mantiqueira mountain range (which extends east-west in the south of Minas Gerais) geographically separate northern muriquis from their sister species, southern muriquis (B. arachnoides), or southern woolly spider monkeys.
The lack of physical contact with one another makes interbreeding impossible, thereby creating two distinct populations.
Wildlife biologists who have studied both species posit that the differing forest composition of the two populations is testament to muriquis’ resiliency for surviving in diverse habitat. Habitat loss has gravely impacted the future of both species, however.
Size, Weight, and Lifespan
Muriquis are South America’s largest monkeys and are the two largest species of New World monkeys. When hanging from tree branches with their long arms outstretched, they give the illusion of being nearly five feet tall! Their true stature is a bit less. Nevertheless, muriquis possess a commanding physicality.
Northern muriquis are slightly shorter and weigh a bit less than the southern species.
Head-to-body length for adult males ranges from 1.57 to 1.63 ft (47.8–49.7 cm). A long, fully prehensile tail adds another 2.41–2.53 ft (73.4–77.3 cm), with an average tail length of 2.43 ft (74.2 cm). Northern muriqui males weigh between 20.4 and 21.2 lb (9.25–9.6 kg), with an average weight of 20.7 lb (9.4 kg).
Head-to-body length for adult females ranges from 1.5 to 1.7 ft (46–51.4 cm). A long, fully prehensile tail adds another 2.4–2.6 ft (74–80 cm), with an average tail length of 2.55 ft (77.8 cm). Northern muriqui females weigh between 15.2 and 20.5 lb (6.9–9.3 kg), with an average weight of 18.4 lb (8.3 kg).
Lifespan for northern muriquis is 40 years old in the wild.
These monkeys look like personable individuals with whom you’d like to hang out and have a nice chat—if you could hang from a high tree branch in a Brazilian forest. But you probably can’t . . . and you shouldn’t.
Both northern and southern muriquis are similar in appearance. Poofy, woolen coats cover their bodies, including their spindly limbs and pot bellies (accoutrements and physical attributes that may lend themselves to the species’ alternative name of woolly spider monkeys). Coat colors vary from gray to yellow-brown to a creamy, light toffee. The head of the northern species might be darker than the pelage covering its torso and limbs.
But the most obvious distinction between the two species is the face. Both have black, hairless faces. However, while the face of the southern muriqui is entirely black, the face of the northern muriqui is distinguished by splotches of pink pigments, giving it a mottled appearance. Northern muriqui infants are born with black faces that become mottled with pink pigmentation as the monkeys age. The large scrotums of adult male northern muriquis are also mottled with pink pigmentation.
Another major distinction between the two species is the thumb, or lack thereof. Mother Nature has fitted the northern muriqui’s claw-like hand with a rudimentary thumb, only 0.12–0.75 in (0.3–1.9 cm) in length, a mere stub (known as a “vestigial” thumb). But she has completely removed this apparatus from the southern muriqui’s physique.
Muriquis are total herbivores. They eat mostly leaves and fruits. Northern muriquis eat more leaves than fruits, along with a small amount of flowers, tree bark, buds, and seeds. (The southern species eats more fruits than leaves.)
Northern muriquis consume food from 132 plant species. One plant species favored by the northern muriqui belongs to the genus Anadenanthera. The seeds of this tree, jokingly called “magic beans,” are known to produce hallucinogenic, psychoactive effects (begging a lighthearted conjecture, perhaps, for contributing to the muriqui’s playful characterization as a “hippie monkey.”)
Muriquis’ diets vary according to the season, linked to availability of their preferred food.
They practices geophagy, or “soil consumption.” They eat dirt. Researchers believe that certain minerals found in the soil—including zinc and iron—may supplement the monkeys’ dietary nutrients. Geophagy is also believed to increase digestion and improve nutrient absorption in the body.
What Does It Mean?
A species selected to act as an ambassador, icon, or symbol for a defined habitat, issue, campaign or environmental cause.
Genus (plural, genera):
A biological classification, or ranking, of living beings that includes a group(s) of species that are structurally similar or “related” to one another through evolution
The time of pregnancy from conception until birth.
A particular way or method of doing something, especially one that is characteristic or well-established.
A forest that grows on the slope of a mountain, regardless of altitude or latitude, within a specific climate, just below the subalpine zone.
The fur, hair, or wool of a mammal.
A mating system in which both males and females have multiple mating partners during a breeding
The tail of an animal that has adapted to be able to grasp or hold objects.
Also termed old-growth forest, virgin forest, or primeval forest—a forest that has attained great age without significant disturbance and thereby exhibits unique ecological features and might be classified as a climax community, an ecological community in which populations of plants or animals remain stable and exist in balance with each other and their environment.
A forest that has regrown after a major disturbance, such as fire or timber harvest, but has not yet reached the mature state of primary forest.
Vegetation that has regrown after a major disturbance, such as fire or clearance.
A botanical term referring to plants that lose their foliage for a short period of time; as leaves fall, new ones start growing.
An individual that has passed through the juvenile period but not yet attained typical adult characteristics; an animal who is not fully adult.
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Behavior and Lifestyle
Northern muriquis are diurnal (active during daylight hours) and arboreal (spending most of their time in trees). Daily life for these monkeys consists of foraging, feeding, socializing, playing, embracing—and resting. About half their day is spent resting! Overnight, they sleep close together in the dense canopies of large trees.
Brachiation—swinging by their arms, hand over hand, from one tree limb to the next—is the main mode of travel for both species as they advance through the forest canopy. The northern species’ rudimentary thumb comes along for the ride, not actually contributing anything to the effort. Both species are assisted by their long, prehensile tail. A bare patch of skin on the underside at the tip of the tail functions as a gripping pad, providing stability as the monkeys move about the forest and allowing them to hang solely by their tail as they feed.
It’s easy to understand why northern muriquis have been dubbed “hippie monkeys.” They typically begin their day with a group hug, they are constantly touching and showing affection to one another, and they abide by an egalitarian doctrine; that is, their social system lacks a dominance hierarchy. Competitive and aggressive behavior with one another is not the norm. Peace, love, and group hugs is their modus operandi.
An abundance of leaves keeps members of a group together in a big, loving tangle while they eat. Even when northern muriquis separate into smaller groups to forage for fruit and other food sources—sometimes leaving behind smaller all-male groups while the females forage together or with their offspring—a sense of decorum prevails. The males exhibit no animosity toward one another as they patiently wait for the females to return and then politely queue for their turn to mate. Should a mild affront occur between males, it is easily resolved with a hug.
In rare hostile encounters between two northern muriqui groups, the males of the larger group collectively protect the females of their group. Seldom does aggressive behavior among individuals of the same group occur. When aggression arises, it is typically related to females of a group chasing away new females who have tried to join the group.
Respectful behavior toward one another preserves northern muriquis’ peaceful reputation.
Daily Life and Group Dynamics
Muriquis are social primates who live in extremely large family groups, or “troops.” Northern populations can include 80 individuals! These impressive troop numbers include adult males and females, subadults, juveniles, and infants. Upon reaching adolescence (between 5 and 7 years old), females leave their natal group (that is, the family group into which they were born), while males remain behind. (More often with nonhuman primate species, males are the ones to depart their natal group.)
The monkeys travel throughout their home range, about 0.8–3.1 sq m (2–8 sq km), when foraging for food. In the course of this activity, northern muriquis are known to demonstrate cooperative behavior, often hugging as they pass one another on a branch (an act of encouragement, maybe?).
Occasionally, muriquis descend to the ground. Of the two species, northern muriquis spend more time on the ground feeding, resting, and socializing; males spend more time on the ground engaging in these activities than females. But it’s primarily habitat destruction that has forced northern muriquis out of the trees. Wherever the dense canopy of late successional forests has been destroyed, the monkeys are not able to get from one place to another; hence, they have no choice but to travel on the ground.
Sympatric species include ocelots, jaguars, and pumas; these wild cats are also natural predators of northern muriquis, along with possibly feral dogs, who share northern muriqui habitat.
“Muriqui” is a native word of Brazil’s Tupi Indians; it means “largest monkey.”
Muriquis’ alternative name of “woolly spider monkey” is misleading, assert some wildlife biologists. They believe that the name incorrectly implies that the monkeys are hybrids of woolly and spider monkeys—which they are not. Since 1995, the northern and southern muriqui has each been classified as a unique species.
Both northern and southern muriquis use a wide variety of vocalizations to communicate; many of these vocalizations are similar between the two species.
Neighs are contact calls, used to maintain group cohesion and to inform individuals about group movement. Barks indicate alarm. Shrill yips are given as an alert. Chuckles and warbles are often heard during embraces; the former is used to give reassurance, while the latter indicates excitement. Chirps convey a variety of meanings: at short distances, they can be used as “food calls” (think of ringing the dinner bell!); an individual who sounds a loud chirp is letting his group know that he has just discovered a great foraging area. Infants chirp (think of a human infant “cooing”) and keen (plaintively cry) when trying to get their mothers’ attention. Staccato (sharp, disjointed notes) chutter-whinnies are used during reunions. Northern muriqui adult females are thought to use staccato vocalizations to ensure adequate space between themselves and other females during feeding, with a goal of maximizing their energy intake while avoiding competition. Twitters are sounded by females who are trying to attract a male to mating partner. “Kh-kh-kh” is a sound of contentment, heard by resting individuals. Screaming is heard from individuals who are lost.
Northern muriquis engage in what is known a “sequential exchange”; that is, individuals take turns answering one another.
Tactile communication is certainly a part of muriquis’ daily life. All the hugging that occurs between individuals helps to establish strong social bonds. Grooming instills strong bonds between a mother and her infant.
Pheromones leave important scent clues when it comes to mating.
Reproduction and Family
Muriquis are promiscuous, whether they are from the north or south. Both sexes enjoy multiple partners, making them polygynandrous.
In the egalitarian society of northern muriqui populations, there are no dominant or subordinate individuals. Males appear to use their large testes to catch the eye of breeding females. The males of southern muriqui populations have been observed consuming their excess ejaculate, perhaps for its high protein content, researchers speculate. Researchers think that northern muriqui males may also engage in this curious behavior; they just have not been caught in the act.
Northern muriqui males gather in large all-male groups to attract more females, thereby improving their chances for mating. Males reach sexual maturity (capable of siring offspring) between 4 and 8 years old; females are between 5 and 11 years old when they reach sexual maturity (capable of conceiving), typically bearing their first offspring at 7 years old. For females who have successfully dispersed from their birth (natal) group just prior to reaching sexual maturity, the average age of their first birth is delayed to 8.7 years old. Muriqui mothers have a two-year interval between births.
To protect their offspring from the negative effects of inbreeding, northern muriqui females deliberately avoid mating with their sons or with other closely related relatives.
Northern muriquis breed during Brazil’s wet season (October to March), and infants are born during the country’s dry season (May to October). Both male and female northern muriquis may remain sexually active and reproductive up to age 30.
The gestation period for northern muriquis is not documented; however, the gestation period for southern muriquis is reported to be 7 months. Northern muriquis produce a single offspring; a maximum birth number has not been reported.
During their first year of life, young muriquis are almost completely dependent on their mothers. From 3 to 6 months old, the infant clings to his mother’s belly as she travels through the forest. At age 6 months, he begins riding on his mother’s back.
Northern muriqui mothers nurse their babies for 14 to 18 months, often in the company of other nursing mothers. Mothers have been observed swapping and nursing one another’s infants.
Young muriquis are considered to be independent at age 2, at which time they no longer get a free ride on their mother’s backs.
Muriquis are important ecological ambassadors. They help to regenerate their forest environment by distributing seeds, via their feces, from the many fruits they eat.
Conservation Status and Threats
The northern muriqui is classified as Critically Endangered—only one step away from Extinction in the Wild—by the International Union for the Conservation of Nature (IUCN 2019), appearing on the IUCN Red List of Threatened Species. Additionally, the northern muriqui has had the ignoble distinction of being featured as one of The World’s 25 Most Endangered Primates. Over the past 60 years (three generations), the overall population has declined 80 percent.
Severe habitat destruction has greatly reduced the northern muriquis’ range, forcing the monkeys into fragmented populations. Once-pristine tracts of forest have been razed for logging, agricultural use, cattle farming, and human development, leaving a devastating effect on the muriqui population.
Northern muriquis face another grave threat: humans hunt them, for both food (the monkey’s flesh is known as bushmeat) and for sport. Because of hunting—even when the monkeys reside within protected areas—two dozen populations have been lost to extirpation, or “local extinction.”
An outbreak of yellow fever that began in 2016 caused the deaths of many northern muriquis.
Today, fewer than 1,000 individuals remain in the wild. They survive in 14 isolated subpopulations in a mixture of private, municipal, state, and national protected areas; even the largest subpopulation has shown severe population fluctuations in recent years. These individuals are in peril.
The northern muriqui is listed in Appendix I of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
As a flagship species for the conservation of the Atlantic Forest hotspot, conservationists call for the restoration of late successional forests to help ensure the northern muriqui’s survival. One important reserve dedicated to the northern muriqui is the Reserva Particular do Patrimonio Natural Feliciano Miguel Abdala, located in the state of Minas Gerais. The reserve is named for its founder, Feliciano Miguel Abdala (1908–2000), a private landowner and passionate conservationist, who established the reserve as the Caratinga Biological Station in 1982. (To learn about the good work going on at the reserve, visit the reserve’s Facebook page: https://www.facebook.com/ProjetoMuriquiCaratinga (Projeto Muriqui de Caratinga / Muriqui Project of Caratinga).
Northern muriquis are also found in the Augusto Ruschi Biological Reserve in the state of Espirito Santo and in a mixture of private, municipal, state, and national protected areas within the species’ geographic range.
Scientists are currently conducting population monitoring and ecological/behavioral studies on the species. A study published in January 2016 reports a previously unknown population of southern muriquis within Caparaó National Park, on the border between the states of Espírito Santo and Minas Gerais.
Additionally, conservation initiatives are underway at six private reserves and state parks: four locations within Minas Gerais and two locations within Espírito Santo.
- Mendes, Francisco D.C., & Ades, César. (2004). Vocal Sequential Exchanges and Intragroup Spacing in the Northern Muriqui Brachyteles Aarachnoides hypoxanthus. Anais da Academia Brasileira de Ciências, 76(2), 399-404.
- https://animaldiversity.org/accounts/Brachyteles_hypoxanthus https://www.newscientist.com/article/2119610-yellow-fever-outbreak-is-killing-off-rare-monkeys-in-brazil
Written by Kathleen Downey, May 2021