Cercocebus sanjei

Geographic Distribution and Habitat

Sanje mangabeys, also known as Sanje River mangabeys and Sanje crested mangabeys, are Old World monkeys native to Tanzania, a sovereign state of eastern Africa. They occur only in Tanzania’s Udzungwa Mountains, residing on the eastern slopes at a wide range of altitudes from 1,312 to 4,266 ft (400-1,300 m) above sea level. Of the world’s entire Sanje mangabey population, about 60 percent lives in the Mwanihana Forest within Udzungwa Mountains National Park; the remaining 40 percent lives within the Udzungwa Scarp Forest Reserve, southwest of Mwanihana Forest. Habitat destruction has isolated the two populations from one another; no interaction between the two groups is known to occur.

The species’s habitat includes mosaic environments (interspersed ecosystems) of tropical, acacia, evergreen, montane, submontane, primary, riverine, seasonal, secondary, and semideciduous forests. Within these ecosystems are microhabitats (smaller habitats differing in character from the surrounding, more extensive habitat) that provide ecological niches where Sanje mangabeys make their homes.


The species’ pale eyelids earn its membership to the “white-eyelid mangabey” group. Five other mangabeys, all within the genus Cercocebus, share this aesthetic distinction and exclusive membership:

White-eyelid mangabeys are closely related to mandrills (Mandrillus sphinx) and drills (Mandrillus leucophaeus).

Sanje mangabey geographic range. Two regions so small they are barely perceptible on the map. Map: IUCN, 2019

Size, Weight, and Lifespan

Sanje mangabeys are medium-sized monkeys. Head and body length for both sexes is between 1.6 and 2.13 ft (50-65 cm). Their tails add another 1.8 to 2.13 ft (55-65 cm).

Males carry much more girth than females. Adult males weigh about 22.7 lb (10.3 kg); adult females weigh considerably less, 12.8 lb (5.8 kg). This significant difference is size is an example of sexual dimorphism.

Lifespan for wild Sanje mangabeys is not well documented. (Information gleaned from a scientific abstract puts it at about 20 years.) For captive Sanje mangabeys, lifespan is 25 to 30 years.


Deep, dark, luminous eyes are made more prominent by a creamy pink coloring that powders the surrounding skin, including the monkey’s high forehead.

At the center of the monkey’s pinkish-beige face sits a modest, gray muzzle. Pale blue hues etch the outer cheeks along the hair line, meeting a bushy head of hair. A tiny glimpse of bare scalp is visible amidst the thick hair on the crown of the head, before the hair swirls outward. It’s as if Mother Nature intended it to part the monkey’s hair, but threw down her comb in frustration. A distinct rim, or crest (lending itself to the monkey’s alternative name of “Sanje crested mangabey”) feathers out above the forehead, tapering to frame the Sanje mangabey’s expressive face.

Coat color in adults varies from a smoky brown or fawn to a silvery gray. Fur covering their abdomen is a pale orange, and the monkey’s long, gray tail is punctuated with a slight tuft at the tip.

Skin on their hands, feet, and ears is dark gray.

Infant Sanje mangabeys are cloaked in dark, grayish-black coats, and the fur covering their abdomen is orange. The skin on their face, ears, hands, and feet is pink.

Photo: © Stefaneakame/iNaturalist/Creative Commons

More than half their diet—which includes 90 species of plants—consists of fruits, and nearly a third of their diet consists of seeds. Seasonal availability determines which fruits they eat. Sanje mangabeys round out their meal plan with a smattering of insects, leaves, flowers, fungi, and the occasional snail or lizard.

The foods they eat are high in fiber, are easily digestible, and provide the energy surge needed to go about their day. To further maximize their nutrient intake, Sanje mangabeys feed selectively on certain types of foods. Subadults and juveniles, whose energy requirements are great, eat a lot of shoots and stems. Adult males and females, whose energy requirements are low, eat more leaves.

Behavior and Lifestyle

Sanje mangabeys are equally arboreal (tree-dwelling) and terrestrial (land-dwelling). Some scientific accounts suggest that the species is more terrestrial than arboreal. When in the trees, they are often found in the lower understory of submontane and montane forests. They use their long tails for balance.

The monkeys are active during daylight hours, making them diurnal. When not climbing, leaping, or moving quadrupedally (on all fours) through the forest, Sanje mangabeys can be found foraging on the forest floor. Their home range size is approximately 1.5-2.3 sq mi (4-6 sq km), and it may overlap with the home range of neighboring Sanje mangabey groups. The monkeys forage closer to their sleeping sites during the rainy season and travel farther distances during the dry season. Researchers speculate that longer travels increase the monkeys’ chances of finding rare and delectable food items such as figs, termites, and unusual fruits.

Fun Facts

Considered a species that is relatively new to science, the Sanje mangabey was “discovered” by happenstance in 1979. A somewhat anecdotal account describes this momentous event. While collecting plants in the forest, two wildlife researchers heard an unusual primate call. Their Tanzanian guide described the animal they had heard as a “n’golaga,” the Swahili name for the species. The next day, researchers caught a glimpse of the monkey in the forest canopy. But it wasn’t until their guide escorted them to the village of Sanje to meet an orphaned “n’golaga” that the researchers were certain they had discovered a new species. The young monkey was being kept as a pet by local children, whose father had shot and killed the monkey’s mother after mistaking her for a yellow baboon.

The white-eyelid mangabey’s genus name Cerocebus means “tail monkey” in Greek.

The name Udzungwa comes from the Kihehe word “Wadzungwa,” which means “the people who live on the sides of the mountains,” or in this case, the word refers to the Udzungwa Mountains’ most iconic nonhumane primate: the Sanje mangabey.

Daily Life and Group Dynamics

Some Sanje mangabeys prefer their own company to the company of others and live as solitary individuals; others are more social and live together in multimale/multifemale groups. Group size ranges from 10 to 60 individuals. The larger groups typically have one adult male to every 4 or 5 adult females (as documented by wildlife biologists who studied one 47-member group). A clear dominance hierarchy exists among adult males; less of a hierarchy is evident among a group’s females. In what is described as “emigration,” adult males leave their birth group upon reaching maturity to seek out or establish a group where they can be an alpha male. However, resident males are also known to form coalitions with one another.

Large groups often break into smaller foraging parties and then reform as one group in the evening, an example of a fission-fusion society. Overnight, Sanje mangabeys sleep in tall trees that are clumped closely together.

During their foraging expeditions, Sanje mangabeys may associate with other primate species, including Udzungwa red colobus monkeys (Piliocolobus gordonorum) and Sykes’ monkeys (Cercopithecus albogularis).

Nonprimate associations include duikers (small African antelopes) and crested guinea fowl (Guttera pucherani).

Although they do not associate with human primates, Sanje mangabeys residing in Udzungwa Mountains National Park are habituated to humans; that is, they are acclimated to the presence of humans in their habitat, whether these humans are tourists or wildlife researchers.

Major predators of the species include leopards, pythons, and African crowned eagles. However, in a rare and documented account from 2006, an adult Sanje mangabey emerged the victor in a battle with an African crowned eagle. The eagle, an adult female, was intercepted and bitten by an adult Sanje mangabey when the eagle tried to attack a young Sanje mangabey who was feeding in a tree. Both the eagle and Sanje mangabey fell 82 ft (25 m) to the forest floor. Miraculously, the Sanje mangabey survived. The eagle, however, died from her injuries.

Humans are the major predators of those Sanje mangabeys residing in Udzungwa Scarp Forest Reserve.


When approaching one another, Sanje mangabeys often arch their long tail over their back, with the tip nearly touching the top or side of their head. They bat their white eyelids and raise their eyebrows to convey a range of meanings.

Their vocal communication includes several calls. To check in with one another, particularly when members are spread out over an area while resting, the monkeys give low-volume, low-frequency contact calls.

Adult males emit a “whoop-gobble” call, most often at dawn from a group’s sleeping site high in the trees, before the group begins its daily foraging. The call, which can be heard for a distance up to 0.6 m (1 km), announces a group’s location and is also intended to intimidate outsider Sanje mangabeys who are intent on causing trouble.

Should they perceive a threat, the monkeys emit high-pitched alarm barks given in succession.

Females have calls of their own. To indicate her readiness to mate, a female emits a pre-copulatory call to nearby males. She may be vocal while mating, as well. After engaging in sex, she moves hurriedly away from her male partner and elicits a post-copulation call.

Reproduction and Family

Sanje mangabeys mate year-round. A female copulates with multiple adult males from her group. Lower-ranking males have a better chance of “getting lucky”—that is, experiencing greater mating success—when a group’s number of receptive females increases. Known as female synchrony—the greater number of ovulating females—the more challenging it is for a dominant male to succeed in monopolizing a harem all to himself. He is simply unable to simultaneously tend to each fertile female, unavoidably inviting breeding competition from other males in his group. (Conversely, when the number of receptive females is small, dominant males have the breeding advantage.)

Mating behavior and male mating “skew” was the subject of a 22-month study (published in 2017) that examined the impact of two outside Sanje mangabey males who infiltrated and took over an established group. Prior to the takeover, the group’s dominant male was “copulator-in-chief” (responsible for 75 percent of copulations with the females of his harem). But the arrival of the two outsider males, who achieved dominance, caused a temporary instability—a skew—in  in the group’s hierarchy when they became chief copulators of the harem.

When a female is in estrus (the period when she is capable of conceiving), her genital area swells, indicating to her male suitor that she is physically ready to copulate. She presents her rear end to the male and may wave her head. Enticed, the male sniffs or touches the female’s genitals before mounting her. He mounts her by holding onto her ankles with his feet while grabbing her hips with his hands. (Researchers have dubbed this technique the “double-foot clasp.”) The female may arch her head back and make eye contact with her male partner, or she may reach back to grasp his leg, shoulder, face, or hair.

Soon after she copulates with one male, a female emits a pre-copulatory call to attract her next partner. She may mate with as many as seven males in one day! Whether her behavior is considered promiscuous or is a means to ensure conception—males are only single-mount ejaculators—is a topic open to scientific speculation (or gossip!). A female’s “promiscuousness” might also be savvy feminine strategy. Having multiple male partners makes paternity questionable, which, in turn, may reduce the occurrence of infanticide and may garner help with parental care from a fecund female’s male suitors.

Gestation period (pregnancy) in the species is between 5-1/2 and 6 months.

Mangabeys give birth to a single offspring, who is born with soft fur and with eyes open.

Like all mangabeys, infant Sanje mangabeys cling to their mother’s belly as she travels. Between 3 and 5 weeks of age, infants begin to grasp and mouth objects. At 12 weeks they begin engaging in social play and can walk, run, and jump. Young mangabeys are considered weaned between 7 to 10 months of age, but they remain near their mother until she gives birth to a new sibling.

Male Sanje mangabeys participate in co-parenting by carrying and protecting a group’s infants (this shared infant care is known as “alloparenting”). But they might have an underlying motive to their attentiveness. Apparently, males who more often help with infant care are granted more opportunities by the females to copulate.

Photo: © markusgmeiner/iNaturalist/Creative Commons
Ecological Role

Thanks to their penchant for fruits, Sanje mangabeys help to replenish their habitat through the seeds they disperse via their feces.

Conservation Status and Threats

The Sanje mangabey is listed as Endangered by the International Union for Conservation of Nature, appearing on the IUCN Red List of Threatened Species (IUCN, 2016). Its total population is decreasing.

Most recent available population estimates range from 1.300 to 3,500 individuals. Some estimate 1,750 to 2,100 individuals residing in Mwanihana Forest within Udzungwa Mountains National Park and between 1,050 and 1,400 in Udzungwa Scarp Forest Reserve. But researchers caution that more a precise census is needed to accurately determine the species’ total population. Increased use of camera trapping in recent years indicate that occupancy is higher for the population in the Udzungwa Mountains National Park, at about 60% of the total population, than in the Uzungwa Scarp Nature Reserve.

Habitat loss, due to deforestation, is a major threat to the species’ survival. Timber and charcoal production have destroyed pristine habitat. Tanzania’s policy regarding agricultural use favors conversion of forest habitat to alternative uses. But conservation is a complex issue in Tanzania. Expansion of agricultural tracts of land is intended to increase per capita production to meet the needs of Tanzania’s poor. Approximately 60 percent of Tanzania’s human population lives on the equivalent of $1 U.S. a day.

Poaching is a serious threat to the Sanje mangabey population residing within Udzungwa Scarp Forest Reserve, where they are hunted and slaughtered for “bushmeat.” Locals, for whom poaching is a dubious livelihood, occasionally use dogs to hunt the monkeys; more typically, they use snares. Sometimes, the mangabeys are caught in snares that have been set to capture other species.

Conservation Efforts

The Sanje mangabey is listed in Appendix II 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; the species is listed as Class B by the African Convention on the Conservation of Nature and Natural Resources. However, as is often the case with endangered species, laws created to protect them are largely ignored and difficult to enforce.

Most wildlife studies of the Sanje mangabey have occurred in Udzungwa Mountains National Park. Known as “The Galapagos of Africa,” Udzungwa Mountains National Park is a world biodiversity hotspot and is recognized by the World Wide Fund for Nature as an ecoregion of global critical importance for the extraordinary diversity of plants and animal species found there. Established in 1992, the park covers more than 770 sq mi (1,994 sq km). It was formed from five forest reserves that have survived over 30 million years and were once connected to the Congo Basin and West Africa.

Previous efforts to strengthen and enforce protections with and expand the park’s boundaries have been unsuccessful.

Sanje mangabeys residing in the Udzungwa Scarp Forest Reserve are less habituated to humans and are therefore more elusive.

Researchers are currently using genomic techniques (studying the species’s genetic material) to analyze the DNA from fecal samples. Their goal is to establish the diversity of the Sanje mangabey population and to shine a light on any adaptive differentiation between the individuals residing in Udzungwa Mountains National Park and the individuals residing in Udzungwa Scarp Forest Reserve.

Established in 2006, the Udzungwa Ecological Monitoring Center (UEMC) is a field station in Tanzania’s Udzungwa Mountains National Park that promotes ecological monitoring and environmental education programs to increase awareness and appreciation of the park and its species. Wildlife researchers who stay at the center participate in various conservation initiatives—including a primate monitoring program. A key tenet of UEMC is community education.

An intensive camera trapping assessment (“capturing” the monkeys using surveillance cameras) has been underway since 2016, and demographic surveys of the species began in 2017.

Working with locals is imperative to wildlife researcher Dr. David Fernandez, who has dedicated himself to saving the Sanje mangabey. In the course of monitoring Tanzania’s Sanje mangabey population, Fernandez has engaged the local community by partnering with local primary schools to improve infrastructure and raise awareness of conservation issues. He has authored or co-authored several recent articles about the species. Fernandez and his fellow researchers advocate for the enforcement of existing anti-hunting laws in nature reserves, and they stress the importance of preventing further habitat destruction.

our thanks

Our thanks to Dr. David Fernandez, wildlife conservation biologist and Senior Lecturer at the University of the West of England, Bristol, for providing rare photos of the Sanje mangabey—quite a few years ago—and, more recently, for so generously sharing his extensive field research with us so we could bring you this profile. We are immensely grateful.

  • Fernandez, David. “Sanje Mangabey.” Email, October 2018.

Written by Kathleen Downey, August 2018. Conservation status and population details updated November 2020.