Cercocebus sanjei

Geographic Distribution and Habitat

The Sanje mangabey, also known as the Sanje River mangabey or the Sanje crested mangabey, can be found in the highlands of Tanzania. They often live in mountainous and sub-montane forests, and spend over half of their time on the forest floor. These regions rest between 951 to 6,561 feet (290 to 2,000 m) above sea level. Sanje mangabeys can be found in the Mwanihana Forest and Udzungwa Scarp Nature Preserve, and are often observed moving throughout the varied, sometimes human-occupied landscape. While some populations are occasionally found in lowland forests, they occur at much higher densities in high-elevation forests. This may be because there have been fewer anthropogenic, or human-caused, disturbances there. 


The term mangabey is an informal name for a branch of the Cercocebus genus. They are characterized by their short hair, light-colored eyelids, and propensity for time on the forest floor. The Sanje mangabey was made known to Western scientists in the 1980s, with the first recording happening in the Udzungwa Mountains in Tanzania. Mangabeys light eyelids are used as a marker for a small group, aptly named the white-eyelid mangabeys. There are five other mangabeys in this group, along with Sanje mangabey. 

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 a sexually dimorphic monkey species, meaning their body size and attributes differ depending on the monkey’s sex. In the Sanje mangabey’s case, the males are larger than the females. Head and body length ranges from 1.6 to 2.13 feet (50-65 cm) long, and their tails are almost the same length at 1.8 to 2.13 feet (55-65 cm). Sexual dimorphism is more noticeable regarding weight, with males weighing almost twice as much as females. Males are approximately 22.7 pounds (10.3 kg) and females are 12.8 pounds (5.8 kg). This gives the males a much more robust appearance. 

While their life span is not well recorded, most likely due to the difficulty of long-term studies in their native habitat, we have a loose approximation for life expectancy in the wild. Other mangabeys live to be 15 to 20 years old in the wild, so we may assume that this is similar for the Sanje mangabey.


The Sanje mangabey has light to dark brown hair on its back and often has a blonde or golden underside. Their light eyelids help them stand out among the forest crowd. This lighter coloration surrounds their eyes, almost as if they are wearing a mask. This lighter coloration is abruptly cut off by the Sanje mangabey’s distinct crest at the brow. The hair on the top of their heads is often coarse and tends to stick straight up in the air, leaving them with a distinguished hair-do. Underneath their thick pelage, they have light to dark grey skin. This coloration can be seen on their hands, feet, and parts of the face. 

Infant Sanje mangabeys have a darker coloration than the adults in the group. Their backs and heads are often dark brown or grey, and their underside is golden or orange.

Photo: © Stefaneakame/iNaturalist/Creative Commons

The Sanje mangabey is a frugivore, which is to say that their diet consists heavily of fruit. They eat other foods as well, such as nuts, seeds, fungi, leaves, and some small invertebrates. However, they largely prefer sweet, ripe fruit. In fact, more than half of their diet consists of fruit, with seeds being their favored secondary food. While the Sanje mangabey prefers fruit, their diet varies depending on the season and food availability.

It has been observed that diet varies, depending on life stage too. Juveniles and subadults eat more high-energy foods to support their growth and development. By contrast, the adult diet includes lower-energy supplemental foods, such as leaves. 

Sanje mangabeys have been observed engaging in geophagy, which is the deliberate ingestion of soil. While there are many aspects of geophagy that scientists do not understand, it is thought to help relieve gastrointestinal distress, help rid the body of parasites, or act as a supplement for minerals. Geophagy is often seen in primates that consume a lot of leaves, presumably to help digest the plant’s tough cellulose. Yet, it was observed most often in Sanje mangabeys during seasons of increased ripe fruit availability. It has been hypothesized that the high fructose levels from the fruit may cause some gastrointestinal distress, which is then alleviated or eased by soil consumption.

Behavior and Lifestyle

The Sanje mangabey is agile in the canopy and comfortable on the forest floor. It is estimated that these monkeys spend over 50% of their time on the forest floor, foraging and socializing. Their home range is 1.5-2.3 square miles (4-6 sq km), and they can be found venturing to the farthest boundaries of this range during the dry season. During the rainy season, they tend to stay closer to the heart of their territory. 

They are a diurnal species, meaning they are awake during the day and sleep at night — just like humans. They spend these days resting, socializing by allogrooming or playing, and searching for food.

Daily Life and Group Dynamics

Sanje mangabeys live in multi-male, multi-female groups. Females usually outnumber males, and their group size can vary from 10 to 60 individuals. Females are philopatric, meaning they stay in the group they were born into. Males, conversely, disperse once they reach sexual maturity. This allows for a healthy gene flow between populations, and the avoidance of inbreeding. Because males are the sex that enters into a new group, they must compete for dominance and female favor. Females have a less rigid hierarchy, but their bonds and alliances run deep. 

When foraging, Sanje mangabeys use a fission-fusion system. This is when a large group of monkeys may break into small foraging parties, then reconnect briefly throughout the day or sometimes not until nighttime. Once they have ‘fusioned’ back together at the end of the day, they gather close to sleep, which allows them to keep warm and stay safe from predators. 

Sanje mangabeys live sympatrically with other primate species, specifically the Udzungwa red colobus monkey (Piliocolobus gordonorum) and blue monkey (Cercopithecus mitis). They are known to occasionally associate with these monkeys, sometimes foraging or resting in the same area. They have also been seen associating with non-primate friends, such as duikers and crested guinea fowl. 


Sanje mangabeys communicate through a variety of vocalizations and physical cues. They communicate through difficult-to-see terrain by using low-frequency calls, which act as a check-in with other members of their group.  

Like many primate species, the morning is marked by the group’s dominant males vocalizing loudly. This call can be heard up to 0.6 miles away (1 km). This acts as a reminder to neighboring groups that the group is there, and a warning to stay in their respective range. 

Females are known to communicate vocally of their interest in copulation and are often vocal throughout sexual encounters.

Reproduction and Family

Sanje mangabey males and females mate with multiple partners to maximize reproductive success. As mentioned in the Communication section, females vocally invite males to copulate. Additionally, when they are in estrus (the time when they can procreate), their genitals swell dramatically, signaling to potential mates that they are receptive to copulation. Female Sanje mangabeys copulate with multiple partners while in estrus, and often multiple partners per day. She will choose partners that show some advantage or dominance in their environment, thereby endowing her offspring with traits that will be useful to their environment. 

Females reach sexual maturity at around the age of 2 years, however, they will not have their first birth until they are around 4 years old. Females give birth to singlets, and nurse for the first year of life. It will take the female close to two years to give birth again. The infant monkey travels everywhere with its mother, clinging to her belly for protection. Males participate in alloparenting, which is the caretaking of infants. This may serve a reproductive purpose, with these parenting consortships often ending in copulation advantages for the male.

Photo: © markusgmeiner/iNaturalist/Creative Commons
Ecological Role

As a frugivore, the Sanje mangabey is a vital seed disperser in their environment. Through ingestion and defecation, as well as their messy eating habits, they spread the seeds of fruit in their environment. This helps with the delicate balance of the forests and benefits many species.

Conservation Status and Threats

The International Union for Conservation of Nature lists Sanje mangabey as Endangered (IUCN, 2016), appearing on the IUCN Red List of Threatened Species.

The Sanje mangabey population is thought to be between 1,500 and 3,500 individuals, although a thorough and widespread assessment has not been conducted. This species has split into two primary populations, with around half being in the Udzungwa Mountains National Park and the other half being in the nature reserve. This population fragmentation presents complications for gene flow and species health since the population numbers are already so greatly reduced. The Sanje mangabey experiences pressures due to loss of habitat, habitat fragmentation, poaching, and predators. These monkeys are considered habituated to humans, as they live in the same geographic area as humans. This presents issues regarding disease transmission and susceptibility to hunting. They are hunted for meat and subsistence purposes. They fall prey to non-human animals as well, such as leopards, crowned eagles, and large snakes.

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.

While the Sanje mangabey is protected both nationally and locally, park rangers have had a difficult time protecting them locally. Previously a Forest Reserve, the Nature Reserve is due to receive more funding. This will allow for initiatives, more protection, and ecotourism opportunities, which will benefit all in the community. There are large-scale camera trapping projects happening, which will help with population estimates as well as assessing the extent of habitat loss, and those implications. Presently, enforcing laws that protect these monkeys is the most pressing matter.


Written by Robyn Scott, May 2024