TANA RIVER MANGABEY
Geographic Distribution and Habitat
The Tana River mangabey (Cercocebus galeritus), also known as the Tana River crested mangabey, is endemic to a small stretch of floodplain forests along the Tana River in Kenya. They occupy about 27 small (under 1,200 acre/5 km²) forests, many of which are fragmented, both naturally and anthropogenically (from man made activities and events). The species’ entire range is less than 6,400 acres (26 km²). Floodplain forests are very unique ecosystems, as they are highly dependent on the height of the groundwater table and on the natural river processes of flooding, erosion, and nutrient cycling. Unfortunately, human activities such as building dams interrupts these sensitive natural processes and degrades the forest. The Tana River region boasts high biodiversity, with many endemic plant and animal species with small ranges. Unfortunately, it is a highly threatened environment due to sensitivity of the ecosystem and the rampant human-caused degradation that occurs there.
Size, Weight, and Lifespan
Female Tana River mangabeys weigh 12 lbs (5.4 kg) on average, and their head and body length is 17–21 inches (44–53 cm), with a tail length of 20 inches (50 cm) on average. Males are larger, weighing 21 lbs (9.5 kg) on average, with a head and body length of 19–25 inches (49–63 cm) and a tail averaging 27 inches (68 cm) in length. Lifespan has not been well-documented, but based on other species in their genus, they likely live to about 19 years in the wild.
Tana River mangabeys are medium-sized monkeys with long, light gray hair over most of their bodies. On the bellies, their hair transitions to a cream color. Their forearms and hands are darker than the rest of their bodies. Their faces are dark gray, except for their white eyelids, and framed by a crest that parts in the middle. Males are overall larger than females, and their tail is about 33% longer. Besides that, they do not exhibit any other sexual dimorphism. Infants have pink faces, ears, and limbs, and do not have a crown. Tana River mangabeys have jaws and teeth that are specially adapted to their diet. They have very thick enamel on their molars and an extremely powerful jaw that can provide the crushing force needed for seed crushing.
Tana River mangabeys are frugivores, primarily eating fruits, and supplementing their diet with stems, leaves, insects, and fungi. They are flexible eaters, adapting to what the season has to offer. This flexibility has been crucial as their habitat continues to become fragmented and degraded.
Behavior and Lifestyle
Tana River mangabeys are diurnal (active during the day) and mostly terrestrial, spending about 56% of their time on the ground, 32% in trees up to 33 feet (10 m), and 12% in trees above 33 feet (10 m). They tend to sleep in tall trees with full crowns that are inaccessible to non-arboreal species. They usually sleep in branch forks near the main trunk of the tree. Most of their time on the ground is spent foraging, which usually occurs around the middle of the day. They travel an average of about 1 mile (1.5 km) per day in search of food. In the months when food is scarce, Tana River mangabeys spend more of their time foraging and moving. When food is more abundant, they spend more time socializing with group members. Although their habitat is highly fragmented, they can and will travel between different patches of forest, another example of how they have adapted to the degradation of their habitat.
Because of their highly fragmented habitat, Tana River mangabeys serve as host to an unusually diverse variety of gastrointestinal parasites, including ten species of nematodes and three species of protozoans. Further research is needed to determine whether these parasites have a significant negative impact on the species.
Highly social animals, Tana River mangabeys live in multi-female social groups with an average size of 27 individuals. Typical group composition is two adult males, seven adult females, two subadult males, two subadult females, ten juveniles, and four infants. There is typically one alpha male who dominates mating opportunities, and sometimes a beta male that is tolerated.
Groups occupy home ranges of 114 acres (0.46 km²) in area on average, about one-third of which may overlap with other groups. Their home ranges are flexible, expanding when food is scarce, another adaption to habitat degradation. Relations between different groups vary depending on the time of year. When food is scarce, groups tend to stick to non-overlapping parts of their territory, with adult males using loud calls to ensure that groups are spaced apart. When groups do interact during these months, there may be a confrontation, although generally Tana River mangabeys are quite peaceful. During months when food is abundant, multiple groups may move and feed together. Groups may even come together during times of food abundance as a way to avoid inbreeding, since this also tends to be a time of increased conception.
Antagonistic displays are common among competing males in a group, and include eyelid flashing, lunging, chasing, biting, branch shaking, and vocalizations. In the mornings, males frequently exhibit a loud “whoop-gobble” call that can be heard almost a kilometer away, which is used to signal their position to other groups and maintain spacing between them. Amicable displays between a male and a female include grooming and play. Females often vocalize after copulation.
Tana River mangabeys have a polygynous reproductive system, in which the females of a group typically only mate with the alpha male. Females have large estrous swellings every month for about four or five days, which signals to the male that she is fertile and receptive to copulation. Their mounting position is unique, with the male grasping the female’s ankles with his feet and placing his hands on her hips. Females also exhibit post-conception swelling. This may serve to mimic a female’s estrous swellings and trick males into believing she is fertile. This then confuses the paternity of the offspring and encourages paternal investment by all males with whom she mated. For example, if she conceives from copulation with the alpha male, her post-conception swellings may trick the beta male into believing she is fertile and mating with her. When she gives birth, both males have incentive to provide parental care, because they both mated with her around the time she conceived.
Gestation lasts 180 days, after which a single offspring is born. During the birth, other members of the group give plenty of space to the new mother, only approaching about 20 minutes after the offspring is born. More births tend to occur between August and April, the time of greatest food availability. Females give birth about every 18 to 24 months, and nearly two-thirds of adult females in a group give birth in a given year. Based on other species in their genus, Tana River mangabey females likely become sexually mature at about three years of age, breeding for the first time when they are six or seven. Males likely become sexually mature when they are about five, and breed for the first time when they are about seven. Little is known about parental care, although females tend to have the most sustained interest in young and likely provide most parental care.
Tana River mangabeys are sympatric with Tana River red colobus monkeys, blue monkeys, Kenya coast galagos, and northern greater galagos. About 32% of their diet is composed of seeds, making them important seed dispersers. Their predators include African rock pythons, African crowned eagles, martial eagles, and Nile crocodiles.
The International Union for Conservation of Nature lists the Tana River mangabey as Critically Endangered (IUCN,2019), appearing on the IUCN’s Red List of Threatened Species. This is the last step before extinction in the wild. There are believed to be approximately 1,000 individuals in the wild today, and this number is predicted to be reduced to under 200 over the next 30 years.
Tana River mangabeys face a slew of threats, most of which are threats against their habitat. Dam construction in the Tana River area has resulted in drastic changes to vegetation, changes to the water table, and changes to the frequency and severity of floods, all of which negatively impact the forests that they rely on for survival. These forests are also threatened by deforestation for agriculture, wildfires, degradation due to livestock, and the unsustainable collection of wood and other forest products. One of Tana River mangabeys’ top food plants, the wild date palm, is also collected at an unsustainable rate, impacting the monkeys’ food security. Tana River mangabeys are sometimes hunted as crop pests. As forests continue to be exploited and the availability of natural resources dwindles, it’s likely that hunting of the species will increase. Political instability in the area has also caused violence, insecurity, and inadequate law enforcement in the area that has negatively impacted the species. High human population growth rates indicate that habitat encroachment and demand for natural resources is likely to increase.
Tana River mangabeys are listed on Appendix I of the Convention on International Trade in Endangered Species (CITES), and on Class A of the African Convention on the Conservation of Nature and Natural Resources.
Despite the critical state of the species, there is currently no legal habitat protection available to Tana River mangabeys. The Tana River Primate National Reserve, which was established in 1976 and was home to 56% of all Tana River mangabeys, was “de-gazzetted” in 2007 by the High Court of Kenya, meaning it had all legal protection stripped. This decision was made because it was found that the reserve was not founded in accordance with the law. A small number of Tana River mangabeys may live in the Tana River Delta, which is recognized as a “Wetland of International Importance” (also known as a Ramsar Site) under the international Convention on Wetlands, but otherwise is not protected. The Ndera Community Conservancy and the Northern Rangeland Trust are attempting to preserve habitat in the Tana River region.
While the Tana River mangabey is an adaptable species, it illustrates the fact that even flexible species are greatly at risk when it comes to human encroachment on habitat. It is clear that for this charismatic, unique species to have a chance at surviving the 21st century, significant work needs to be done to protect the Tana River habitat.
We’d like to express our sincere thanks to Dr. Julie Wieczkowski at the Anthropology Department of SUNY (State University of New York) Buffalo State College for so generously providing us with an array of beautiful Tana River mangabey photos to select from to accompany this profile. For many years Dr. Wieczkowski conducted field studies on Tana Rive mangabeys in Kenya. We are grateful for her generosity.
- Mbora, D.N., Wieczkowski, J. and E. Munene. 2009. Links between habitat degradation, and social group size, ranging, fecundity, and parasite prevalence in the Tana River mangabey (Cercocebus galeritus). Am. J. Phys. Anthropol., 140: 562-571. https://doi.org/10.1002/ajpa.21113
- Ndera Community Conservancy. 2019. Conservancy Management and Community Development Plan 2019-2024). Northern Rangelands Trust.
- Wieczkowski, J. 2009. Brief communication: Puncture and crushing resistance scores of Tana river mangabey (Cercocebus galeritus) diet items. Am. J. Phys. Anthropol., 140: 572-577. https://doi.org/10.1002/ajpa.21132
- Wieczkowski, J. 2010. Tana River Mangabey Use of Nonforest Areas: Functional Connectivity in a Fragmented Landscape in Kenya. Biotropica, 42: 598-604. https://doi.org/10.1111/j.1744-7429.2010.00627.x
Written by K. Clare Quinlan, December 2020