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Episode 364: Exploring Gelada Monkeys

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The gorgeous Gelada monkey, or bleeding heart baboon (image from the New England Primate Conservancy)

Gelada monkeys, also called the bleeding heart baboons thanks to the reddish patch of fur on their chest, are only found in the Ethiopian highlands, and are unique in the primate world as they are the only grass grazing monkeys! Although they are sometimes called baboons and resemble them physically, evolutionarily and biologically speaking they are not true baboons, but are actually old world monkeys. 

90% of their diet consists of grass, both the blades and seeds, and the remaining 10% is made up of flowers, fruits, small plants, bushes and thistles. This diet has developed as their highlands of Ethiopia are free of forests or more complex vegetation, although in contrast to many other monkeys the lack of large predators in their habitat makes the Gelada monkeys lives far easier than their lowland counterparts, despite their limited diet. The monkeys are diurnal, spending their nights sleeping on the rugged highland cliffs and traveling to the plateaus during the day to forage and socialize. Unlike monkeys in other areas, gelada monkey do not experience the harshness of the dry season and the loss of food that accompanies it as the highlands are far cooler and less arid. However, they can experience some frost in the dry season and hailstorms during the wet season that can affect their ability to forage. 

Gelada monkeys have been placed in their own genus, theropithicus. Extinct members of this genus have been found in the fossil record, and revealed that they once ranged throughout Africa to the modern day Mediterranean and into Asia. A study of their genetics from the 90’s revealed that the closest relatives of the Gelada monkey is the mangabey, a family of old world African monkeys. 

Monkey business: the social lives of Gelada monkeys 

Gelada monkeys live in complex, multilevel societies. The lowest level is called a reproductive unit, and consists of around 12 females, 1-4 males and a few infants. At this level there are all male groups, which consist of anywhere between 2-15 males that have not paired up for the breeding season. The next social level are called bands that are made up of 2-27 reproductive units, several of which will be all male units. After this, the next social level are called herds, which can be as many as 60 reproductive units, and tend to form only briefly. 

Females in reproductive units are usually closely related with strong bonds that they maintain through grooming and other social interactions and they will generally stay in these natal units for life. The higher ranking a female is, the more reproductively successful she will be. Aggression is very rare amongst groups of Gelada monkeys, and is generally mostly seen between females in different units. 

A reproductive unit of Gelada monkey in their Ethiopian highland home (image from Stony Brook University)

As with many species, Gelada monkeys display female philopatry, meaning that males will disperse away from their natal group after being with their reproductive unit for anywhere between 1-5 years. Once males disperse away, they will form all male units where they will fight to determine who will be dominant. After 2-4 years in an all male unit, males will begin to establish themselves in their own groups by trying to take dominance of a reproductive unit. Their success in this endeavor will depend largely on the females in the unit; they may be kicked out or supported by the females; talk about girl power! Even if a male cannot assert dominance, he may be kept around in the unit as a babysitter, helping to care for any infants in the unit. Unlike other primate species, such as baboons, males will maintain their position in the group through grooming females, rather than through aggression and dominance displays. 

Only the dominant male will breed, even if there are multiple males in one reproductive unit. However, despite this dominance, South African researchers observing bands found that Gelada monkeys can ‘cheat’ on their partners and will even attempt to cover it up! They observed that subordinate males would at times mate with females in the unit, and both parties would suppress their usual loud mating calls. If the dominant males would discover this, the pair would be attacked in one of the only displays of aggression seen within units in this species. This is the first known example of cheating and a fear of discovery in the animal world, and an interesting finding that primates are capable of deceit, once thought to be solely the province of humans. 

Geladas also display a phenomenon called the Bruce Effect, first described by the zoologist Hilda M Bruce in 1959. She discovered that female mice will terminate their pregnancies when exposed to the scent of unfamiliar male urine due to chemical signals that kickstart a biological process in the females that leads to the disruption of pregnancies. This is very common in mice and has been seen in lions, and in Gelada monkeys occurs when a new male takes over a reproductive unit, causing the abortion of any offspring conceived with the previous male dominant. Because of this effect, there is little incentive for males to commit infanticide as females will abort their pregnancies if they are pregnant, and if they have just given birth will be immediately back in oestrus. Occasionally though, in some isolated regions, occasional infanticide has been seen, and no one is entirely sure of the reason why. 

Gelada monkeys have pretty good success with their offspring, with 85% of their infants surviving to their 4th year. This is largely due to the fact that the empty rolling grasslands of highland Ethiopia are not rich enough to support many other animals, and especially not large predators that would hunt monkeys in rainforests and savannahs. Occasionally, Gelada monkeys can be predated by domestic dogs from local settlements or by leopards, servals and hyenas that will venture into higher ground. 

Living the high life: the current status of the Gelada monkey 

Gelada monkeys are listed as least concern by the IUCN red list, although there is some concern that their populations are beginning to decline. In Simien National Park, the largest national reserve in Ethiopia, Gelada monkeys are considered crop pests, and can cause huge damage to crops such as barley, potentially devastating the livelihoods of the poor farmers in the region. In response to this Gelada monkeys are often sadly shot by frustrated farmers. Although the highlands of Ethiopia have been relatively untouched compared to other ecosystems in Africa, agricultural expansion  and subsequent soil erosion still threatens the monkeys as they are often pushed into areas with less resources to support them.

The Simien Mountains National Park (image from TripSavvy)

Although they are not the sole focus of any conservation charities, the African Wildlife Foundation, a not for profit based in Kenya, has set up the Simien Mountains Cultural Tourism Project that is aiming to set up better infrastructure in the area for ecotourism. This will not only provide a sustainable source of income to support conservation initiatives in the area, but will also give local communities a chance for different livelihoods to improve their living standards. The AWF also works with local communities to better plan the long term usage of land, improving the lives of both the people and the Gelada monkeys. You can learn all about the Simien Mountains project here Simien Mountains | African Wildlife Foundation (awf.org), and learn more about the work of the African Wildlife Foundation all throughout Africa. 

Awesome videos!


Dunbar, R. (1980) ‘Determinants and evolutionary consequences of dominance among female gelada baboons.’ Behavioural Ecology and Social Biology 

Dunbar, R. (1977) ‘Feeding ecology of gelada baboons: a preliminary report.’ Primate Ecology

Dunbar, R. (1983) ‘The Social Ecology of Gelada Baboons.’ Ecological Aspects of Social Evolution

Elia, K. Roberts, A.L. Bergman, T.J. Beehner, J.C. (2012) ‘A Bruce Effect in wild geladas.’ Science 

Hughes, K. Elton, S. O’Regan, J (2008) ‘Theropithecus and ‘Out of Africa’ dispersal in the Plio-Pleistocene.’ Journal of Human Evolution

Iwamoto, T. Dunbar, R. (1983) ‘Thermoregulation, habitat quality and the behavioural ecology of gelada baboons.’ Journal of Animal Ecology

Le Roux, A. Snyder-Mackler, N. Roberts, E.K. Beehner, J.C. Bergman, T.J. (2013) ‘Evidence for tactical concealment in a wild primate.’ Nature Communications

Theropithecus gelada (Gelada) (iucnredlist.org)

Yihune, M. Bekele, A. Teferea, Z. (2009) ‘Human-gelada baboon conflict in and around the Simien Mountains National Park, Ethiopia.’ African Journal of Ecology 


November 29, 2023
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