For those of you who are avid fans of Disney’s The Lion King, you may be familiar with a mischievous monkey who is one of the primary characters guiding Simba on his journey to becoming king. Rafiki, the wise rascal beloved by many fans, is based on a real species of primate called a mandrill (Mandrillus sphinx). If you ever wanted to know more about the species that inspired this interesting and eclectic character, this article is for you!

Mandrills are the largest bodied monkey species in the world, which means apart from the large-bodied apes (gorillas, chimpanzees, orangutans, and humans) they are the largest primate species you may encounter if you were to saunter through the tropical rainforests of Cameroon, Gabon, Guinea, or the Democratic Republic of Congo [1]. They are one of the most sexually dimorphic mammals in terms of both their coloration and body size. This means males are significantly larger than females (the average male weighs 71 lbs., while the average female weighs 27 lbs.) and males have distinct red-blue coloration on their faces and rear-ends (see photo below). The coloration of a male’s face and ano-genital regions is correlated with his rank and, subsequently, his testosterone levels [2]. Alpha males have the brightest coloration, which is a signal to females of their status and value as reproductive partners. The coloration of mandrills is so infamous that Charles Darwin even wrote about them in his book, The Descent of Man, stating “no other member of the whole of the class of mammals is as extraordinarily colored as the adult male mandrill.” Before 1989, mandrills were classified in the same genus as baboon species (Papio), largely due to the similarities in their body size, coloration, and social structure to baboon species. Since 1989 however, this species has occupied its own genus (Mandrillus) after it was discovered that this species is actually more closely related to mangabeys (genus Cercocebus) than any of the extant baboon species.

Mandrills have been noted to be omnivorous generalists [3], meaning they can exploit a wide variety of food sources and are flexible in the ecological niches they inhabit. They have been observed to consume over 100 different plant species, but they prefer to feed on items such as fruits, leaves, stems, lianas, and other fibers. They also will consume multiple types of invertebrates including ants, beetles, spiders, snails, and termites, as well as small vertebrate species such as birds, frogs, porcupines, rats, shrews, and even tortoises. The inhabitants of gallery forests and equatorial jungles are essentially an all-you-can-eat buffet for a generalist species like the mandrill.

While mandrills are the predators of many species in the equatorial forests of Africa, they are also the prey of some of the primary predators in these regions as well. Their primary predator is known to be the leopard (Panthera pardus) [4]; however, reports of predation events by Crowned Eagles (Stephanoaetus coronatus) and African rock pythons (Python sebae) have been made for juvenile mandrills as well as old or sick mandrills. These reports put mandrills firmly in the center of the food chain in equatorial Africa.

The social organization of the Mandrill is highly complex and one of the least understood social systems among terrestrial old world monkeys. Some research suggests that this species exhibits a multi-tiered social system consisting of one-male units (social grouping that consist of one adult male, several adult females, and their offspring) that congregate into larger amalgamations termed “hordes” [5,6]. Hordes have been recorded to reach an average size of ~620 individuals, with some reports indicating hordes can reach up to about 845 individuals. This makes the mandrill horde the second largest social grouping of any primate species, only outnumbered by the social organization of geladas (Theropithecus gelada) in the Ethiopian highlands. Geladas also exhibit a complex multi-level social structure, with family units amalgamating into herds of up to 1,200 individuals! The main difference allowing gelada herds to outnumber mandrill hordes is the type of food that these species prefer to consume. While mandrills eat a variety of distributed foods, geladas are dietary specialists that eat mostly grass (and are the only primate species to do so). Since grass is a much more common and widely distributed food resource than any of the plant or animal species that mandrills consume, this allows geladas to congregate in larger social units, as there is less competition among individuals within a population over food resources.

There is still much that is unknown about mandrill social organization. Terrestrial monkey species are some of the most well-studied species of primate, mainly because they are easier than arboreal (tree-dwelling) species to observe consistently. Mandrills are generally considered an exception to this rule. Although they are terrestrial, the thick vegetation and difficult terrain within the forests that they preside in makes them more difficult to observe and study as opposed to other terrestrial monkey species. While the conceptualization that mandrills form hordes resulting from the fusion of one-male units has been well studied, there are conflicting reports of other behavioral dynamics at play in this species. Some reports have indicated there are populations of mandrills that never surpass 50 individuals as the maximum size of their social group. Other reports suggest that mandrill males join a horde during mating season but are solitary outside the mating season. Bachelor groups (all-male groups with no reproductive access) have never been observed in this species, but it is not entirely certain to what degree mandrill males exist in one-male units vs. as solitary individuals outside the mating season [7]. Conflicting hypotheses exist to explain the dynamics of horde formation in this species, with some evidence suggesting hordes form during the mating season while other evidence suggests that hordes form when food resources are abundant in the environment. Whether or not this species can be considered a “fission-fusion” society (a social group that comes together and separates dynamically) is also hotly debated. It may be that mandrills form consistent hordes with the same groups of individuals throughout their lives, or that hordes can be made up of different individuals each time they are formed. The former case would be evidence of this species exhibiting the characteristics of a fission-fusion society, while the latter would be evidence of a unique social group formation strategy among primates. There is not enough evidence at this time to determine which is a more accurate portrayal of this species’ social dynamics, largely due to the difficulties researchers encounter in trying to individually identify and study these monkeys when they are in a large horde, moving through the thick vegetation and complex landscapes of African tropical rainforests.

The mandrill is listed by the IUCN (The International Union for the Conservation of Nature) as having a conservation status of “vulnerable,” meaning they are not quite endangered yet, although they still are liable to experience significant population decreases if their habitat continues to be destroyed. Much like any other non-human animal species, the decisions of human organizations and industrial complexes can have far-reaching consequences on the survival and success of Mandrills. Such a beautiful and interesting organism, that can capture the wonder of the natural world and be modeled as a fan-favorite character in media, deserves the right to coexist on this planet with us. As with any phenomenon in nature, perhaps by further studying it and learning to respect the careful balance that must be struck between human advancement and preservation of our precious natural resources, we can learn more about these fascinating creatures and just maybe, we can learn a bit more about ourselves.

---

J.P. Calcitrai is a first year PhD student in the Animal Behavior program at UC Davis. His research focuses on the evolution of sociality in the primate order. Outside of academia, JP enjoys hiking and backpacking, writing short stories, and tinkering with computers and other electronic devices.

---

References:

[1] Harrison MJS. 1988. The mandrill in Gabon’s rain forest-ecology, distribution and status. Oryx 22(4):218-28.

[2] Setchell JM, Dixson AF. (2001). Changes in the secondary sexual adornments of male mandrills (Mandrillus sphinx) are associated with gain and loss of alpha status. Horm Behav 39(3):177-84.

[3] Rogers ME, Abernethy KA, Fontaine B, Wickings EJ, White LJT, Tutin CEG. (1996). Ten days in the life of a mandrill horde in the Lopé Reserve, Gabon. Am J Primatol 40(4):297-313.

[4] Henschel P, Abernethy KA, White LJT. 2005. Leopard food habits in the Lopé National Park, Gabon, Central Africa. Afr J Ecol 43(1):21-8.

[5] Gartlan JS. 1970. Preliminary notes on the ecology and behavior of the mandrill, Mandrillus leucophaeus. In: Napier JR, Napier PH, editors. Old world monkeys: evolution, systematics, and behavior. New York: Academic Press, pp. 445-80.

[6] Jolly CJ. 2007. Baboons, mandrills, and mangabeys: Afro-papionin socioecology in a phylogenetic perspective. In: Campbell CJ, Fuentes A, MacKinnon KC, Panger M, Bearder SK, editors. Primates in perspective. New York: Oxford U Press, pp. 240-51.

[7] Abernethy KA, White LJT, Wickings EJ. (2002). Hordes of mandrills (Mandrillus sphinx): extreme group size and seasonal male presence. J Zool (Lond.) 258(1):131-7.

[Edited by Alexandra Dwulit]