baboon (genus Papio), any of five species of large, robust, and primarily terrrestrial monkeys found in dry regions of Africa and Arabia. Males of the largest species, the chacma baboon (Papio ursinus), average 30 kg (66 pounds) or so, but females are only half this size. The smallest is the hamadryas, or sacred baboon (P. hamadryas), with males weighing about 17 kg and females only 10, but this still places them among the largest monkeys. Baboons range from 50 to 115 cm (20–45 inches) long, not including the 45–70-cm tail, which is carried in a characteristic arch. All species have long snouts with the nostrils located at the end; male baboons have long daggerlike canine teeth.

Anubis, or olive, baboon (Papio anubis).Norman Myers/Photo ResearchersUnlike most monkeys, few baboons live in tropical forests; most are found in savanna and semiarid regions, where they rove on the ground. They regularly climb trees, however, and here they sleep, keep watch, and sometimes feed. Baboons eat a variety of plants and animals, including grass and grass seeds, fruit, pods, roots, and tubers that they dig out of the ground. They also eat rodents, birds, and even gazelle fawns that they find hiding in long grass. In South Africa they are said to kill lambs, and everywhere they are known as crop raiders.

The five species of baboon replace each other geographically across the open country of Africa from parts of the Sahara southward. The chacma, which lives in southern Africa south of the Zambezi River, is brown or blackish in colour. The much smaller yellow baboon (P. cynocephalus) is found from the Zambezi northward to the Kenya coast and Somalia. The anubis, or olive baboon (P. anubis), is only slightly smaller than the chacma and olive in colour; the male has a large mane of hair over the head and shoulders. The anubis baboon has a wide range, from the hinterland of Kenya and Ethiopia through the grasslands and Sahel westward to Mali. It is also found in the less-arid highlands of the Sahara, such as Tibesti and Aïr. The small red Guinea baboon (P. papio) is restricted to far western Africa, and males have a cape of hair. These four species are often referred to collectively as savannah baboons, and they have much in common. All live in large cohesive troops numbering from 10 to several hundred. In most cases, females will remain with the troop in which they are born, but males nearing maturity will leave and try to join a different troop. Within each troop there is a dominance hierarchy among adult males. They threaten each other and often fight, and the dominance ranking is constantly changing. The dominance hierarchy of females is much more stable; females are genetically related to each other and rarely fight. Dominant members of each sex have the first choice of favoured foods and mating partners; they also keep order within the troop, chasing and threatening subordinates that are fighting or otherwise causing a disturbance.

The female baboon has a menstrual cycle lasting about 35 days. As estrus approaches, she develops large cushionlike swellings incorporating the vulva, perineum, and anus; these make her attractive to the males, which then compete to consort with her. The most dominant male will mate with more estrous females than will any other males and thus will father a high proportion of the next generation. After ovulation, the female’s swellings rapidly subside, and she is no longer attractive to the adult males. A single infant is born after a gestation of five to six months. The young are conspicuously black in colour and are permitted a great deal of freedom in their behaviour.

The fifth species, the hamadryas baboon (P. hamadryas), differs in appearance from the savannah baboons in that females are brown, but males are silvery gray with an enormous cape of hair on the neck and shoulders. Hamadryas baboons live in the semidesert hills along the African and Arabian coasts of the Red Sea. The small groups of hamadryas rove on their own during the day, then come back together in the evening and sleep on cliffs (not in trees, like most savannah baboons).

The ranges of hamadryas and anubis baboons meet in Africa, and here the two species hybridize. Observations of these hybrids have shown that much of the difference in behaviour between the two species is genetic. For example, the ability of hybrid males to herd females seems to be greater the more they physically resemble hamadryas. Along the Awash River of Ethiopia, the hamadryas/anubis border and its hybrid zone moves back and forth according to climate; after a run of dry years, the hamadryas area moves upriver, whereas a run of wetter years results in the anubis’s expanding downstream. This study (begun in the mid-1960s) and others like it have provided significant information on various evolutionary processes, including adaptation, genetics, and gene flow between different but related species.

Baboons belong to a large family of Old World monkeys (Cercopithecidae). Other long-faced monkeys, especially the mandrill and drill, the gelada, and the Celebes macaque, are sometimes referred to as baboons, but among these only the gelada is closely related. In the Guyanas and Belize, the term baboon is colloquially applied to the howler monkey.