Bonobo

• 1. Introduction

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  The bonobo, known to science as Pan paniscus, lives in the wild in just one country, the Democratic Republic of the Congo. Its home is a region of roughly 500,000 square kilometres in the Congo Basin. Primary forest, secondary forest and seasonally flooded swamp forest all shelter it. Nobody has an exact headcount, but estimates run between 29,500 and 50,000 individuals, and the species is listed as Endangered.

  This great ape shares the genus Pan with the common chimpanzee. Together the two are the closest living relatives of humans. Political instability in the region, and the shyness of the animals themselves, mean that few researchers have studied bonobos in the forest.

  This documentary asks how scientists came to tell the bonobo apart from the chimpanzee, and how a river may have shaped its origin. It asks whether the bonobo's gentle reputation holds up under scrutiny. And it asks what the Congo rainforest would lose if the animal disappeared.

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• 2. A Skull Misfiled

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  A skull in the Tervuren Museum in Belgium had been filed as that of a juvenile chimpanzee. In 1928 the German anatomist Ernst Schwarz looked again and decided it belonged to a different animal. He published his findings in 1929 and placed it as a subspecies of chimpanzee, under the name Pan satyrus paniscus.

  The same year, Schwarz gave the animal a common name, the pygmy chimpanzee, because the skull looked so small beside chimpanzee skulls. The label misled, since a bonobo's body size is similar to a chimpanzee's. In 1933 the American anatomist Harold Coolidge raised the bonobo to full species status.

  Behaviour told a parallel story. In the 1920s the American psychologist and primatologist Robert Yerkes noticed an unexpected and major behavioural difference. He did not know that a separate kind of ape existed. The Austrian zoologist Eduard Paul Tratz and the German biologist Heinz Heck discussed such differences in detail in the early 1950s.

  The word bonobo first appeared in 1954, when Tratz and Heck proposed it as a new term for the pygmy chimpanzees. It is thought to come from a misspelling on a shipping crate. The crate came from Bolobo, a town on the Congo River near where the first specimens were collected in the 1920s. English speakers usually stress the second syllable, in both the United States and Britain, though some stress the first.

  Schwarz's 1927 paper announcing the animal carried the French title Le Chimpanzé de la Rive Gauche du Congo. Readers have taken that title as a wink linking the Parisian Left Bank, with its bohemian culture, to the left bank of the Congo and its unconventional ape. That river, and not the Paris district, turns out to explain where bonobos came from.

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• 3. Cousins Across the River

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  Between 1.5 and 2 million years ago the Congo River took shape, and it may have split one ancestral population into two. Neither bonobos nor common chimpanzees swim well. Bonobos therefore ended up south of the river, while the ancestors of chimpanzees stayed north of it.

  DNA evidence complicates that tidy picture. It places the split between the two species at roughly 890,000 to 860,000 years ago, possibly driven by acidification and the spread of savannas. The river already existed by then. Ancestral Pan may have crossed it using corridors that have since vanished.

  The line leading to humans parted from the ancestors of both apes about 8 million years ago. Since then the muscles and skeleton of the two apes have changed remarkably little. Bonobos show no anatomical change since separating from common chimpanzees, which makes them a better model for the ancestor shared with humans.

  The timing of that shared ancestor is contentious. DNA comparisons suggest that Pan and Homo groups kept interbreeding after they diverged, until about 4 million years ago. The first Pan fossils were reported only in 2005, from the Middle Pleistocene of Kenya, alongside early Homo fossils.

  The anthropologist A. Zihlman observed that bonobo body proportions closely resemble those of Australopithecus. The evolutionary biologist Jeremy Griffith took this to suggest that bonobos may be a living example of our distant ancestors. The Australian anthropologists Gary Clark and Maciej Henneberg argue that human ancestors passed through a bonobo-like phase of reduced aggression, exemplified by Ardipithecus ramidus.

  The first bonobo genome was published in June 2012, from a female at Leipzig Zoo. The National Human Genome Research Institute had confirmed that it differs from the chimpanzee genome by about 0.4 percent. A high-quality reference genome arrived only in 2021. It puts the divergence at 0.421 percent for autosomes and 0.311 percent for the X chromosome.

  That reference genome predicts 22,366 full-length protein-coding genes. Sequencing of cDNA confirmed 20,478 protein-coding and 36,880 noncoding genes, close to the count annotated in the human genome. Compared with humans, 206 protein-coding genes sit in gene families that contracted, and 1,576 in families that expanded.

  Central and eastern chimpanzees share more genetic material with bonobos than other chimpanzee subspecies do. Admixture is believed to have happened at least twice within the past 550,000 years. In the wild, the Congo River now keeps the two populations isolated, so hybridization does not occur.

  In captivity it has happened. Between 1990 and 1992, researchers followed five pregnancies conceived between a male bonobo and two female chimpanzees. The first two were aborted because of environmental stressors. The next three produced hybrid offspring.

  A hybrid called Tiby was featured in the 2017 Swedish film The Square. Tiby's parents, a male bonobo and a female chimpanzee, went on to have several more offspring.

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• 4. Matriarchs and Mothers

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  At the top of a bonobo community stands a coalition of high-ranking females and males, usually headed by an old and experienced matriarch. She acts as decision-maker and leader. Bonobos are unusual among apes for this arrangement, though the overlap between male and female hierarchies leads some researchers to call them gender-balanced instead.

  Female bonobos earn rank through experience, age and the alliances they build with other females, rather than through physical intimidation. Top-ranking females protect immigrant females from male harassment. Females also carry sharper canines than female chimpanzees. A male bonobo is dominant over a female in a one-on-one encounter, yet socially bonded females can be co-dominant with males or outrank them.

  Bonobos keep no fixed territory, and communities travel over a wide range. Females wander widely and food is spread evenly through the forest. Males therefore gain little by banding together or defending a home range, as chimpanzees do. Some males still reach high rank and serve as partners to the alpha female, often taking the lead in coordinating the group's movements. Certain males raise the alarm about predators such as Central African rock pythons and leopards.

  A male bonobo draws his status from his mother. The mother-son bond often lasts for life. A mother supports her grown son in conflicts with other males and helps him build ties with females. She may even physically stop other males from breeding with the females she wants her son to mate with. Researchers have confirmed one case of a grown male adopting his orphaned infant brother.

  A community of about 100 follows a fission-fusion pattern. It splits into small groups to search for food by day, then reunites at night to sleep in nests built in the trees. Females usually feed before males, and a female with several allies is extremely successful at monopolizing a food source. In the wild, females lead hunts for duikers and defend their catch from males.

  In a study published in November 2023, scientists reported that two distinct groups at the Kokolopori Bonobo Reserve, Ekalakala and Kokoalongo, cooperate with each other. Over two years, researchers witnessed 95 encounters between them. The groups groomed, shared food and defended together against threats such as snakes. Despite the friendliness, the two groups kept separate identities, with no sign of interbreeding or blended cultures.

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• 5. Sex as Diplomacy

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  When bonobos come upon a new feeding ground, the excitement often leads to communal sexual activity. Researchers presume it eases tension and encourages peaceful feeding. Some scientists see such behaviour as serving several roles in bonobo society: a greeting, a way of forming social bonds, a means of resolving conflict and a way of reconciling afterward.

  Bonobos are the only non-human animal observed engaging in tongue kissing. They and humans are the only primates that typically mate face to face, although a pair of western gorillas has also been photographed in that position. Bonobos form no permanent monogamous partnerships. They do not seem to choose partners by sex or age, with the possible exception of mothers and their adult sons.

  Females often rub their genitals together, a behaviour scientists call genito-genital rubbing. It is thought to bond females socially and to form a female nucleus of the society. The ethologist Jonathan Balcombe reported that a session lasts ten to twenty seconds, and that females engage in it about once every two hours on average. Adolescent females often leave their native community to join another, and this behaviour helps establish them in their new group. Their migration also mixes the gene pools and so provides genetic diversity.

  Males have their own forms of contact. After a conflict, two males may stand back to back and rub their hindquarters together as a gesture of reconciliation. Kitamura recorded such rump contacts in 1989, and Takayoshi Kano saw similar practices among bonobos in their natural habitat.

  The pace of reproduction is slow. Wild females give birth for the first time at 13 or 14 years of age. Gestation averages 240 days, and a mother carries and nurses her young for four years. She gives birth on average every 4.6 years. Oestrus swelling lasts 10 to 20 days, and females resume the swelling cycle sooner after birth than chimpanzees do.

  Because many males and females mate with each other, a male cannot know which offspring are his. Mothers therefore take on all parental care. This strategy may counter infanticide, since males who cannot identify their own young lose the incentive to kill them. Infanticide occurs in every other great ape except orangutans. How the bonobo avoids simian immunodeficiency virus and its effects remains unknown.

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• 6. The Peace Debate

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  The primatologist Frans de Waal describes bonobos as capable of altruism, compassion, empathy, kindness, patience and sensitivity. He called their society a gynecocracy, meaning a matriarchy. Other primatologists countered that his data reflect only captive bonobos, and that wild bonobos are about as aggressive as chimpanzees. De Waal replied that captivity makes for a fair comparison, since the two species behave differently even under identical conditions.

  A 2014 study also found bonobos less aggressive than chimpanzees, particularly eastern chimpanzees. Its authors credited ecological factors for the relative peacefulness of bonobos and western chimpanzees. Then in April 2024, biologists reported that bonobos behave more aggressively than earlier thought.

  The contrast with chimpanzees is still real. Chimpanzee males patrol for lone males from neighbouring communities, attack them and often kill them. Bonobos prefer sexual contact to violent confrontation with outsiders. Among wild males, bonobos show about three times the rate of aggressive acts that chimpanzees do. Their squabbles are more frequent but less intense, and lethal aggression is essentially absent. Only one killing in the wild has been inferred, and one lethal attack confirmed in captivity. In both, females were the attackers and males the victims.

  Bonobo aggression is also not confined to their own kind, since humans have been attacked and seriously, though not fatally, injured. Males fight each other for access to females and can inflict missing digits, damaged eyes and torn ears. Some of those injuries come from a large male being swarmed by a mob of females after threatening a high-ranking one.

  Brain scans point to a biological basis for the milder temperament. Bonobos have more grey matter in regions linked to empathy, sensing distress and anxiety, including the right anterior insula and the hypothalamus. They also have a thick connection between the amygdala, which can spark aggression, and the ventral anterior cingulate cortex, which helps humans control impulses. One proposal is that abundant nutritious vegetation lets bonobos forage in large parties, which supports a calmer life.

  De Waal himself warns against romanticizing them. In his words: "All animals are competitive by nature and cooperative only under specific circumstances." He adds that he spoke of "sex for peace" because bonobos had plenty of conflicts to resolve.

  In 2008, Surbeck and Hohmann showed that bonobos sometimes hunt monkeys. They observed five incidents in Salonga National Park that seemed to be deliberate cooperative hunts. Three succeeded, and infant monkeys were captured and eaten.

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• 7. Mirror of Ourselves

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  A male bonobo averages 45 kilograms and stands 119 centimetres tall upright, while a female averages 33 kilograms and 111 centimetres. Body mass across the species ranges from 34 to 60 kilograms. The animal has a black face, pink lips, small ears, and long hair on its head that forms a parting. Compared with chimpanzees, it has a slim upper body, narrow shoulders, a thin neck and long legs. A tail tuft persists into adulthood.

  In captivity bonobos live about 40 years, while the wild lifespan is unknown but almost certainly shorter. Bipedal walking makes up less than 1 percent of their ground travel in the wild. In captivity it ranges from 3.9 percent of movement bouts when spontaneous to nearly 19 percent when food is abundant. Their faces vary from one individual to the next, as human faces do, which suits recognition in social life.

  Two bonobos at the Great Ape Trust, Kanzi and Panbanisha, learned to communicate on a keyboard of geometric symbols called lexigrams. Kanzi's own vocabulary covered more than 500 English words, and he understood around 3,000 spoken ones. He learned by watching people try to teach his mother. He began performing tasks she was taught, some of which she had failed to learn. The philosopher and bioethicist Peter Singer argues that such results qualify great apes for rights to survival and life.

  In the 1990s the researchers Kathy Schick, Nicholas Toth and later Gary Garufi taught Kanzi to make and use simple stone tools. Kanzi produced flakes, but not as humans do by holding a core in one hand and striking it with the other. Instead he threw the cobble against a hard surface or against another cobble.

  A 2013 study examined what bonobos do after a fight. When a bystander spontaneously comforted the victim, the victim's anxiety fell, measured by rates of self-scratching. Comfort came mostly to kin, then friends, then acquaintances, mirroring the empathic gradient described for humans. The authors concluded that consolation in bonobos may rest on empathy.

  Bonobos pass the mirror test for self-awareness, and wild ones use the same call to mean different things in different settings. Their listeners must read the context. When tickled, bonobos laugh at a higher pitch than human babies, yet with a similar spectrographic pattern. Two 2010 studies found that bonobos were better at tasks involving social causality, while chimpanzees did better with tools and physical causality. A study published in February 2025 found that bonobos could tell when humans did not know something, supporting the idea that they may have theory of mind.

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• 8. Seed Planters Under Threat

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  Each bonobo is estimated to ingest and disperse nine tons of seeds in its lifetime, from more than 91 species of lianas, grasses, trees and shrubs. Fruit makes up 57 percent of its diet. Leaves, honey, eggs, invertebrates and meat from small vertebrates such as anomalures, flying squirrels and duikers fill out the rest. The truffle species Hysterangium bonobo is on the menu too.

  Among frugivores in the Congo rainforest, only elephants are larger. Seeds spend about 24 hours in a bonobo's gut and are deposited intact, on average 1.3 kilometres from the parent tree and at most 4.5 kilometres. They germinate better and faster than seeds that have not passed through. The plant Dialium may even depend on bonobos to activate germination. Bonobos disperse the seeds of 40 percent of the tree species in these forests, or 11.6 million seeds over a single bonobo's life.

  Few species could take over that work, and bonobos could not replace elephants. Losing them would harm the forest itself.

  The pressures are heavy. Habitat destruction, human population growth and movement, and civil unrest all threaten the species, but commercial poaching is by far the most prominent danger. Hunting for bushmeat rose dramatically during the first and second Congo Wars, when heavily armed militias moved even into remote protected areas. The population is believed to have declined sharply over the last 30 years.

  Salonga National Park is the only national park in bonobo habitat, and it has little local involvement. Surveys since 2000 show that bonobos, forest elephants, okapi and other rare species have been devastated there by poachers. Many Congolese resist new parks, because indigenous communities were previously driven from their forest homes to create them. In some areas, bonobos and biodiversity still thrive without park borders, thanks to indigenous beliefs and taboos against killing bonobos.

  In 1995 the Zoological Society of Milwaukee published the Action Plan for Pan paniscus, drawing on 20 years of research at sites across the bonobo's range. The society later built schools, hired teachers, supplied some medicines and started an agriculture project so that local people could depend less on hunting wild animals. In 2002 the Bonobo Conservation Initiative began the Bonobo Peace Forest Project, which works with communities to set up linked reserves managed by local and indigenous people. It has helped bring agreements to protect over 50,000 square miles of habitat. Amy Parish says it "is going to be a model for conservation in the 21st century".

  Starting in 2003, the United States government allocated $54 million to the Congo Basin Forest Partnership. The port town of Basankusu, on the Lulonga River, serves as a base for conservation efforts because it is the last port of substance before the bonobo heartland of the Lopori Basin and Lomako River. The African Wildlife Foundation established World Bonobo Day in 2017, and it falls on the 14th of February, Valentine's Day.

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