Bonobo

Etymology

Formerly the bonobo was known as the "pygmy chimpanzee", despite the bonobo having a similar body size to the common chimpanzee. The name "pygmy" was given by the German zoologist Ernst Schwarz in 1929, who classified the species on the basis of a previously mislabeled bonobo cranium, noting its diminutive size compared to chimpanzee skulls.

The name "bonobo" first appeared in 1954, when Austrian zoologist Eduard Paul Tratz and German biologist Heinz Heck proposed it as a new and separate generic term for pygmy chimpanzees. The name is thought to derive from a misspelling on a shipping crate from the town of Bolobo on the Congo River near the location from which the first bonobo specimens were collected in the 1920s.

In English the word is generally stressed on the second syllable, in both the US and UK, though sometimes it is instead stressed on the first syllable.

Taxonomy

The bonobo was first recognised as a distinct taxon in 1928 by German anatomist Ernst Schwarz, based on a skull in the Tervuren Museum in Belgium which had previously been classified as a juvenile chimpanzee (Pan troglodytes). Schwarz published his findings in 1929, classifying the bonobo as a subspecies of chimpanzee, Pan satyrus paniscus. In 1933, American anatomist Harold Coolidge elevated it to species status. Major behavioural differences between bonobos and chimpanzees were first discussed in detail by Tratz and Heck in the early 1950s. Unaware of any taxonomic distinction with the common chimpanzee, American psychologist and primatologist Robert Yerkes had already noticed an unexpected major behavioural difference in the 1920s.

Bonobos and chimpanzees are the two species which make up the genus Pan, and are the closest living relatives to humans (Homo sapiens).

Bonobos and common chimpanzees show remarkable evolutionary stasis in musculoskeletal anatomy since their split from humans 8 million years ago, with bonobos exhibiting no changes since diverging from common chimps ~2 million years ago, making them a better anatomical model for the last common ancestor of humans and chimps/bonobos.

Nonetheless, the exact timing of the Pan–Homo last common ancestor is contentious, but DNA comparison suggests continual interbreeding between ancestral Pan and Homo groups, post-divergence, until about 4 million years ago. DNA evidence suggests the bonobo and common chimpanzee species diverged approximately 890,000–860,000 years ago following separation of these two populations possibly because of acidification and the spread of savannas at this time. Currently, these two species are separated by the Congo River, which had existed well before the divergence date, though ancestral Pan may have dispersed across the river using corridors which no longer exist. The first Pan fossils were reported in 2005 from the Middle Pleistocene (after the bonobo–chimpanzee split) of Kenya, alongside early Homo fossils.

According to A. Zihlman, bonobo body proportions closely resemble those of Australopithecus, leading evolutionary biologist Jeremy Griffith to suggest that bonobos may be a living example of our distant human ancestors. According to Australian anthropologists Gary Clark and Maciej Henneberg, human ancestors went through a bonobo-like phase featuring reduced aggression and associated anatomical changes, exemplified in Ardipithecus ramidus.

Genetics and genomics

Relationships of bonobos to humans and other apes can be determined by comparing their genes or whole genomes. While the first bonobo genome was published in 2012, a high-quality reference genome became available only in 2021. The overall nucleotide divergence between chimpanzee and bonobo based on the latter is 0.421 ± 0.086% for autosomes and 0.311 ± 0.060% for the X chromosome. The reference genome predicts 22,366 full-length protein-coding genes and 9,066 noncoding genes, although cDNA sequencing confirmed only 20,478 protein-coding and 36,880 noncoding bonobo genes, similar to the number of genes annotated in the human genome. Overall, 206 and 1,576 protein-coding genes are part of gene families that contracted or expanded in the bonobo genome compared to the human genome, respectively, that is, these genes were lost or gained in the bonobo genome compared to humans.

Hybrids

Researchers have found that both central (Pan troglodytes troglodytes) and eastern chimpanzees (Pan troglodytes schweinfurthii) share more genetic material with bonobos than other chimpanzee subspecies. It is believed that genetic admixture has occurred at least two times within the past 550,000 years. In modern times hybridization between bonobos and chimpanzees in the wild is prevented as populations are allopatric and kept isolated on different sides of the Congo river.

Within captivity, hybrids between bonobos and chimpanzees have been recorded. Between 1990 and 1992, five pregnancies were conceived and studied between a male bonobo and two female chimpanzees. The two initial pregnancies were aborted because of environmental stressors. The following three pregnancies however led to the birth of three hybrid offspring.

A bonobo and chimpanzee hybrid called Tiby was also featured in the 2017 Swedish film The Square. This same male bonobo and female chimpanzee had several offspring.

Description

The bonobo is commonly considered to be more gracile than the common chimpanzee. Although large male chimpanzees can exceed any bonobo in bulk and weight, the two species broadly overlap in body size. Adult female bonobos are somewhat smaller than adult males. Body mass ranges from 34 to 60 kg (75 to 132 lb) with an average weight of 45 kilograms (99 lb) in males against an average of 33 kg (73 lb) in females. The total length of bonobos (from the nose to the rump while on all fours) is 70 to 83 cm (28 to 33 in). Male bonobos average 119 cm (3.90 ft) when standing upright, compared to 111 centimetres (3.64 ft) in females. The bonobo's head is relatively smaller than that of the common chimpanzee with less prominent brow ridges above the eyes. It has a black face with pink lips, small ears, wide nostrils, and long hair on its head that forms a parting. Females have slightly more prominent breasts, in contrast to the flat breasts of other female apes, although not so prominent as those of humans. The bonobo also has a slim upper body, narrow shoulders, thin neck, and long legs when compared to the common chimpanzee.

Bonobos are both terrestrial and arboreal. Most ground locomotion is characterized by quadrupedal knuckle-walking. Bipedal walking has been recorded as less than 1% of terrestrial locomotion in the wild, a figure that decreased with habituation, while in captivity there is a wide variation. Bipedal walking in captivity, as a percentage of bipedal plus quadrupedal locomotion bouts, has been observed from 3.9% for spontaneous bouts to nearly 19% when abundant food is provided. These physical characteristics and its posture give the bonobo an appearance more closely resembling that of humans than the common chimpanzee does. The bonobo also has highly individuated facial features, as humans do, so that one individual may look significantly different from another, a characteristic adapted for visual facial recognition in social interaction.

Multivariate analysis has shown bonobos are more neotenised than the common chimpanzee, taking into account such features as the proportionately long torso length of the bonobo. Other researchers challenged this conclusion.

Behaviour

Primatologist Frans de Waal states bonobos are capable of altruism, compassion, empathy, kindness, patience, and sensitivity, and described "bonobo society" as a "gynecocracy" (i.e. a matriarchy). Primatologists who have studied bonobos in the wild have documented a wide range of behaviours, including aggressive behaviour and more cyclic sexual behaviour similar to chimpanzees, even though bonobos show more sexual behaviour in a greater variety of relationships. An analysis of female bonding among wild bonobos by Takeshi Furuichi stresses female sexuality and shows how female bonobos spend much more time in oestrus than female chimpanzees.

Some primatologists have argued that De Waal's data reflect only the behaviour of captive bonobos, suggesting that wild bonobos show levels of aggression closer to what is found among chimpanzees. De Waal has responded that the contrast in temperament between bonobos and chimpanzees observed in captivity is meaningful, because it controls for the influence of environment. The two species behave quite differently even if kept under identical conditions. A 2014 study also found bonobos to be less aggressive than chimpanzees, particularly eastern chimpanzees. The authors argued that the relative peacefulness of western chimpanzees and bonobos was primarily due to ecological factors. Bonobos warn each other of danger less efficiently than chimpanzees in the same situation.

Nonetheless, in April 2024, biologists reported that bonobos behave more aggressively than thought earlier.

In a study published in February 2025, scientists determined that bonobos could tell when humans did not know something. The findings advance researchers' proposal that like humans, chimpanzees and bonobos—humans' closest evolutionary cousins—may also possess theory of mind.

Social behaviour

Bonobos are unusual among apes for their matriarchal social structure (extensive overlap between the male and female hierarchies leads some to refer to them as gender-balanced in their power structure). Bonobos do not have a defined territory and communities will travel over a wide range. Because of the nomadic nature of the females and evenly distributed food in their environment, males do not gain any obvious advantages by forming alliances with other males, or by defending a home range, as chimpanzees do. Female bonobos possess sharper canines than female chimpanzees, further fueling their status in the group. Although a male bonobo is dominant to a female in a dyadic interaction, depending on the community, socially-bonded females may be co-dominant with males or dominant over them, even to the extent that females can coerce reluctant males into mating with them.

At the top of the hierarchy is a coalition of high-ranking females and males typically headed by an old, experienced matriarch who acts as the decision-maker and leader of the group. Female bonobos typically earn their rank through experience, age, and ability to forge alliances with other females in their group, rather than physical intimidation, and top-ranking females will protect immigrant females from male harassment. While bonobos are often called matriarchal, and while every community is dominated by a female, some males will still obtain a high rank and act as coalitionary partners to the alpha female, often taking initiative in coordinating the group's movements. These males may outrank not only the other males in the group, but also many females. Certain males alert the group to any possible threats, protecting the group from predators such as Central African rock pythons, leopards and possibly African golden cats and crowned eagles.

Aggressive encounters between males and females are rare, and males are tolerant of infants and juveniles. A male derives his status from the status of his mother. The mother–son bond often stays strong and continues throughout life. While social hierarchies do exist, and although the son of a high ranking female may outrank a lower female, rank plays a less prominent role than in other primate societies. Relationships between different communities are often positive and affiliative, and bonobos are not a territorial species. Bonobos will also share food with others, even unrelated strangers. Bonobos exhibit paedomorphism (retaining infantile physical characteristics and behaviours), which greatly inhibits aggression and enables unfamiliar bonobos to freely mingle and cooperate with each other.

Diet

The bonobo is an omnivorous frugivore; 57% of its diet is fruit, but this is supplemented with leaves, honey, eggs, meat from small vertebrates such as anomalures, flying squirrels and duikers, and invertebrates. The truffle species Hysterangium bonobo is eaten by bonobos. In some instances, bonobos have been shown to consume lower-order primates. Some claim bonobos have also been known to practise cannibalism in captivity, a claim disputed by others. However, at least one confirmed report of cannibalism in the wild of a dead infant was described in 2008. A 2016 paper reported two more instances of infant cannibalism, although it was not confirmed if infanticide was involved.

Cognitive comparisons to chimpanzees

In 2020, the first whole-genome comparison between chimpanzees and bonobos was published and showed genomic aspects that may underlie or have resulted from their divergence and behavioural differences, including selection for genes related to diet and hormones. A 2010 study found that "female bonobos displayed a larger range of tool use behaviours than males, a pattern previously described for chimpanzees but not for other great apes". This finding was affirmed by the results of another 2010 study which also found that "bonobos were more skilled at solving tasks related to theory of mind or an understanding of social causality, while chimpanzees were more skilled at tasks requiring the use of tools and an understanding of physical causality". Bonobos have been found to be more risk-averse compared to chimpanzees, preferring immediate rather than delayed rewards when it comes to foraging. Bonobos also have a weaker spatial memory compared to chimpanzees, with adult bonobos performing comparably to juvenile chimpanzees.

Similarity to humans

Bonobos are capable of passing the mirror-recognition test for self-awareness, as are all great apes. They communicate primarily through vocal means, although the meanings of their vocalizations are not currently known. However, most humans do understand their facial expressions and some of their natural hand gestures, such as their invitation to play. The communication system of wild bonobos includes a characteristic that was earlier only known in humans: bonobos use the same call to mean different things in different situations, and the other bonobos have to take the context into account when determining the meaning.

Two bonobos at the Great Ape Trust, Kanzi and Panbanisha, were taught how to communicate using a keyboard labeled with lexigrams (geometric symbols) and they could respond to spoken sentences. Kanzi’s vocabulary consisted of more than 500 English words, and he had comprehension of around 3,000 spoken English words.

Kanzi is also known for learning by observing people trying to teach his mother; Kanzi started doing the tasks that his mother was taught just by watching, some of which his mother had failed to learn. Some, such as philosopher and bioethicist Peter Singer, argue that these results qualify them for "rights to survival and life"—rights which humans theoretically accord to all persons (See great ape personhood).

In the 1990s, Kanzi was taught to make and use simple stone tools. This resulted from a study undertaken by researchers Kathy Schick and Nicholas Toth, and later Gary Garufi. The researchers wanted to know if Kanzi possessed the cognitive and biomechanical abilities required to make and use stone tools. Though Kanzi was able to form flakes, he did not create them in the same way as humans, who hold the core in one hand and knap it with the other; Kanzi threw the cobble against a hard surface or against another cobble. This allowed him to produce a larger force to initiate a fracture as opposed to knapping it in his hands.

As in other great apes and humans, third party affiliation toward the victim—the affinitive contact made toward the recipient of an aggression by a group member other than the aggressor—is present in bonobos. A 2013 study found that both the affiliation spontaneously offered by a bystander to the victim and the affiliation requested by the victim (solicited affiliation) can reduce the probability of further aggression by group members on the victim (this fact supporting the Victim-Protection Hypothesis). Yet, only spontaneous affiliation reduced victim anxiety—measured via self-scratching rates—thus suggesting not only that non-solicited affiliation has a consolatory function but also that the spontaneous gesture—more than the protection itself—works in calming the distressed subject. The authors hypothesize that the victim may perceive the motivational autonomy of the bystander, who does not require an invitation to provide post-conflict affinitive contact. Moreover, spontaneous—but not solicited—third party affiliation was affected by the bond between consoler and victim (this supporting the Consolation Hypothesis). Importantly, spontaneous affiliation followed the empathic gradient described for humans, being mostly offered to kin, then friends, then acquaintances (these categories having been determined using affiliation rates between individuals). Hence, consolation in the bonobo may be an empathy-based phenomenon.

Distribution and habitat

Bonobos are found only south of the Congo River and north of the Kasai River (a tributary of the Congo), in the humid forests of the Democratic Republic of Congo. Ernst Schwarz's 1927 paper "Le Chimpanzé de la Rive Gauche du Congo", announcing his discovery, has been read as an association between the Parisian Left Bank and the left bank of the Congo River; the bohemian culture in Paris, and an unconventional ape in the Congo.

The ranges of bonobos and chimpanzees are separated by the Congo River, with bonobos living to its south and chimpanzees to the north.

Ecological role

In the Congo tropical rainforest, the very great majority of plants need animals to reproduce and disperse their seeds. Bonobos are the second largest frugivorous animals in this region, after elephants. It is estimated that during its life, each bonobo will ingest and disperse nine tons of seeds, from more than 91 species of lianas, grass, trees and shrubs. These seeds travel for about 24 hours in the bonobo digestive tract, which can transfer them over several kilometers (mean 1.3 km; max: 4.5 km), far from their parents, where they will be deposited intact in their feces. These dispersed seeds remain viable, germinating better and more quickly than unpassed seeds. For those seeds, diplochory with dung-beetles (Scarabaeidae) improves post-dispersal survival.

Certain plants such as Dialium may even be dependent on bonobos to activate the germination of their seeds, characterized by tegumentary dormancy. The first parameters of the effectiveness of seed dispersal by bonobos are present. Behaviour of the bonobo could affect the population structure of plants whose seeds they disperse. The majority of these zoochorous plants cannot recruit without dispersal and the homogeneous spatial structure of the trees suggests a direct link with their dispersal agent. Few species could replace bonobos in terms of seed dispersal services, just as bonobos could not replace elephants. There is little functional redundancy between frugivorous mammals of the Congo, which face severe human hunting pressures and local extinction. The defaunation of the forests, leading to the empty forest syndrome, is critical in conservation biology. The disappearance of the bonobos, which disperse seeds of 40% of the tree species in these forests, or 11.6 million individual seeds during the life of each bonobo, would have consequences for the conservation of the Congo rainforest.

Conservation status

The IUCN Red List classifies bonobos as an endangered species, with conservative population estimates ranging from 29,500 to 50,000 individuals. Major threats to bonobo populations include habitat loss and hunting for bushmeat, the latter activity having increased dramatically during the first and second Congo Wars in the Democratic Republic of Congo, due to the presence of heavily armed militias (even in remote, "protected" areas such as Salonga National Park). This is part of a more general trend of ape extinction.

As the bonobos' habitat is shared with many people, the ultimate success of conservation efforts still relies on local and community involvement. The issue of parks versus people is salient in the Cuvette Centrale, within the bonobos' range. There is strong local, and broad-based Congolese, resistance to establishing national parks, as indigenous communities have previously been driven from their forest homes by the forming of parks. In Salonga National Park (the only national park in bonobo habitat), there is no local involvement, and surveys undertaken since 2000 indicate the bonobo, the African forest elephant, the okapi, and other rare species have been devastated by poachers and the thriving bushmeat trade. In contrast, areas do exist where the bonobo and ecological biodiversity still thrive without any established park borders, because of the indigenous beliefs/taboos against killing bonobos and other animals.

During the wars in the 1990s, researchers and international non-governmental organisations (NGOs) were driven out of the bonobo habitat. In 2002, the Bonobo Conservation Initiative initiated the Bonobo Peace Forest Project (supported by the Global Conservation Fund of Conservation International), in cooperation with national institutions, local NGOs, and local communities; the Peace Forest Project works with local communities to establish a linked constellation of community-based reserves managed by local and indigenous people. This model, implemented mainly through DRC organisations and local communities, has helped bring about agreements to protect over 50,000 square miles (130,000 km2) of the bonobo habitat. According to Amy Parish, the Bonobo Peace Forest "is going to be a model for conservation in the 21st century".

The port town of Basankusu is situated on the Lulonga River, at the confluence of the Lopori and Maringa Rivers, in the north of the country, making it well placed to receive and transport local goods to the cities of Mbandaka and Kinshasa. With Basankusu being the last port of substance before the wilderness of the Lopori Basin and the Lomako River—the bonobo heartland—conservation efforts for the bonobo use the town as a base.

In human culture

World Bonobo Day is February 14 (Valentine's Day), established in 2017 by the African Wildlife Foundation.