The bonobo (Pan paniscus) belongs to the Pongidae family of the Primate order and is restricted to the central Zaire basin, south of the Zaire River in Africa. They live in forested areas and are often called “pygmy chimpanzees.” They are closely related to common chimpanzees (Pan troglodytes) but are not necessarily smaller.
Bonobos spend their time on the ground and in the trees. In the trees, they are able to move quadrupedally, bipedally, and suspensory. On the ground, they knuckle-walk like their chimpanzee relatives.
The earliest description of a wild bonobo is from the 1641 journal of a Dutch anatomist, Nicolas Tulp. His observation of what was thought to be a small chimpanzee includes a description of toes that are webbed between the second and third digits. This trait is common among bonobos, but not chimpanzees.
The first-known captive bonobo lived for only 6 years, from 1911 to 1916, and died of pneumonia in the Amsterdam Zoo. Mafuca, as he was named, was being raised with an infant chimpanzee. In 1916, the Dutch naturalist Anton Portielje suspected that Mafuca was not a chimpanzee, but it was not until 1929 in a Belgium museum, that what was thought to be an infant chimpanzee skull was more closely examined. Due to the suture closure, German anatomist Ernest Swarby determined the skull could not possibly belong to an infant chimpanzee. Swarby accurately assigned bonobos to their own species based on these observations.
Robert Yerkes is also credited with early observations of the unchimpanzee-like characteristic differences in the bonobo he worked with, named “Prince Chimpsky.” Yerkes described the bonobo as more gentle, graceful, and intelligent than any chimpanzee he had ever known.
During the 1930s, Eduard Tratz and Heinz Heck at the Hellabrunn Zoo in Munich conducted the first comparative study of chimpanzees and bonobos. Their list of trait comparisons is still relevant. Alison and Noel Badrian began studying wild bonobos in 1974 in the Lomako forest, which is still a study site. In the same year, Takayoshi Kano from Kyoto University began studies in Wamba, the most productive study site also still in use.
Chimpanzees and bonobos are human’s closest living relatives, sharing more than 98% of the same DNA. We are not more closely related to one or the other. Our lineages separated about 6 million years ago. We are also their closest relatives of all of the living primates. Bonobos and chimpanzees are thought to have shared a common ancestor from whom they split approximately 3 million years ago.
Bonobos are commonly called “pygmy chimpanzees”; however, the overall size difference is negligible. The basic anatomy is the same. The average male bonobo weighs only a few pounds less than the average male chimpanzee. This point is especially true when bonobos are compared with smaller-stature colonies of chimpanzees, like those who live in Gombe, Africa.
There is less sexual dimorphism, size difference between males and females, in bonobos than chimpanzees. A female bonobo’s weight is approximately 85% that of her male counterpart, which is the same difference as in humans.
The main anatomical difference between bonobos and chimpanzees is in limb length, head size, hairiness, and upright ability. Bonobos have longer and more slender limbs, a longer torso, and a leaner, more gracile appearance than chimpanzees. The head of a bonobo is smaller than the head of a chimpanzee, and they have a crop of longish black hair with a central part. Bonobo infants are born with dark-skinned faces, compared with the pale appearance of chimpanzee newborns.
When it comes to bipedal ability, bonobos surpass chimpanzees. Bonobos have an upright position that makes them appear more humanlike when walking. In mechanical observations of the efficiency of bipedal walking, bonobos and chimpanzees are equal in energy expenditure and speed when walking upright. However, bonobos seem to have mastered bipedal efficiency and have been observed wading upright in streams and rivers while fishing for small shrimps and insect larvae, walking upright with an arm around a partner’s shoulder, and walking upright with both hands full of sugar cane from research observation stations.
The bonobos dental formula is 2123/2123 = 32, the same as all great apes. However, their molars are smaller than chimpanzees.
Bonobos live in colonies of up to 150 individuals. Colony membership is determined by sex. Female bonobos migrate out of their birth group when they reach sexual maturity. Bonobos forage in a group of 6 to 15 individuals that often has an equal number of males and females. They eat mainly fruit, pith, and leaves. Bonobos have been observed eating small ungulates, insects, and fish. They are known to wade in small rivers to obtain food. Bonobo hunting, especially of other primates, has not been observed, as it has been for chimpanzees.
Takayoshi Kano of Kyoto University is one of the pioneering researchers in the study of wild bonobos. He and his collaborator, Suehisa Kuroda are working in Wamba, Zaire. Kano first arrived in Wamba in 1973 and has continued research in the region for the past two decades. Kano has played a major role in uncovering the behavioral peculiarities of the bonobo. Among the more important behaviors, Kano describes long-term mother-infant bonding, especially between mother and son; the use of sexual contact as a social tool and as aggression substitute; and a female-centric social structure.
In bonobo society, the mother-son bond is the most important social tool that is use by a male bonobo in order to rise through the ranks. This is not in order to obtain dominance over the entire troop, which does not happen in bonobo society, but more to curry favor with other high-ranking adult females who rule bonobo society in an almost egalitarian manner with the adult males. Bonobos do not have as formalized or ritual displays of dominance as chimpanzees do. Rank within the bonobo hierarchy seems to be based more on age and the amount of time spent within a group, for females, and who one’s mother is, for the males.
Kano described the adult alpha male and female as being almost equally dominant. The male of the highest rank is well-known, but the hierarchy blurs after that. Unrelated females bond in bonobo society and control the hierarchy. This is one of the most important of the bonobo behaviors. It allows females to act collectively to protect offspring and monopolize resources against males. The female-male relationships are also important and are most evident when a major shift in the hierarchy takes place. The idea of a female-centric social organization contrasts sharply with the known male-dominated social hierarchy that rules chimpanzee life, often by force.
Another striking characteristic of bonobo social organization is the use of sexual contacts and relationships to bond the troop. Even though bonobos have the same birthrate as chimpanzees, about one infant every 5 years, they have several sexual interactions every day. Bonobos employ sexual contact as more than strictly an act of reproduction. All members of a bonobo troop engage in sexual contact of some form with each other. Males will rump rub to console one another, defuse a stressful situation, and cement friendships—whereas females engage in genital-to-genital, or G-G, rubbing, while face-to-face with friends, as well as male-female sexual activities. Males also engage in rump rubbing, self- and mutual manual and oral stimulation, “penis-fencing,” and they present their erect penises to one another, embrace and excitedly rub together. A major display of these behaviors can be observed when a bonobo troop encounters a food source. Everyone gets excited, engages in sexual contact with a troop mate, and then proceeds to share the food.
Bonobo sexual activity is not restricted to the female estrous cycle or to individuals of the same age set. Bonobo females are continually receptive to sexual contact and will mate face-to-face with males, other females, and juvenile males. They even continue sexual activity during pregnancy and lactation, which chimpanzee females do not do. This continual receptivity makes it difficult for males to know which infants they have fathered. In turn, this has reduced the value of infanticide to dominant males. Infanticide has not been observed or suspected to occur in bonobo communities.
Bonobos do observe traditional sexual taboos, such as interactions between fathers and daughters. Since paternity is sketchy, incest is best avoided by the practice of female bonobos transferring out of their natal group when they reach sexual maturity. Also, primatologist Amy Parish found that if captive females were not transferred to a new community, reproductive maturity was delayed by several years.
Zoologist Frans De Waal has studied the sexual behavior of bonobos in relationship to conflict resolution and coalition building among captive colonies. Aggression is not completely absent from bonobo society, but it is mild and restrained when compared with the elaborate and aggressive behaviors of chimpanzees. Bonobos prefer peaceful resolution and are quick to appease each other with sexual contact as opposed to violent physical attacks.
Bonobos also use other familiar social tools, such as pout and play faces and hand gestures similar to chimpanzees, but in conjunction with sexual contact. They reach their hands out to one another for play invitations, food begging, and to get someone else to come near them for an embrace. These actions are most often done with the right hand, like humans. It may suggest a lateralized brain structure, where the left half of the brain controls communication that is not necessarily language specific. While the physical and facial gestures are very similar, bonobos gesticulate and vocalize more often than chimpanzees. They have higher-pitched voices, which are easily distinguished from chimpanzees.
Primate Language Studies
Bonobos have been shown to surpass chimpanzees in their ability to acquire language skills. This is also not to say that bonobos are more intelligent than chimpanzees, for their relatives often surpass them in other cognitive areas. At the Language Research Center of Georgia State University, in Atlanta, Sue Savage-Rumbaugh has been working with Kanzi, one of the bonobo stars of language studies. Kanzi was able to learn 200 vocabulary symbols and construct rudimentary sentences of words and gestures by the age of 6. He was also very good at responding to verbal commands.
Savage-Rumbaugh’s research suggests that bonobos and chimpanzees may have separate cognitive strengths. While working with Panshiba, a bonobo, and Panzee, a chimpanzee, she observed differences in their abilities and the speed with which they learned. Panshiba consistently led in the acquisition of language symbols, while Panzee was always better at spatial and object manipulation, such as mazes.
The bonobos always seem more aware of the social factors surrounding them. Panshiba has been known to ask for extras of the treats she receives and then for them to be given to Kanzi and his sister. When in a group, the bonobos tend to sit together and protest if one of them is removed for a separate task. The chimpanzees tend to be less concerned with what others are doing, be they other chimpanzees or the researchers.
Unlike chimpanzees, wild bonobos have never been observed making or using tools. It is likely that bonobos do not need tools in the wild. The colonies that have been researched the most live in areas of abundant resources that do not require a tool to obtain. The fact that wild bonobos have not been observed using tools may also be a result of the relatively recent commitment to their study. Bonobos have only recently become widely known, and longevity in their research may provide evidence of still unknown behavior, such as tool use.
In captivity, however, bonobos will make use of objects at hand. They have been observed to use halved red peppers as a water scoop, tennis balls as a water sponge, wood-wool as toilet paper, sticks to pole vault, and smaller sticks as honey “fishing poles,” like chimpanzees. Kanzi, the language studies ape, was also taught how to make and use a rudimentary stone tool, which he used to cut through a rope on a food box. Although he developed his own method to produce the flint tool, different from the one he had been taught, he showed an understanding of how to make and use the sharp edge to cut the rope.
Bonobos’ cognitive ability is demonstrated by their ability to pass the self-recognition mirror test, like chimpanzees. Bonobos also display a surprising amount of sensitive behaviors that suggest a cognitive specialization in sympathy, empathy, and understanding others’ needs.
This ability is oddly demonstrated in the bonobo nest-building habits. Like chimpanzees, bonobos build nests of intertwined tree branches to sleep in for the evening. They also build them during the day for napping. What is interesting about the bonobos’ habit is that the nest seems to denote individual space, which is not to be entered by any other without permission. Anecdotal observations include a mother bonobo in the process of weaning her young bonobo and not allowing the youngster into her nest despite incessant pleas. Also, individuals will build a rudimentary nest as a retreat when food is encountered. They are left alone regardless of the amount of food they have that could be shared. In another case, a male bonobo built a quick nest to avoid a charging display by another individual, who stopped at the base of the tree containing the nest. Instead of charging up the trunk, he walked away. These examples demonstrate an understanding of both actors, each wanting something different and understanding the other’s intentions and position.
Kanzi, the language studies bonobo, understands many verbal commands and has demonstrated the ability to tell the difference between commands intended for himself and those intended for others, such as his sister, Tamuli, who has little experience with experiments. In one session, the instructor clearly asked Tamuli to groom Kanzi. Tamuli did not understand what was being asked, but Kanzi did. He also appeared to understand that his sister did not know what to do. Kanzi repeatedly grasped her hand and put it to his chest, holding it in place with his chin. Kanzi demonstrated his own understanding of the command, the fact that it was not meant for him to act, but also that his sister was not capable of understanding what to do but he could show her.
The sensitivity to others is not restricted to interactions between bonobos. At the Twycross Zoo, in England, Betty Walsh observed a bonobo named Kuni capture a starling. Concerned for the bird, the caretaker encouraged Kuni to release the bird. Kuni made several attempts before climbing to the top of a tree, carefully spreading the bird’s wings and tossing it as far as she could. When this failed as well, Kuni stood near the bird, protecting it from others until it eventually flew away.
The ability to see what another individual needs or wants is a cognitive skill possessed only by apes and humans. It provides the skills necessary for social relationships that while ultimately based on resource acquisition and reproduction, are far more complicated than in any other mammals.
Research observations of wild and captive bonobos are ripe with evidence of their understanding of other’s emotions and needs. This humanlike predisposition to sensitivity seems to be the foundation of their civilized social interactions and the apparent ease with which a bonobo community functions. This, in turn, appears to be the result of an abundant environment, with plenty for all, and may be one of the most important models for the evolution of our human ancestors. Unfortunately, human encroachment on bonobo habitats for agriculture, forest products, and hunting for the bushmeat trade threaten the existence of the bonobo, as well as the other great apes of Africa. Plans for expanding the bonobo sanctuaries around the Wamba and Lomako research stations are being petitioned for, but the need for funding and public education still exists. Protection of the bonobos must ultimately lie in the hands of those with whom they share their habitat.
- Boesch, C., & Hohmann, G. (Eds.). (2002). Behavioural diversity in chimpanzees and bonobos. Cambridge: Cambridge University Press.
- D’Aout, K., Aerts, P., DeClercq, D., DeMeester, K., & Van Elsacker, L. V. (2002). Segment joint angles of hind limb during bipedal and quadrupedal walking of the bonobo (Pan paniscus). American Journal of Physical Anthropology, 119, 37-51.
- De Waal, F. (1997). Bonobo, the forgotten ape. Berkeley: University of California Press.