Published: March 2013
The Left Bank Ape
An exclusive look at bonobos
By David Quammen

In a remote forest sector of the Democratic Republic of the Congo, along the north bank of the Luo River, 50 miles by dirt trail from the nearest grass airstrip, lies the Wamba research camp, a place that’s quietly renowned in the annals of primatology. Wamba was founded in 1974 by a Japanese primatologist named Takayoshi Kano for the study of the bonobo, Pan paniscus, a species of simian unlike any other.

The bonobo, in case you haven’t heard, carries a reputation as the “make love, not war” member of the ape lineage, far lustier and less bellicose than its close cousin, the chimpanzee. Modern studies of zoo populations by the Dutch-American biologist Frans de Waal and others have documented its easy, pervasive sexuality and its propensity for amicable bonding (especially among females), in contrast with chimpanzee dominance battles (especially among males) and intergroup warfare. But the bonobo’s behavior in the wild has been harder to know, and Takayoshi Kano, operating out of the Primate Research Institute of Kyoto University, was among the first scientists aspiring to study it there. Apart from several interruptions, including a hiatus during the Congo wars of 1996-2002, observations at Wamba have continued ever since.

Early one morning I followed a researcher named Tetsuya Sakamaki, also from Kyoto University, into the forest. Promptly I saw things that, according to the popular image of the species, I might not have expected. Bonobos quarreled. They hunted for meat. They went hours at a stretch without having sex. This was the animal so renowned for its lubricious, pacific social life?

As Sakamaki and I watched a party of bonobos feeding on the fruits of a boleka tree—small, grapelike morsels with papery husks—he identified the individuals by name. That female there, with the sexual swelling, we call her Nova, he said. She last gave birth in 2008; the gaudy inflation of her genital area, like a pink sofa cushion taped to her rump, advertised her readiness to breed again. This female is Nao, he said, very old, very senior. Nao has two daughters, of which the elder has so far remained in this group. And that female there, that’s Kiku, also very senior, with three sons in the group. One of those sons is Nobita—easy to identify, Sakamaki explained, by his great size and the digits missing from his right hand and both feet and by the blackness of his testes. Missing digits suggest a mishap in a snare, not unusual for bonobos facing the hazards of human proximity. Nobita seems to be the alpha male, insofar as bonobo groups recognize alpha males.

By now we had followed the bonobos into a grove of musanga trees, and they were stuffing their mouths with fruit, pulpy and green. Suddenly a screechy altercation broke out between Nobita and another male, Jiro. Kiku, Nobita’s elderly mother, charged over to support her son. Cowed by the two of them, Jiro retreated. He sulked in a nearby tree. It’s interesting, Sakamaki noted, that Nobita is the largest male in this group, and yet his mother helps him in a fight. Even a high-ranking adult male such as Nobita seems to hold his status partly on the merits of his mama.

Forty minutes later, when the screeching began again, Sakamaki drew my attention to the focus of excitement: an anomalure (a gliding rodent, like a flying squirrel), scrambling for its life on a tree trunk while several bonobos converged around it. As the bonobos came closer, the anomalure launched itself into space and glided away. Then we noticed a second one, clinging secretively to the east side of another large bole while a bonobo named Jeudi sat clueless just 15 feet to the west. This anomalure, pink eared and pale eyed, held its place on the bark more patiently, frozen, not giving itself away. Within a moment, though, other bonobos spotted it, and the group closed in, shrieking with predatory menace. One bonobo climbed upward, struggling to find grips. The anomalure skittered 20 feet higher, ascending as easily as a gecko on a wall. When it was entirely surrounded with bloodthirsty apes, the little rodent launched itself and sailed away through the limbs and undergrowth to safety. We never even saw where it hit the ground; neither did the bonobos. Wow, I thought. Nicely done.

“Hunting behavior—it’s very rare,” said Sakamaki. “So you are very lucky.”

Not yet noon on my first day at Wamba, and already my notion of bonobos had been confounded with realities, contrasts, and complications.

Bonobos have been confounding people ever since they first came to scientific attention. Back in 1927 a Belgian zoologist named Henri Schouteden examined the skull and skin of a peculiar animal, supposedly an adult female chimpanzee, from the Belgian Congo. The skull, he reported, was “curieusement petit pour une bête de semblables dimensions”—oddly small for an animal of such size. The following year a German zoologist, Ernst Schwarz, visited Schouteden’s museum and measured that skull as well as two others, concluding that they must represent a distinct form of chimp, unique to the south side—the left bank—of the Congo River. Schwarz announced his discovery in a paper titled “Le Chimpanzé de la Rive Gauche du Congo.” So from the beginning there was at least a subliminal association between the Left Bank culture at the center of the Francophone world—the bohemian artists and writers and philosophers of la rive gauche in Paris, south of the Seine—and this newly identified, unconventional Congolese ape. Soon after, the left-bank ape was recognized as a full species and took its modern name, Pan paniscus.

Another label that fell upon it was “pygmy chimpanzee,” despite the fact that it’s not much littler than the common chimpanzee, the one already widely known, Pan troglodytes. The bonobo’s head is smaller in proportion to its body than a chimp’s, its physique more slender, its legs longer. But in overall size, both male and female adult bonobos fall generally within the same weight range as female chimps. Scientists today tend to avoid the term pygmy chimpanzee; “bonobo” better suggests that this creature is not a miniaturized version of something else.

The major distinctions between bonobos and chimps are behavioral, and the most conspicuous do involve sex. Both in captivity and in the wild, bonobos practice a remarkable diversity of sexual interactions. According to de Waal: “Whereas the chimpanzee shows little variation in the sexual act, bonobos behave as if they have read the Kama Sutra, performing every position and variation one can imagine.” For instance, they mate in the missionary position, something virtually unknown among chimpanzees. But their sexiness isn’t just about mating. Most of those variations are sociosexual, meaning that they don’t entail copulation between an adult male and an adult female during her fertile period. The range of partners includes adults of the same sex, an adult with a juvenile of either sex, and two juveniles together. The range of activities includes mouth-to-mouth kissing, oral sex, genital caressing by hand, penis-fencing by two males, male-on-male mounting, and genito-genital rubbing (G-G rubbing is the shorthand term) by two estrous females, who moosh their swollen vulvas back and forth against each other in a spate of feverish sisterly cordiality. Usually there’s no orgasm culminating these activities. Their social purpose seems to be communication of various sorts: expression of goodwill, calming of excitement, greeting, tension relief, bonding, solicitation of food sharing, and reconciliation. To that list of benefits we might also add sheer pleasure and (for the juveniles) instructional play. Varied and frequent and often nonchalant, sex is a widely applied social lubricant that helps keep bonobo politics amiable. De Waal again: “The chimpanzee resolves sexual issues with power; the bonobo resolves power issues with sex.”

Sexiness isn’t the only big difference between bonobos and chimps, though it’s probably linked to other differences, either as cause or as effect. Females, not males, hold the highest social rankings, which they seem to achieve by affable social networking (such as G-G rubbing) rather than by forming temporary alliances and fighting, as male chimpanzees do. Bonobo communities don’t wage violent wars against other bonobo communities adjacent to their territory. They forage during daytime in more stable and often larger parties, with sometimes as many as 15 or 20 individuals moving together from one source of food to another, and they cluster their nests at night, presumably for mutual security. Their diet, which is similar to the usual chimpanzee diet in most respects—fruit, leaves, a bit of animal protein when they can get it—differs in one signal way: Bonobos eat a lot of the herby vegetation that is abundant in all seasons—big reedy stuff like cornstalks and starchy tubers like arrowroot—which offers nutritious shoots and young leaves and pith inside the stems, rich in protein and sugars. Bonobos, then, have an almost inexhaustible supply of reliable munchies. So they don’t experience lean times, hunger, and competition for food as acutely as chimpanzees do. That fact may have had important evolutionary implications.

Bonobos do share one distinction with chimpanzees: Together they are the two closest living relatives of Homo sapiens. Back about seven million years ago, somewhere in the forests of equatorial Africa, lived a kind of proto-ape that was both their direct ancestor and ours. Then our lineage diverged from theirs, and by about 900,000 years ago, those two apes had diverged from each other. No one knows whether their last shared ancestor resembled a chimp, in anatomy and behavior, or a bonobo—but solving that uncertainty might say something about human origins too. Do we come from a long line of peace-loving, sex-happy, and female-dominant apes, or from a natural heritage of warfare, infanticide, and male dominance?

Also: What happened in evolutionary history to make Pan paniscus the unique creature it is?

Richard Wrangham has a hypothesis. Wrangham is a distinguished biological anthropologist and a professor in the Department of Human Evolutionary Biology at Harvard with more than four decades of experience studying primates in the wild. His work on chimpanzees dates back to his Ph.D. research at Tanzania’s Gombe National Park in the early 1970s and continues at Kibale National Park in Uganda. He addressed the subject of bonobo origins in a 1993 journal paper and then in a popular 1996 book, Demonic Males, co-authored with Dale Peterson. The crucial point in his hypothesis is the absence of gorillas, over the past one or two million years, from the left bank of the Congo River.

The reasons for that absence are uncertain, but the evolutionary consequences seem rather clear. On the river’s right bank, where chimps and gorillas shared the forest, the gorillas ate what gorillas still eat, mainly herby vegetation, and the chimps ate a chimp diet, mainly fruit, tree leaves, and occasionally meat. On the left bank dwelled that other chimpish animal, privileged by circumstance to be free of gorilla competition. “And that’s the formula,” Wrangham told me by phone from his office at Harvard, “that makes a bonobo.” The left-bank creatures, bolstering themselves on a rich chimpanzee diet when it was available and sustained by those staple gorilla foods when it wasn’t, lived a steadier life; they weren’t forced to break into small and unstable foraging groups, diverging, rejoining, scrambling for precious but patchily available foods, as right-bank chimps often are. And that fateful difference in food-finding strategy carried consequences for social behavior, Wrangham explained. The relative stability of foraging groups within a larger bonobo community means that vulnerable individuals usually have allies present at any given moment. This tends to dampen dominance battles and fighting. “Specifically,” he added, “females have other females as well as males available to protect them from those that might want to bully them.”

Another result of the foraging-group stability, he noted, involves the sexual rhythms of bonobo females. Unlike chimp females, they aren’t obliged by circumstance to present themselves always as extremely attractive, extremely ready for mating with all possible males during just short, periodic windows of time. “If you are a bonobo,” Wrangham said, and you live in a larger and more stable foraging group, “then you can afford to have a long period of sexual swelling.” A bonobo female doesn’t need to attract gaggles of frantically horny males on a short-term basis. She’s continually attractive, continually ready. “That greatly reduces the importance to the males of competing for dominance and bullying the females.” So the famed amity and sexiness of bonobo social life has, by Wrangham’s hypothesis, an unexpected source: the availability of gorilla foods uneaten by gorillas.

And why are gorillas absent on the left bank? Wrangham suggested a scenario, speculative, he said, but plausible. Sometime after about 2.5 million years ago, severe drying seems to have hit central Africa. In the equatorial lowlands on both sides of the Congo, herby vegetation—gorilla habitat—shriveled away. Chimps could survive by finding fruit in riverside forests, but the right-bank gorillas were forced into highland refuges, such as the Virunga volcanoes in the northeastern part of the drainage and the Crystal Mountains in the west. On the left bank, though, there were no such highland refuges. The land is flat. So if gorillas had ever lived on that side, the Pleistocene drought may have killed them off.

Bonobo behavior is exceptional among apes, and there are exceptions to the exceptions. You can’t paint their portrait with a broad brush. No researchers have been more punctilious about this than Gottfried Hohmann and Barbara Fruth, a married couple based at the Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany, who have studied bonobos in the wild for more than two decades. Their work began in 1990 at a site called Lomako, in northern Congo, and they enjoyed uninterrupted field seasons until war started in 1998 and stopped everything for four years. Hohmann and Fruth then established a new field camp farther south, at a place known as Lui Kotale, in an excellent piece of forest just outside Salonga National Park. They arranged a compact with the local community within whose traditional territory the forest lies: In exchange for a fee, the local people agreed not to hunt or cut trees at Lui Kotale.

To get there, you land at another grass airstrip, walk an hour into a village, pay your respects to the elders, and then keep walking for five more hours. You cross the Lokoro River in a dugout canoe, wade up a black-water stream, climb a bank, and find yourself in a neat, simple camp of thatched ramadas and tents, with two solar panels to power the computers.

Hohmann arrived back at this place, on a June day last year, like a man very glad to be in the forest again after too many months deskbound in Leipzig. He’s a robust 60-year-old, blue-eyed and bony, long conditioned to the steeplechase rigors of field primatology, and if I hadn’t been pushing to stay at his heels, the six-hour hike would have taken me seven.

One morning I rose with the early crew, two lean young volunteers named Tim Lewis-Bale and Sonja Trautmann. We reached the bonobo nests at 5:20 a.m., before the drowsy animals began to stir. Their first act of the morning: a good piss. Lewis-Bale and Trautmann each stood beneath a nest tree, catching urine in a leaf. They pipetted this harvest into small vials, recorded the identity of each pisser, and then we were off on our morning chase.

That afternoon Hohmann and I sat beneath one of the thatch roofs discussing bonobo behavior. Few other researchers have seen bonobos in the act of predation, and those few reports generally involve small prey such as anomalures (only at Wamba) or baby duikers. Animal protein, insofar as bonobos get any, had seemed to come mainly from insects and millipedes. But Fruth and Hohmann reported nine cases of hunting by bonobos at Lomako, seven of which involved sizable duikers, usually grabbed by one bonobo, ripped apart at the belly while still alive, with the entrails eaten first, and the meat shared. More recently, here at Lui Kotale, they have seen another 21 successful predations, among which eight of the victims were mature duikers, one was a bush baby, and three were monkeys. Bonobos preying on other primates: “This is a regular part of the bonobo diet,” Hohmann said.

Sexiness, on the other hand, seemed to him less manifest than others, such as de Waal, had claimed. “I could show Frans some of the behaviors that he would not think are possible in bonobos,” Hohmann said. Infrequent sex, for instance. Yes, there’s a great diversity of sexual acts in the bonobo repertoire, but “a captive setting really amplifies all these behaviors. Bonobo behavior in the wild is different—must be different—because bonobos are very busy making their living, searching for food.”

Hohmann mentioned other points of conventional wisdom against which he and Fruth dissent, including the notion that bonobo society is held together as a genial sisterhood by female bonding (they consider mother-son bonding at least as important) and the notion that bonobos aren’t aggressive toward one another. Aggression may be rare and muted, he said, but that doesn’t make it unimportant. Consider how subtle human aggression can be. Consider how a single violent act, or merely a mean one, can stick in a person’s memory for years. “I think this is just what applies to bonobo behavior,” he said. Life as a bonobo may be more stressful than it appears. Evidence of hidden anxieties has begun emerging from a hormone study by one of his postdocs, Martin Surbeck.

Analyzing fecal and urine samples, such as the ones gathered that morning by Lewis-Bale and Trautmann, Surbeck has found a surprising pattern: high levels of cortisol, a stress-related hormone, in some bonobo males. Cortisol levels have been especially elevated among high-ranking males in the presence of estrous females. What does it imply? That a high-ranking bonobo male, walking a fine line between not enough machismo (which could cost him his status among males) and too much machismo (which could cost him his mating opportunities with imperious females), feels stressed by his complex situation. Bonobos eschew crude aggression and violence, but they’re not carefree; they use sociosexual behaviors, diverse and relatively frequent, as a means of conflict management. “This is what makes them different,” Hohmann said, “not that everything is peaceful.”

The bonobo is classified as endangered, and though protected by Congolese law, it continues to suffer from all-too-familiar problems, especially hunting for bush meat and habitat loss. Perhaps 15,000 to 20,000 bonobos remain in the wild, some of which are harbored within national parks and reserves, such as Salonga National Park and the Lomako-Yokokala Faunal Reserve. These “protected” areas may or may not provide effective security for bonobos and other wildlife, depending upon realities on the ground—for instance, whether or not guards have been hired and trained, paid their salaries, and supplied with adequate weapons to face poachers. Congo suffered severely from its seven decades of Belgian colonialism, followed by three decades of Mobutu’s kleptocracy, followed by war; the context that frames all conservation efforts is institutional dysfunction. Among the hostages to this situation is the bonobo, a species native to no country in the world except Congo. If it doesn’t survive in the wild there, it will survive in the wild nowhere.

Two people who believe that it can survive are John and Terese Hart, conservationists who came originally to the Congo Basin in the early 1970s. Nowadays the Harts work with a young Congolese staff and a wide range of Congolese partners on a large project known as the TL2 Conservation Landscape, a region that straddles three rivers in eastern Congo and holds not just bonobos but also forest elephants, okapis, and a peculiar, newly discovered monkey called the lesula. Bonobos are still being poached at TL2, John told me, their carcasses often transported to market by bicycle. With park status for part of TL2, antihunting regulations, support from local people, and enforcement at just a few checkpoints, he explained, that trade could be choked off. TL2 has magnificent potential, but the constraints are formidable, even for such an irrepressible, experienced man as John Hart.

In Kinshasa I joined John and Terese, and we flew into Kindu, a provincial capital in eastern Congo (and a jumping-off point to TL2) on the west bank of the Lualaba River, which defines the eastern limit of bonobo distribution. In Kindu we finally got approval for a little five-day expedition through TL2. Around four p.m.—late for a departure, but we were concerned not to lose another day—we climbed into a large dugout canoe before the officials could change their minds. We were joined by two of the Harts’ trusted Congolese colleagues, plus a visiting biologist, and a colonel and a soldier (both with Kalashnikovs) as our military escorts. There was also a man from the immigration directorate, assigned at the last minute to shadow us. The immigration man wore street shoes and carried his change of shirt in a briefcase. We’ll be out about 30 days, and you’ll need to help us kill crocodiles for food, John teased him, as the outboard pushed us weakly away from Kindu, and we set our course midstream down the Lualaba.

The river was brown, flat, and a thousand yards wide. The sun, sinking low behind the dry-season haze, looked like a great bloody yolk. I watched a pair of palm-nut vultures pass overhead and then, to the east, a flock of fruit bats circling their roost. Dusk faded quickly to dark, and the river glowed sepia with reflection from a waxing crescent moon. The air cooled; we pulled on jackets. Hours later we grounded at a village on the left bank that marked our trailhead for this hike into bonobo country. It had to be the left bank, I knew. There were no bonobos, anywhere, on the right.

Montana-based David Quammen writes frequently about evolution. Panama-based Christian Ziegler specializes in nature photography.

Society Grant Research on bonobos was funded in part by your National Geographic Society membership.