Meanwhile, bonobo studies began to gain momentum. Other scientists followed Horn into the Congo Basin, and they set up two primary field sites. One, at Lomako, three hundred miles northeast of Lake Tumba, came to be used by Randall Susman, of Stony Brook University, and his students. Further to the east, Takayoshi Kano, of Kyoto University, in Japan, made a survey of bonobo habitats on foot and on bicycle, and in 1974 he set up a site at the edge of a village called Wamba. Early data from Wamba became better known than Lomako’s: the Japanese spent more time at their site and saw more bonobos. Susman, however, can take credit for the first bonobo book: he edited a collection of papers given at the first bonobo symposium, in Atlanta, in 1982.

In the winter of 1983-84, in an exploration that was less gruelling but as influential as any field research, Frans de Waal turned his attention from chimps to bonobos, and spent several months observing and videotaping ten bonobos in the San Diego Zoo. He had recently published “Chimpanzee Politics: Power and Sex Among Apes” (1982), to great acclaim, and, as de Waal recently recalled, “Most people I talked to at the time would say, ‘Why would you do bonobos if you can do real, big chimpanzees?’ ” Among the papers that drew on his studies in San Diego, one was particularly noticed in the academy. In “Tension Regulation and Nonreproductive Functions of Sex in Captive Bonobos,” de Waal reported that these apes seemed to be having more sex, and more kinds of sex, than was really necessary. He recorded seventeen brief episodes of oral sex and four hundred and twenty equally brief episodes of face-to-face mounting. He also saw forty-three instances of kissing, some involving “extensive tongue-tongue contact.”

In the late nineteen-eighties, Gottfried Hohmann was an ambitious scientist in his thirties; he spent nearly three years in southern India, researching vocal communication in macaques and langurs. “But it was difficult then to get funding for India,” Hohmann told me. “And the bonobo thing was just heating up. Frans’s paper really affected everyone” in the scientific community. “Tongue-kissing apes? You can’t come up with a better story. Then people said to me, ‘We want you to go in the field.’ ” Hohmann ran his hand back and forth over his head. “So,” he said.

We were sitting on a wooden bench at the edge of a forest clearing barely larger than a basketball court, talking against a constant screech—an insect tinnitus that the ear never quite processed into silence. Trees rose a hundred feet all around, giving the impression that we had fallen to the bottom of a well. Two days after our plane touched down, we had reached Lui Kotal. In the intervening hours, which were inarguably more challenging for the three newcomers than for Hohmann, we had first camped in a violent rainstorm, then followed some unflagging porters on a trail that led through the hot, soupy air of the forest, and along waist-high streams that flowed over mud. We had then camped again, before crossing a fast-flowing river in an unsteady canoe.

Now, at five in the afternoon, the light at Lui Kotal was beginning to fade. People who work there make do with little sun—and with a horizon that is directly overhead. Around us, the wall of vegetation was solid except where broken by paths: one led back to the village; another led into that part of the forest where Hohmann and his team have permission to roam—an area, six miles by five, whose boundaries are streams and rivers. In the clearing stood a dozen structures with thatched roofs and no walls. Some of these sheltered tents; a larger one was a kitchen, where an open fire was burning; and another was built over a long wooden table, beside which hung a 2006 Audubon Society calendar that had been neatly converted—with glue, paper, and an extravagant superfluity of time—into a 2007 calendar. At the table sat two young American volunteers who were not many weeks away from seeing the calendar’s images repeat. Pale, skinny men in their twenties, wearing wild beards, they looked like they needed rescuing from kidnappers. Three others were less feral, and had been in the camp for a shorter time: a young British woman volunteer, an Austrian woman who had recently graduated from the University of Vienna, and a Swiss Ph.D. student attached to the Max Planck Institute.

Hohmann, shirtless, was in an easy mood, knowing that much of the logistical and political business of the trip was now done. Before leaving the village, I’d seen something of a bonobo researcher’s extended duties. The men of Lompole had convened around him, their arms crossed and hands tucked into their armpits. Hohmann remained seated and silent as an angry debate began—as Hohmann described it, between villagers who were unhappy about the original deal that compensated the village for having to stop hunting around Lui Kotal (this had involved a bulk gift of corrugated iron, to be used for roofs) and those who worked directly for the project and saw the greater advantage in stability and employment. Hohmann had finally got up and delivered a forceful speech in Lingala, Congo’s national language. He finished with a moment of theatre: he loomed over his main antagonist, wagging his finger. “It’s good to remind him now and then how short he is,” Hohmann later said, smiling.

By 1989, Hohmann told me, he had read enough bonobo literature to be tempted to visit Zaire. Even if one left aside French kissing, he said, “the bonobo allured me. I thought, This is a species.” By then, thanks to field and captive studies, a picture of bonobo society had begun to emerge, and some peculiar chimpanzee-bonobo dichotomies had been described. Besides looking and sounding different from chimpanzees (bonobos let out high whoops that can seem restrained alongside chimpanzee yelling), bonobos seemed to order their lives without the hierarchical fury and violence of chimpanzees. (“With bonobos, everything is peaceful,” Takeshi Furuichi, a Japanese researcher who worked with Kano at Wamba, told me. “When I see bonobos, they seem to be enjoying their lives. When I see chimpanzees, I am very, very sorry for them, especially for the high-ranking males. They really have to pay attention.”) In captivity, at least, male bonobos never ganged up on females, although the reverse sometimes occurred. The bonds among females seemed to be stronger than among male chimpanzees, and this was perhaps reinforced by sexual activity, by momentary episodes of frottage that bonobo experts refer to as “genito-genital rubbing,” or “g-g rubbing.” And, unusually, the females were said to be sexually receptive to males even at times when there was no chance of conception.

“We said, ‘We have to answer: Why is it like this?’ ” Hohmann said. “The males, the physically superior animals, do not dominate the females, the inferior animals? The males, the genetically closely related part of any bonobo group, do not coöperate, but the females, who are not related, do coöperate? It is not only different from chimpanzees but it violates the rules of social ecology.”

Hohmann flew to Zaire and eventually set up a small camp in Lomako Forest, a few miles from the original Stony Brook site. His memory is that Susman’s camp had been unused for years, but Susman told me that it was still active, and that Hohmann was graceless in the way that he took over the forest. And although Hohmann said that he worked with a new community of bonobos, Susman said that Hohmann inherited bonobos that were already habituated, and failed to acknowledge this research advantage. Whatever the truth, the distrust seems typical of the field. The challenges of bonobo research call for chimpanzee vigor, and this leads to animosities. Susman told me that Hohmann was the kind of man who, “if he was sitting by the side of the road and needed a filter for his Land Rover, people would drive right by. Even if they had five extra filters in the trunk.”