The individual stories in the book are compelling, and they lead towards an interesting conclusion about the way we project our own attributes onto other species. How much should we anthropomorphize animals like our pets or apes like Brian? As much as it helps us help them. If treating Brian like a human psychiatric patient helped Prosen treat the suffering animal, then it makes sense to project that level of humanness onto the creature.

Prosen began with a full psychiatric history of Brian. He'd been born at the Yerkes National Primate Research Center at Emory University in Atlanta. Bonobos are famously, polymorphously, perversely sexual—but they don't generally engage in sexual violence. And yet Brian's father, who had suffered his own traumas as a research animal, sodomized Brian for years. During his seven years at Yerkes, Brian started to stick his own hand into his rectum, causing bleeding and—over time—thickening of the tissue there. It was a horrifying situation.

In 1997, when Brian arrived, the bonobo crew at the Milwaukee County Zoo, which was the largest captive troop in the United States, was unusually stable and nice, seemingly due to the calming presence of two apes, Maringa, and Brian's friend, Lody. The troop had already helped other animals recover from mental disturbances, which is one reason that Brian had been sent there. But he seemed beyond natural recovery.

Prosen first prescribed Paxil, to help with Brian's anxiety, occasionally supplemented by Valium, on the bad days. "The beauty of the drug therapy," Prosen told Braitman, "was that the other bonobos could start to see him for who he really was, which was really a cool little dude."

Meanwhile, Prosen and the zookeeping staff began Brian's therapy, focusing on making changes to their own behavior and his environment. They spoke quietly and moved slowly and consistently. No sudden movements or loud noises. They made each of his days exactly the same, and only introduced new things slowly and deliberately. They had Brian hang out with apes who were younger than him, so that he could learn what he'd never been taught as a kid: play.

"Interacting with adult females, to whom he’d had no exposure as a youngster, caused him all sorts of anxiety," Braitman writes. "This was confusing to the rest of the troop because Brian looked like an eight- or nine-year-old young male, but developmentally he acted like a five- or six-year-old."

By 2001, after four hard years of therapy and improvement, Brian had begun to integrate into the Milwaukee troop. The zookeepers saw it as significant that a new mother let him touch her 10-day-old baby, and over the the next few years, his behavior became more and more socially aware. They peg his 16th birthday, in 2006, as the time when he "started acting his age." He loves carrying around the babies in the troop, and even managed to have his own children. And, as his keeper Barbara Bell recalled, he went off Paxil at some point, after he took to sharing it (!) with the other apes.