A chimp called Mike lies on his back in a hammock of branches, legs bent, one ankle crossed atop the opposite knee. One arm is behind his head, the other is crooked at the elbow, the hand hanging slack from the wrist, in the manner of a cowboy slouched against a fence. We stare at each other for a full ten seconds. Partly because his pose is so familiarly human and partly because of the way he holds my gaze, I find myself feeling a connection with Mike.

I confess this to Pruetz, who admits to similar feelings. She cares about the Fongoli chimps as one cares about family. She sends excited emails when a baby is born and worries when the elderly and nearly blind Ross disappears for more than a week. But she does not reveal this side of herself at conferences. There it's all lingo and statistics, pairwise affinity indexes and "blended whimper pouts." "Especially with male chimp researchers," she says.

One of the first things primatology students are taught is to avoid anthropomorphism. Because chimps look and act so much like us, it is easy to misread their actions and expressions, to project humanness where it may not belong. For example, I catch Siberut looking toward the sky in what I take to be a contemplative manner, as though pondering life's higher meaning. What he's actually pondering is life's higher saba fruits. Pruetz points some out in the branches above Siberut.

Yet it is impossible to spend any time with chimpanzees and not be struck by how similar they are to us. I've been keeping a list of things I have seen or read or heard Pruetz say that drive home this point in unexpected ways. I had not known that chimpanzee yawns are contagious—both among each other and to humans. I had known that chimps laugh, but I did not know that they get upset if someone laughs at them. I knew that captive chimps spit, but I hadn't known that they, like us, seem to consider spitting the most extreme expression of disgust—one reserved, interestingly, for humans. I knew that a captive ape might care for a kitten if you gave one to it, but had not heard of a wild chimpanzee taking one in, as Tia did with a genet kitten. The list goes on. Chimps get up to get snacks in the middle of the night. They lie on their backs and do "the airplane" with their children. They kiss. Shake hands. Pick their scabs before they're ready.

The taboo on anthropomorphizing seems odd, given that the closeness—evolutionary, genetic, and behavioral—between chimpanzees and humans is the very reason we study chimps so obsessively. Some thousand-plus studies have been published on chimpanzees. As a colleague of Pruetz's once said to her, "A chimp takes a crap in the forest, and someone publishes a paper about it." (No exaggeration. One paper has a section on chimpanzees' use of "leaf napkins": "This hygienic technology is directed to their bodily fluids (blood, semen, feces, urine, snot). ... Their use ranges from delicate dabbing to vigorous wiping."

As for the chimps, they are not nearly as intrigued by the ape-human connection. While we've been observing them, they have largely ignored us, occasionally shooting a glance over one shoulder as they move through the brush. There is no fear in this glance, but neither is there curiosity or any sort of social overture. It is a glance that says simply, Them again.

Even Mike. He just turned away from my gaze and pointedly, or so it seemed, rolled over to turn his back on me. In hindsight I would have to say that the reason Mike had been looking at me was that I happened to be in his line of vision.

The chimps begin making their nests, breaking off leafy branches and dragging them into the treetops. Pruetz will wait until all are bedded down before turning to head back. We sit and listen to their "nest grunts"—soft, breathy calls that seem to express nothing more than the deep contentment one feels at the end of a day, in a comfortable bed.