The chimp named Sissy sits motionless and hunched at a low termite mound twenty feet from us.

Only her right arm moves, pushing a saba vine probe into a hole and gently withdrawing it, with termites clinging to it. She raises it carefully to her mouth like a pensioner spooning soup. The mound is across an open lay of pebbly, brick-colored laterite that gives the ground the look of a clay tennis court.

Like fly-fishing, termite fishing is a meditative, deceptively nuanced activity. I tried it a few times and could not even find an active hole. My probe never sinks farther than an inch or so; the chimps regularly bury theirs a foot or more. They can find active holes by smell, inserting a probe and then sniffing the end of it for the smell of soldier termite pheromone.

Fongoli chimps eat termites year-round, not just in the dry season, when other foods are scarce. Termites make up, at bare minimum, 6 percent of the Fongoli chimps' diet. We know this because most evenings at six o'clock research assistant Sally Macdonald sits down with a set of sieves and buckets, and one or two ziplock bags of the chimp feces that the researchers bring back most days. She scans the fruit seeds, estimates the percentage of fiber from leaves and shoots, and takes note of bones and termite pincers. "Science in all its glamour," deadpans Macdonald, whose mother sends ziplock bags but does not know their fate.

A quick glimpse into the bucket reveals that saba fruit is the chimps' mainstay this time of year, an adult averaging 30 to 40 a day. The Fongoli record for saba seeds in a single fecal sample—499, compared with an average of 75—probably belongs to a male named Mamadou. Which may explain why Mamadou is, quoting Pruetz, "especially gassy."

Pruetz's Ph.D. student Stephanie Bogart says part of the reason chimps fish termites is that they're an exceptionally calorific food. A 3.5-ounce serving of termites has 613 calories, compared with chicken's 166. But 3.5 ounces of soldier termites is hundreds of insects, fished piecemeal from a mound. It's like eating cake one crumb at a time. The chimps must really like them.

Sissy gets up from her spot at the termite mound to select a new tool. She breaks off a length of vine, inspects it. Satisfied, she sticks it in her mouth and carries it back to the mound like a seamstress holding pins between her lips. Pruetz and others argue that female chimps are not only more skilled than males at crafting and using tools, but also more diligent. Craig Stanford agrees that it might well have been our female ancestors who first steered the culture toward tool use. Early tools for foraging, he imagines, gave way to tools for scavenging meat from carcasses killed and abandoned by large carnivores. These tools in turn may have paved the way for implements for killing prey. Which makes Pruetz's observations of chimps sharpening sticks and using them to whack bush babies all the more arresting: Fongoli's females seem to have skipped ahead to the killing tools. Barbecue tongs can't be all that far behind.

Pruetz and I are sitting along a forested ravine where the chimps rest during the day's hottest hours. The vegetation is thicker here. We watch a slender green vine snake move through the grass. Birds are calling over our heads. One says cheerio; one actually says tweet. A third says whoop whoop whoop whoop whoop, like Curly of the Three Stooges. (When I ask what that one is, Pruetz replies, not at all sarcastically: "a bird." She is a woman of singular interests.)

Pruetz directs my gaze to a tangle of saba vines. Where I see a dark mass, she is able to distinguish six animals. The woman has chimp vision. (It's a condition that lingers long after she gets back to Iowa. "I get home and I'm looking for chimps on campus.") The animals can be so well hidden and so quiet that even Pruetz has trouble finding them. She sometimes locates them by smell—"chimp" being a potent variant of B.O. "Yesterday I thought I smelled chimp," Pruetz says, "but it was me."

The scene in the vines is one of drowsy, familial contentment. Yopogon is grooming Mamadou. Siberut is leaning against a tree trunk, rubbing his two big toes together, as he often does. A pair of youngsters swing on vines, flashing in and out of an angled shaft of sun. One uses a foot to push off from a tree trunk, spinning himself around. The other swings from vine to vine, Tarzan-style. They are almost painfully cute.