Documenting the carnage has had some effect. In October 2007 the BBPP convinced Equatorial Guinea's president, Teodoro Obiang Nguema Mbasogo, to issue a ban against the hunting, selling, and consumption of primate meat. It had been in place for two months when the BBPP team arrived in January. How were the monkeys faring? And how would they react to humans who wanted to count rather than shoot them? It didn't take long for the biologists to find out.

It was dusk, a time when monkeys chatter as they settle in for the night, but the caldera forest was oddly quiet. Butynski had anticipated an especially lively chorus because all seven monkey species live in the caldera. But there was little to hear aside from the trilling hum of insects and frogs. Butynski kept to a trail, stopping every 50 feet or so to look and listen. "Well," he said at last, clearly baffled, "maybe the monkeys have moved to another part of the forest for the night."

No sooner had he finished speaking than two fur bombs the size of large dogs hurtled overhead. They splashed into the leafy crown of a nearby tree and then plunged into another, as if diving from one green pool to the next. Finally, having uttered not a single call, they disappeared over the edge of a river gorge and into the forested twilight.

Butynski, who has made dozens of primate surveys throughout Africa, pulled out his notebook to record the sighting. "That's a surprise—two drills," he said. "They must have been in the trees sleeping when they heard my voice."

The next morning Butynski set out into the caldera again. The instant he spotted a troop of red-eared monkeys, they began to give hack calls and chirps, eyes wide with terror. Mothers clutched babies to their breasts; branches bounced and limbs heaved as the monkeys scurried to get away.

Such fleeting glimpses were all the scientists came to expect. In the first three days of the survey, as the team hiked from the island's southern shore 2,000 feet up the caldera, all the monkeys they spotted gave alarm calls before vanishing into one of the steep river gorges that cut through the caldera.

On the fourth day, however, the monkeys were less afraid. After hiking up and down steep, muddy trails littered with rough lava cobbles, the team reached the caldera's northern end, its inner sanctum. Bioko receives more than 400 inches of rain a year, and although this was supposed to be the dry season, daily thunderstorms unleashed torrents. Between storms, the sun shone fierce and bright, and large beads of sweat rolled off everyone's foreheads and noses.