Despite increased hunting, the forest canopy in this distant part of the crater fairly exploded with monkeys. In their leafy shelter a dozen red-eared monkeys leaped in alarm, trailing their long copper-colored tails along the branches and shouting their nasal call of warning. Forty feet farther on, a smaller group of gnomelike black colobus interrupted their leaf breakfast to race away. Just beyond them, a single charcoal-colored Preuss's monkey jumped from a low bush where he'd been feeding into a towering mahogany tree, then leaped into a neighboring tree, his dark tail curled in a shepherd's crook over his back. In the distance troops of red colobus gave honk calls, and crowned monkeys made their throaty booms.

Occasionally, red Ogilby's and blue duikers crashed through the tangled undergrowth. Dozens of butterflies in brilliant hues and patterns to rival a Missoni gown flitted along the trail, while Jurassic Park–size earthworms and millipedes slithered into damp ravines, and pairs of gray parrots pirouetted in the sky.

Butynski jotted down each monkey troop and duiker, and stopped to inspect flowers, leaves, and fruits that monkeys had nibbled. Sometimes a strong, ammonia scent filled the air—the calling card of a troop of red colobus, one of the rarest of Bioko's monkeys. But it was the drills—even scared drills—that we all most wanted to see.

Finally we spotted a small troop of drills below us on the far side of a river feeding in a tree. The distance and rushing water extinguished the sounds and smells of our little clutch of humans, and the drills went about their business as if we weren't there. This was when we sat down to watch.

All had bushy, gray-brown pelts, and all but one were adult females or adolescents. The sole adult male was nearly twice as big as the others. He was simultaneously muscled and rotund, his Buddha belly at odds with his sharp-featured, obsidian black face. So sculpted were the angles of his cheeks, brows, and nose that he looked as if he wore a mask. White fur bristled around his face; his rump shone red, blue, and purple. Whenever he moved, the other drills got out of his way. At last, when they had eaten their fill, the troop clambered down the tree and vanished into the shadowy forest.

"Isn't it remarkable?" Butynski said after the last drill was gone. For nearly 30 minutes the biologists had been able to observe monkeys that weren't frightened of humans. "No one has studied the ecology and behavior of these animals in the wild," he said. "But that might be possible here now: Someone could habituate a troop of drills to humans and start a long-term study."