A few years ago, while setting up camp deep in the Congolese rain forest, Dave Morgan and Crickette Sanz heard a party of male chimpanzees vocalizing raucously in the distance. The hoots grew louder, and they could tell the group was moving rapidly through the canopy.
The chimps, they realized, were headed straight for their camp and would soon be nearly on top of them. Then, just as the group seemed to be closing its distance to a few dozen yards, the forest went silent. A few seconds passed before Sanz and Morgan heard a gentle hoo from a tree almost directly above them. They looked up and saw a perplexed adult chimp peering down.
When wild chimps encounter humans, they typically flee in panic—understandable given that the relationship between our two species has often been one of prey and predator. This reticence around humans is part of what makes wild chimp research so difficult. Before the animals can ever be studied, they must learn not to bolt at the sight of a person, a process of habituation that requires many years of diligently trailing the animals around the forest.