I began at the source, motoring hundreds of miles upriver in a precarious, pencil-thin canoe in the Democratic Republic of Congo to an area where many bonobos have disappeared. Then I traveled to Thailand, where some of the biggest wildlife gangs operate. When I finally found an ape trafficker and had him on the phone, I stumbled over what, exactly, to say. All my instincts told me to hammer him with questions like: Where are you getting your apes from? Who are you? But my role was to observe his illicit operation firsthand, which meant not scaring him off. It was a journalistic pickle.

The trafficker introduced himself as “Tom,” clearly a fake name. He had been discovered by Daniel Stiles, a well-known wildlife researcher in Kenya and a source of mine for years. Mr. Stiles had seen Tom’s online advertisements and arranged a deal. The plan was to meet in Bangkok, where Mr. Stiles would give Tom around $15,000 for two endangered baby orangutans, enabling the Thai authorities to swoop in and arrest the smuggler. I would report the events as they unfolded, without getting involved in any exchanges or arrests.