Here’s what surprised me most: the Shuar themselves were prolific commercial head shrinkers. Beginning in the mid-1940s, word spread throughout the region that a tsantsa could be traded for a shotgun. Around the same time, anthropologist John Patton told me, the Shuar gained a tactical advantage over the Achuar. The Achuar had long controlled the rivers, affording access to trade routes and opportunities to barter for superior firearms being made in Brazil and traded up through Peru and Ecuador. Because Shuar headhunters faced retaliation from the better-armed Achuar, head-taking raids were sporadic and carefully considered. And then the balance shifted. A critical ­section of border closed, cutting off the Achuar’s access to trade and ammunition. The Shuar got busy.

“A hundred and fifty Shuar warriors would go and take heads, whole families,” says Patton, “partly because they had a commercial outlet for it and also because when the ­Achuar were reduced to using spears it was a lot easier to do.” Patton told me that the Shuar, around that time, would refer to the Achuar as fish—as in, “Let’s go catch some fish.”