Kwykuti, body squat and sinewy under a yellow T-shirt, head sporting a tiara of crested oropendola feathers, guides us into a side channel that feels more as the Amazon should — steaming, fetid and overhung. Soon we have a chupita piranha the size of a dinner plate. “Maijkrumej,” says Kwykuti, “good” in his native Kayapo.