NUEVA OCEANIA, Peru — Sometimes, they imitate the low rumble of the howler monkey or the shrill squawk of the curassow. Often, they block a jungle path with two branches in the shape of an X.

The Mashco Piro, believed to be the planet’s largest Indigenous group still living in voluntary isolation from the outside world, have ways of registering their displeasure at — and fear of — intruders. When they’ve felt threatened in their remote Amazonian territory, the hunter-gatherers, whose number is estimated at 750, have launched six-foot arrows from the bush. They have, on occasion, killed strangers.