DOROUGOK, Indonesia — The older man wore just a loincloth, revealing taut muscles and leathery skin from decades of living deep in the rain forest. Like other members of his tribe, he was covered head to toe in tattoos. Though he appeared strong, he had a pronounced hunch, and a cough from smoking too much tobacco.

The man, Teu Kapik Sibajak, grabbed his ax on a recent morning and went off through the forest to chop down a sago palm tree. Mr. Kapik delivered precise blows before he and a few friends stooped down and rolled pieces of the thick, heavy trunk toward his house. “Hard work, this!” he announced.

But the effort would be worth it: The tree’s leaves provide the roof for his wooden long house; its starchy insides can be cooked and eaten, or fed to the household’s pigs, ducks and chickens.

Mr. Kapik and his wife, Teu Kapik Sikalabai, are among the last of the Mentawai people living traditional lives deep in the forest on the remote island of Siberut.

They, and others like them, have for decades resisted Indonesian government policies that pressured the forest-bound indigenous groups to abandon their old customs, accept a government-approved religion and move to government villages. That shift, along with the inevitable lure the modern world has for their children, has led to major disjunction between generations of Mentawai.