Biruté Galdikas spent two months in the rainforest before she saw an orangutan. She was 25, and she had given up an easy life in Los Angeles for what people told her would be an impossible one in Indonesian Borneo.

Every day, Dr. Galdikas walked up to a dozen miles through a dense wood plagued by fire ants and pit vipers. Leeches fell out of her socks. It rained so much, the swamps swelled up to her armpits. Most nights, she wrote in a 1995 memoir, she arrived back at her bark-walled hut “drenched in sweat and convinced that the wild orangutans had all moved out of the study area simply to spite me.”