There are only about 6,600 orangutans left in Sumatra. Once found throughout the Indonesian island, the orangutans now live in just a few forest patches on the northern tip of the island.

Now a genetics study reveals that the population patches are not as isolated as they seem, and that interbreeding occurs across rivers and mountainous areas.

Protecting the existing corridors among these small populations is important to ensure that the species survives, said Alexander Nater, an evolutionary biologist at the University of Zurich and the study’s lead author. “It is important that all Sumatran orangutans act like a large population for future survival,” he said.

The research, reported in The Journal of Heredity, was part of Dr. Nater’s doctoral work at the University of Zurich Anthropological Institute and Museum.