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.

Sumatran orangutans build new nests each night to sleep in, and the researchers climbed trees to get hair samples from abandoned nests. They used the hair, along with blood and fecal samples, to look at the apes’ genealogy.

By looking at mitochondrial DNA, they were able to study the orangutans’ maternal lineage and found that females always settle close to their mothers.

Autosomal microsatellites (highly repetitive stretches of DNA inherited from both parents) revealed more about the paternal lineage. It is the males who navigate rivers and mountains and mate with females outside their local populations, the researchers found.

While it is a healthy sign that males cross boundaries and improve genetic diversity, deforestation is still a threat to the species.