Orangutan Oops: Rescue Efforts Led to Genetic Mix-up Among Borneo’s Endangered Apes

Crossbreeding between subpopulations of an island’s orangutans could weaken the species, a new study finds.
A baby orangutan hangs onto its mother in Tanjung Puting National Park, on the Indonesian island of Borneo, in this 2001 image. (Photo: Paula Bronstein/Getty Images)
Apr 3, 2016· 2 MIN READ
John R. Platt covers the environment, wildlife, and technology and for TakePart, Scientific American, Audubon, and other publications.

The forests of Tanjung Puting National Park in southern Borneo contain some unusual orangutans that, by all rights, probably shouldn’t even exist.

Scientists say decades-old attempts to rescue, rehabilitate, and release orangutans unintentionally created an unnatural mix of genetics among the park’s orangutans that could pose future health problems for these endangered apes.

Their research was published in February in the journal Scientific Reports.

The situation dates back to the pioneering work begun in the 1970s by orangutan researchers and rescuers Biruté Galdikas and her husband, Rod Brindamour. At that time, scientists still debated whether the orangutans living on the islands of Borneo and Sumatra were different species, something that wasn’t fully accepted until 1996.

Beyond that, what no one back then yet realized was that the orangutans living on Borneo actually represented three different subspecies, each from a different part of the island and each with widely divergent genetics.

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“Bornean orangutans last shared a common ancestor around 176,000 years ago and have markedly differentiated over the last 80,000 years,” said Graham Banes, the lead author of the new paper and a scientist with Germany’s Max Planck Institute for Evolutionary Anthropology. “That’s an incredible amount of independent evolution.”

Galdikas and Brindamour rehabilitated more than 90 orangutans at their famous Camp Leakey research site and released many of them back into the national park. The animals, most of which were rescued from the pet trade, came from all over Borneo and, we know now, were not all from the same subspecies. At least two females released into the park came from the western portion of the island, home to a subspecies different from the orangutans that already lived in the park.

When Banes and his colleagues, including Galdikas, tracked the genetic lines of these two orangutans, they revealed two vastly different stories. One female, known as Rani, had at least 14 descendants, all but two of which are believed to still be alive.

The other, Siswoyo, only had eight descendants, most of which did not survive.

The researchers concluded that these vastly different levels of reproductive success could be the result of what geneticists call “hybrid vigor” or “outbreeding depression,” scenarios where the combination of genetics from two different subspecies can cause either benefits or consequences.

These hybrids may be the tip of the iceberg for Tanjung Puting’s orangutans, as Banes said other people might have illegally released hundreds of additional animals of unknown subspecies into the national park over the years.

If enough of their offspring have thrived, the resulting “cocktail” of unusual genetics “could have serious implications for their health and reproductive success, not only for individuals, but for their offspring and descendants as well,” Banes said.

To assess the full scope of this problem in the wild, the next phase of the team’s research will focus on orangutans in North American zoos. The Orangutan Species Survival Plan organizes and documents all captive breeding in accredited facilities around the world, but it doesn’t track Bornean orangutans by subspecies. That means some of these same genetic “cocktails” and health problems may be present in captive populations.

Banes said zoo orangutans provide an ideal opportunity to investigate the effects of interbreeding different subspecies. “We know who is related to who and we have all their health, reproductive, and necropsy data,” he said.

Examining their genetics and health, he added, could provide clues to save orangutans in the wild and in rescue centers, where orphaned and injured animals often show up after their forest habitats have been chopped down or burned for agriculture and other development.

The researchers might find that some interbreeding doesn’t create problems, or they could find severe consequences. Either way, Banes said, zoo-housed orangutans might prove the salvation of their wild-born counterparts.

The three subpopulations of orangutans on Borneo all evolved

from a common ancestor, but diverged genetically about

80,000 years ago. (Image: Banes et al./Scientific Reports)