Orangutans interbreeding in Indonesia are threatening the already endangered species

Endangered species such as orangutans are often taken to sanctuaries and rehabilitation centres across the globe, with the ultimate aim being to reintroduce them into the wild.

But it appears that reintroducing these endangered species can sometimes have unwanted effects.

A group of researchers has discovered that a non-native subspecies released into a national park in Indonesia has since bred with the park's apes - creating a hybrid the scientists dubbed 'cocktail'.

Orangutans are the two exclusively Asian species of extant great apes.

Native to Indonesia and Malaysia, they are currently found in only the rainforests of Borneo and Sumatra.

When the orangutans were first taken to the Camp Leakey in the Tanjung Puting National Park, Indonesia, in 1971, it was thought all orangutans were the same species.

It has only been since 1985, after around 90 of the great apes were released into the park, that advances in genetic studies have revealed two different species of orangutan – Bornean and Sumatran.

Orangutan subspecies diverged 176,000 years ago, according to researchers, and breeding between the two subspecies could have negative impacts on the populations of the great apes that are already under threat.

Their forest habitat in Indonesia and Malaysia is rapidly disappearing, putting the future of Asia's only great ape in peril.

The Bornean species can be split into three genetically different subspecies that were geographically and genetically isolated from each other.

The researchers, Dr Graham Banes and Dr Linda Vigilant from the Max Planck Institute for Evolutionary Anthropology, used 44 years of data and worked with Biruté Galdikas, who had originally released the orangutans in the 1970s, to determine the extent to which she had released non-native apes into the park.

They found that two non-native females had been rescued from the pet trade, Rani and Siswoyo. 

They were originally captured from Borneo.

Since they were released they have interbred with the native males and produced 22 'hybridised' descendants, who inherited a cocktail of genes that would not have otherwise occurred in the wild. 

Breeding between animals that are genetically different can sometimes be successful if the offspring inherit the benefits of both parents' individual qualities.

But Vigilant said 'offspring born to parents from two genetically distinct populations, which have not been in genetic contact for significant periods of time, have also been shown to suffer poor health and reproductive success in a range of different species.'

One of the orangutans, Rani, had a successful family with 14 descendants. Two died in infancy but the rest are thought to still be alive and healthy.

But in contrast, Siswoyo had fewer surviving offspring than any other female in the park. 

The researchers think this could be because of the interbreeding causing 'outbreeding depression' - when offspring from individuals from different populations have lower fitness than those from individuals from the same population.

Siswoyo only had five first-generation and three second-generation offspring. Two of her offspring died when they were young, while an infection following her last pregnancy meant Siswoyo's died ten days after the birth.

Her only daughter, Siswi, has frequently required care from the vets, including major surgery to treat a perforated intestine. 

She also gave birth to a stillborn offspring, a daughter that died in infancy, and a son that often needed medical interventions.