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 at Camp Leakey in the Tanjung Puting National Park (pictured) now carry a 'cocktail' of genes from different subspecies that would not normally exist in the wild. This could be causing health problems for the genetically different offspring

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.

'OUTBREEDING DEPRESSION' When offspring from individuals in different populations have lower fitness than those coming from individuals in the same population, it is called outbreeding depression. This might have occurred when two subspecies of orangutan bred after a non-native subspecies was introduced to an Indonesian national park. One of the introduced orangutans, Siswoyou, had fewer surviving offspring than any other female in the park. She 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 and gave birth to a stillborn offspring, a daughter that died in infancy, and a son that often needed medical interventions. The researchers warn that releasing the great apes back into the wild without genetic testing could lead to more interbreeding and potentially cause more scenarios like Siswoyo. Advertisement

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.'

The orangutans were first taken to the Camp Leakey in the Tanjung Puting National Park, Indonesia, in 1971, when it was thought that all orangutans were the same species. Since 1985, after around 90 of the great apes were released into the park, that advances in genetic studies have revealed two different species

More than 1,500 orphaned and displaced orangutans are currently waiting to be released from centres on Borneo and Sumatra. The researchers are warning that releasing the great apes back into the wild without genetic testing first could lead to more interbreeding and potentially cause health problems in offspring

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.

The family trees of the two female orangutans, Rani and Siswoyo, the researchers studied are pictured. 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

Two orangutans, 11 and 10 months old, at an animal hospital in Indonesia safari park in 2004. Orangutan subspecies diverged about 176,000 years ago, according to the researchers, and breeding between two subspecies could have negative impacts on the populations of the great apes that are already under threat

'Hybridisation can have positive effects, and also negative effects, and these are not very predictable,' Mark Jobling, Professor of Genetics at the University of Leicester told MailOnline.

'In general, conservationists, as the name suggests, try to keep things the way they are, so of course they want to prevent the meeting and hybridisation of groups that normally would not interact.'

ORANGUTAN 'COCKTAIL' Graham Banes, lead researcher on this project, is now studying the genetics of Bornean orangutans in zoos around the world. 'Hundreds of orangutans have kindly volunteered their faeces, for example, to provide us with a source of their DNA that we can use to help their orphaned and displaced cousins in the wild,' he told MailOnline. 'Given my focus on 'cocktail' orangutans, I also plan to create an orangutan cocktail. Preferably something very alcoholic,' Banes added. 'I will need it to process all these faecals.' The research team is now studying the orangutans in zoos over the world. Advertisement

'It's interesting to note that our own species today is the result of past hybridisations - for example, when Homo sapiens left Africa there was interbreeding with Neanderthals, another hominid from which we'd be separated by hundreds of thousands of years, and also the mysterious Denisovan.

'There were some advantages to these contacts - humans now carry genes that protect us from the effects of cold, high-altitude and infectious diseases that came from these early liaisons,' he added.

'There is no definitive evidence of outbreeding depression among Bornean orangutans,' says Banes, 'but our findings are enough to cause serious alarm.'

More than 1,500 orphaned and displaced orangutans are currently waiting to be released from centres on Borneo and Sumatra.

The researchers are warning that releasing the great apes back into the wild without genetic testing first could lead to more interbreeding and potentially cause more scenarios like Siswoyo. They are calling for international guidelines to be put in place.

'They might look roughly the same, but these orangutans from different subpopulations haven't shared a common ancestor for tens of thousands of years,' Banes added.

'It may be that inter-breeding them has no ill effects at all, but what if it does? Suddenly, for the sake of short-term welfare, we've compromised the viability of wild populations - and we can never take that back,' he said.

The research team is now studying the orangutans in zoos over the world. 'We need to err on the side of caution,' researcher Graham Banes told MailOnline. 'I feel strongly that we shouldn't be hybridising Bornean orangutans until we know for certain how this affects their offspring over multiple generations'

'We need to err on the side of caution,' Banes told MailOnline.

'I feel strongly that we shouldn't be hybridising Bornean orangutans until we know for certain how this affects their offspring over multiple generations.

'However, given that more than 1500 orangutans are awaiting release from rehabilitation centres, we do need to determine precisely what those effects might be on a broader scale.

'It would be much easier to reintroduce these animals if we could just mix them all together, but we have to be certain there won't be any problems that could affect their viability later on.

'Otherwise, we're sacrificing conservation in the longer term for the sake of short-term welfare.'

The research team is now studying the orangutans in zoos over the world.

'We're studying the genetics of Bornean orangutans in more than a hundred zoos worldwide, which have been indiscriminately inter-bred between subspecies for decades,' Banes told MailOnline.