In the 1970s and '80s, more than 90 rehabilitated orangutans – mostly confiscated from illegal captivity in the pet trade – were released into the wild at Camp Leakey in Tanjung Puting National Park in Borneo. At the time, all orangutans were classified as just one species, but we now know that there are at least two species and a handful of genetically distinct subspecies.
Now, researchers reveal that some of the reintroduced orangutans were inadvertently from a non-native subspecies, and they have produced at least 22 descendants carrying a “cocktail” of genes. The findings, published in Scientific Reports this week, cautions that reintroducing displaced mammals may threaten wild populations.
By unintentionally collecting animals from different and sometimes unknown places of origin, sanctuaries – many of which have the ultimate goal of reintroduction – may be mixing individuals from populations that haven’t been in contact for millennia. About 1,516 orangutans are confined in sanctuaries awaiting reintroduction. Based on genetics and recent morphological studies, researchers say orangutans are two species: Pongo pygmaeus of Borneo and Pongo abelii of Sumatra. There are also at least three subspecies on Borneo, and they’re thought to have shared a common ancestor around 176,000 years ago. Nowadays, these orangutans are separated by river and mountain barriers that are insurmountable.
There are about 60 orangutans at Camp Leakey right now, including eight reintroduced females, and all of them have reproduced. While most of the 90 or so animals that were released over 14 years were translocated from areas close to the camp on Central Kalimantan, some did come from much farther away.
A team led by Graham Banes from the Max Planck Institute for Evolutionary Anthropology wanted to know the extent to which orangutans from non-native, geographically and reproductively isolated lineages were reintroduced into the wild population. They looked at 44 years of data and conducted genetic analyses using DNA extracted from fecal samples.
They found that two reintroduced females – Rani and Siswoyo – were from a non-native subspecies, and they have since produced at least 22 hybridized descendants over multiple generations. These descendants, 15 of which are still alive, carry a “cocktail” of genes that wouldn’t normally occur in the wild: maternally inherited mitochondrial DNA specific to P. p. pygmaeus and Y chromosomes inherited from P. p. wurmbii fathers.
Under certain circumstances, the transfer of genetic material between distant individuals may increase the fitness and even survival of a population by augmenting genetic diversity. On the other hand, hybridization between genetically distinct lineages may have negative effects like outbreeding depression – the reduced fitness that happens when divergent populations mate. There’s also the risk of infertility, which is often seen in offspring that result from interbreeding.
The team found no evidence of outbreeding depression, but given the size and scale of their reintroduction program, Rani and Siswoyo, the authors write, may prove to be just the tip of the iceberg.