As a genus, orangutans (Pongo sp) are the most endangered of all the great apes, with only an estimated 70,000 individuals, compared to roughly 100,000 gorillas (Gorilla sp), 275,000 chimpanzees (Pan sp), and almost 8,000,000,000 modern humans (Homo sapiens). To put this in perspective, orangutans currently represent less than 0.001% of all living Hominids and only 15% of all non-human Great Apes. Of the living orangutans, almost 80% are of the Bornean species (Pongo pygmaeus), with 65% of this species and 50% of all orangutans belonging to the central Bornean subspecies (P. p. wurmbii), and the rest of the Bornean species being split 6-1 between the north-eastern (P. p. morio) and north-western (P. p. pygmaeus) subspecies. The other 20% of orangutans are found in Sumatra and most of these (95%) belong to the north Sumatran species (P. abelii) rather than the recently described central Sumatran species (P. tapanuliensis) which boasts fewer than 800 individuals. All orangutan populations are threatened by poaching, illegal animal trading, and habitat destruction for the purpose of palm oil production. Conservation of habitat and legal enforcement against the poaching and selling of orangutans is increasingly necessary. Rehabilitation and reintroduction of injured and orphaned orangutans in their existing range is an established practice, but it would be prudent to consider the creation of new populations as well, in order to safeguard unique lineages against future threats and to re-establish their ecological function where it has disappeared.

           While today restricted to Borneo and northern Sumatra, orangutans were previously much more widespread. In addition to occupying the entirety of Sumatra, there was also an endemic species of orangutan in Java (†P. javensis) and a series of forms on the Asian mainland (†P. hooijeri, †P. weidenreichi, and most recently P. devosi), with fossils found in Cambodia, Laos, (peninsular) Malaysia, Thailand, Vietnam, and southern China. These taxa seem to have disappeared sometime between the Late Pleistocene and Early Holocene. Humans are known to have hunted orangutans in history and prehistory, and this is likely the reason for their collective retreat into denser, less accessible rainforests. Orangutans have demonstrated an ability to live in degraded and secondary forests that would suggest a broader habitat tolerance than they are typically given credit for. While the largest tracts of suitable habitat still remain in Sumatra and Borneo, there may very well be appropriate sites elsewhere as well.

           Probably the first experiments would be in central and southern Sumatra, where there are numerous large National Parks that might be suitable. Central Sumatran orangutans are currently not found within any National Parks, including the adjacent Batang Gadis National Park (from which there are historical records), while north Sumatran orangutans are only found in one, Gunung Leuser National Park. Re-introduction of either variety to existing National Parks would seem like a pragmatic step, and north Sumatran animals are already being used in a reintroduction project in Bukit Tigapuluh National Park. The central Sumatran species would likely be a priority for further reintroductions, but it would also be hard to source founding individuals from a small population that is already threatened by potential inbreeding. Unlike Bornean or north Sumatran orangutans, there is no captive population of central Sumatran orangutans to source from. Existing reintroduction projects source from animals seized from the pet trade, and the reintroduction of orangutans to Batang Gadis using Central Sumatran individuals sourced this way might be possible, but the translocation of a few dozen wild adults would be more pragmatic, so long as attention was given to genetic diversity. Relocation might become necessary if the Batang Toru hydroelectric dam is built, as this project will impact much of the remaining habitat for this species. If successful, similar projects could be implemented to reintroduce orangutans in other National Parks, such as the Kerinci Sablat National Park, which is a little under 300 km south of the Batang Gadis area and also within the historical distribution of orangutans. Kerinci Sablat, at almost 14,000 km2, is also the largest National Park in Sumatra. Orangutans are missing from only one National Park in Borneo, Kayan Mentarang, which is also the largest at almost 14,000 km2. The orangutan’s absence here is likely due to the high density of hunter-gatherer peoples in the area, who are thought to have hunted orangutans to local extinction in prehistory. Future reintroductions might be possible if these activities are no longer practiced, but would obviously be undesirable if they are.

           In addition to the repatriation of orangutans within Sumatra and Borneo, it should be established whether there are suitable sites outside of these two islands, within the genus’ Pleistocene distribution. Using a very conservative orangutan population density of about 1/5 km2, at least 1,000 km2 would be necessary to sustain the minimum population of about 200 needed to maintain genetic diversity, and larger protected areas would be ideal. Java has fewer large, protected areas than either Borneo or Sumatra, and the only sufficiently large one with rainforest is Ujung Kulon, at about 1,200 km2. Peninsular Malaysia might be more practical, with at least one very large National Park, i.e. Taman Negara, at about 4,300 km2. North of Malaysia, sufficiently sized rainforests might be found in several protected areas of Cambodia, Laos, Thailand, and Vietnam. However, thorough habitat suitability evaluation would be necessary, as the local conditions and food species in these countries would differ more significantly from Borneo/Sumatra than would those in Java or Malaysia.

           Orangutans are primarily frugivorous, though different populations may intake differing proportions of other foods depending on the availability of fruits. As such, the availability of important fruit species in potential release sites is a vital indicator for the suitability of said location. Favoured species include paldao (Dracontomelon dao), kundong (Garcinia parvifolia), meranti (Shorea leprosula), durian (Durio kutejensis), ampas (Gironniera nervosa), kempas (Koompassia malaccensis), and terap (Artocarpus elasticus). Figs (Ficus sp) are also very important, especially when other masting species are not in season. These and many other fruiting species are eaten by orangutans, and these genera are found across Southeast Asia. Orangutans will also supplement their diets with leaves, bark, insects, eggs, and even meat (usually small primates like lorises (Nycticebus)). An obstacle to reintroduction may be that an orangutan’s foraging success is dependent on its learned knowledge of the region it lives in, typically taught to it by its mother. As such, projects in areas not currently occupied by orangutans will be best undertaken in areas with high food availability, allowing the apes some leeway to branch out and discover their surrounding without a high risk of starvation. For projects using orphaned individuals, a soft-release period where the animals are taught where to find food and how to access it would be advisable. Tool use is an important part of orangutan culture as well, with nest-building and termite-fishing being vital skills. Predator avoidance is a necessary behaviour as well, though predation on orangutans by non-humans is a rare occurrence. Clouded leopards (Neofelis), being the only sufficiently large arboreal predator, will occasionally hunt orangutans, but most attempts are unsuccessful. Orangutans tend to be food-limited more than anything else, though many of them die of old age or disease as well. The extinction of tigers (Panthera tigris) in Borneo seems to have allowed Bornean orangutans to engage in more terrestrial behaviours than their Sumatran relatives, and it is worth noting that while tigers are no longer present in Java either, they would be present in potential reintroduction sites on the mainland.

           As such, while central Sumatran orangutans are likely the ideal candidate for reintroduction to the remainder of central/southern Sumatra (and possibly Java?), and Bornean orangutans would be the obvious candidates for repatriation elsewhere in Borneo, it could be argued that north Sumatran orangutans would be appropriate for reintroduction projects in Malaysia and elsewhere on the mainland, in part due to their recent history with tigers as potential predators. Other reasons include the priority of other taxa elsewhere, their availability from captivity where they breed quite easily (though wild-born individuals would be preferable), and their morphological similarity to remains from the mainland. Central Sumatran orangutans are known to be genetically closer to Bornean orangutans than to those from North Sumatra, and geographical proximity might suggest that the north Sumatrans were more like those from Malaysia or Thailand. During glacial periods, Borneo, Java, and Sumatra were all connected to the mainland due to lower sea levels, which is how orangutans were able to colonize the islands. As such, orangutans might quite recently have been able to travel between Malaysia and North Sumatra. Sumatran and Bornean orangutans are known to be capable of hybridization in captivity, though this practice has been halted since their status as separate species was established. There is also concern that Bornean orangutans released into the wild may have originated from subspecies not native to the area in question, creating concerns about outbreeding depression. This is likely an imaginary issue, however, as the amount of genetic influence of these animals on the population as a whole would be minimal, there are no significant differences in evolutionary pressures on the different subspecies, and they likely hybridized naturally in the past where their ranges met. If anything, it might be beneficial to introduce new blood to populations that might otherwise start to experience inbreeding depression from habitat fragmentation.

           The drastic increase in habitat loss and resulting human-wildlife conflict forces us to consider extreme solutions to prevent species extinction and maintain ecosystem functionality. Repatriation of threatened and/or keystone species to areas within the prehistoric distributions is one of the most effective, if controversial, ways of doing so. Orangutans are important seed-dispersers and disturbance agents where they survive, and the same was inevitably true for their ancient relatives. Their reintroduction outside their current range would aid in the proliferation of fruiting plants across Southeast Asia, which would, in turn, benefit other species that eat these fruits, but which are not large enough to disperse them. There is also an ethical obligation to safeguard the future of this species, not only for the usual reason that their disappearance has been anthropogenic but also because they are intelligent, self-aware, sapient creatures that are among modern human's closest living relatives. Many countries, quite rightly, consider Great Apes to be legal persons, with the rights to safety and protection from exploitation that come with that. Humans have (in all likelihood) already been responsible for the extinction of at least two orangutan species, in addition to 4 members of our own genus, Homo. Earlier members, such as Homo erectus, may have been the agents behind the disappearance of the orangutan’s relative †Gigantopithecus blacki, a terrestrial ape from South Asia that was the largest primate to ever live. The last of the Australopithecines, †Paranthropus robustus, may have met a similar fate. The remaining Great Apes are increasingly rare, and will only become more so without significant intervention. While the conservation and restoration of existing habitat is certainly important, it is always wise not to have all of one’s eggs in one basket. Orangutans should be present in all protected areas that can support them, even if they have not been present there for centuries, millennia, or even tens of millennia. As controversial as this might be to some, it should not be anymore so than the ethically questionable captivity of apes in zoos, which serves the same purpose. In fact, increasing the natural distribution of orangutans might gradually lessen the need to keep them in captivity as an insurance population. If successful, similar projects might be kickstarted to investigate the potential of creating new populations of chimpanzees or gorillas. Neither species has the benefit of a quality fossil record, as with orangutans and humans, but there are certainly areas in Africa that could support these apes where they do not currently occur. The potential to create additional populations of chimpanzees in savannah habitats, similar to those in Fongoli, Senegal, would also be interesting to investigate. Experimental restoration/translocation is going to be more and more important in the coming decades, and there is no reason to think this should not apply to our fellow Hominids as much as it will for anything else.