Rejuvenating a population by changing its genetic identity
Writer: Vikram Malhi
Editor: Audrey Brabant
Artist: Lucie Gourmet
Gibbons seem to be the one group of apes Hollywood has forgotten ‒ and as the fastest primate in the trees, God knows why. With their arms almost the length of their bodies and their wrist joint resembling that of a shoulder’s, it is no mystery how they perform their incredible acrobatics.
From the 20 species of gibbon identified, 19 have been classed as either endangered or critically endangered by the International Union for Conservation of Nature (IUCN). However, none are in as much trouble as the Hainan gibbon (Nomascus hainanus), which inhabits the island of Hainan off the coast of China. Within this island, which is half the size of Scotland, gibbons are constrained to the 85 km sq. nature reserve of Bawangling. With a human population of 9 million, and more than 6 times as many tourists each year, space is limited and deforestation threatens to remove what little natural environment is left. Human development has decimated the primary rainforests that the Hainan gibbon so desperately depends on, driving the population to its lowest point of less than 10 individuals in the 1970s.
Thankfully, numbers have been increasing since then, with a new pair found in 2019. Despite this sliver of good news, the health of the population, currently below 40 individuals, is very much in peril, with inbreeding posing a serious concern. To tackle this problem, hybridisation is being considered as a last resort.
Hybridisation is the reproduction between two different species. It isn’t common due to multiple physical and genetic barriers. For example, if the parent species have different numbers of chromosomes, the offspring will have an odd number of chromosomes and may not be able to form gametes. This infertility prevents them from persisting in the population. However, most hybrids are aborted in development as having genes in different locations on the chromosomes prevents the necessary proteins from forming. In some animal species, the females have mechanisms in place to abort a hybrid foetus or reject the sperm from a male of another species.
Hybrids are often produced by private zoos in an attempt to make their wonderland more ‘exotic’. Most notably, ligers and tigons (lion and tiger hybrids) were featured in the Netflix show Tiger King.
However, the hybridisation of species in the wild does also occur, especially in plants and amongst closely related species of animals. When the geographic distribution of the hybridising species overlaps, a new species can form, such as the golden-crowned manakin, which emerged from a cross between the snow-capped and opal-crowned manakins.
Nonetheless, hybridisation outside of captivity is normally met with apprehension, as it often involves invasive species. Breeding with invasive species can damage the genetic integrity and identity of the native species by introducing physical features into the offspring that decrease their overall fitness. For example, native trout populations produced fewer eggs when they started breeding with non-native rainbow trout. There is also evidence to show that some hybrids can outcompete the native species, which would have a cascading effect on other species within the ecosystem.
Hybridisation between species of gibbon that overlap in geographic distribution has been observed several times in the wild. Gibbon phylogeny is constantly being revised, partly because genetic variation is exceptionally high within species and notably low between species. As such, successful hybridisation occurs at a more frequent rate than expected.
Unfortunately, the island of Hainan separated from what is now continental Vietnam 250 million years ago, allowing the Hainan gibbons to evolve in isolation. Therefore, the lack of genetic mixing with other gibbons may have led its DNA to diverge to the extent that hybridisation with other species of the same genus is no longer viable. There is a high risk that hybrid offspring will have lower fertility than their parent species, or be completely infertile.
Yet, there is hope. Hainan hybrids have been known to occur in captivity. The most morphologically similar species to the Hainan gibbon, and therefore the most appropriate for hybridisation, are the western crested and cao vit varieties (both of which are critically endangered). Advances in molecular genetics are making it increasingly possible to test the genetic compatibility of species before pairing them. For example, the similarity in the sequence of the cytochrome B gene between the parent species may serve as a predictor of offspring fertility. UCL’s Professor Helen Chatterjee advocates implementing fertility tests and researching the fertility of naturally existing gibbon hybrids before jeopardizing the reproductive potential of the handful of individuals still existing.
A major question is, could the new population still be considered Nomascus hainanus, and more importantly, does it matter? One could argue that ensuring the survival of part of the identity of a dying species through the creation of a hybrid population is better than its complete disappearance. This initiative aligns with the current fight against the sixth mass extinction. Whilst the taxa Nomascus hainanus would become less abundant, there would be more gibbons. Gibbons are essential to their ecosystems as seed dispersers, so trees depend on them for survival.
Beyond the possibility of hybridisation, the security of this population in the future is still very much in question. Currently, strategies such as continuous monitoring and supplementary feeding are in place to protect the Hainan gibbon population. To help gibbons across Southeast Asia more generally, deforestation – primarily driven by palm oil production – needs to be arrested. For environment-conscious consumers, the IUCN recommends buying products stamped with RSPO and FSC logos, which indicate sustainable palm oil and timber practices respectively.
As the threats to biodiversity become ever more apparent, it is imperative that we take conservation seriously and explore new methods of saving what wildlife remains. Even if hybridisation turns out not to be feasible for the Hainan gibbon, new genetic techniques hold promise for resolving some of the many profound problems in conservation.