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Only 25 Hainan gibbons remain – what next for the world’s rarest primate?

Hello, I’m 4% of the global population. Jess Bryant, Author provided

Most of the world’s primate species are threatened with extinction, according to a disturbing new study. But the threat is more urgent and critical for some species than others.

Of all 504 primate species, the rarest and most vulnerable is found only on one tropical island at the southern tip of China. The majestic and enigmatic Hainan gibbon is confined to a single small patch of forest. Only around 25 remain.

Gibbons are small apes found throughout Southeast Asia. They live in forests, and are able to swing from tree to tree at incredible speeds. With all but one of the 19 recognised species officially listed as endangered or critically endangered their future is far from certain.

Not only is the Hainan gibbon (or Nomascus hainanus) the world’s rarest ape and rarest primate, it’s one of the rarest mammals of all. The entire species now consists of a single population of around 25 individuals, which separates into smaller social groups. The animals are restricted to just two square kilometres of remnant rainforest in Bawangling National Nature Reserve on Hainan Island in the South China Sea.

Sadly, numbers have plummeted over the past 50 years. As with so many other species, this precipitous decline is the result of decades of deforestation and an expanding human population which hunts the gibbons for food or poaches them for traditional medicines and the illegal pet trade.

The gibbons are ‘sexually dichromatic’ … males are black and females are yellowy-brown. Jessica Bryant, Author provided

Southeast Asian animals are particularly vulnerable to these so-called anthropogenic pressures. The deforestation rate is the highest among the world’s tropical regions and alarmingly it is predicted that up to 85% of the biodiversity is likely to disappear by 2100.

How to save the Hainan gibbon

To protect a very rare species on the brink of extinction, you first have to understand its genetic health. Has the inbreeding that inevitably occurs in a very small population caused any unwanted traits, such as weakness to a disease, to become more common?

My former student Jessica Bryant recently led an assessment of the genetic health of the Hainan gibbon. She gathered genetic samples of the current population from faeces and compared them to historical samples found in museums. Her work showed the Hainan gibbon has seen a major drop in genetic health, which can in turn lead to ill-health or fertility problems.

The study shows that genetic diversity has declined since the 19th century and even further within the last 30 years, representing declines of around 30% diversity from historical levels.

There is also evidence for a recent population bottleneck and an earlier bottleneck in the late 19th century when numbers were already pretty low. Such bottlenecks can occur when there is a dramatic reduction in the size of a population which leads to a reduction in the variation in the gene pool of that population, which in turn can affect the offspring of that population.

Those 25 gibbons are now closely related. Individuals will find even other social groups are filled with their half or full siblings, and inbreeding is likely to increase in the future – there simply aren’t enough of these gibbons for them to mate with anyone but a close relative.

This genetic study and other recent research by an international team from UCL, the Zoological Society of London, Bawangling National Nature Reserve and Kadoorie Conservation China, suggest that the long-term recovery of the Hainan gibbon will require intensive management and new strategies may have to be developed to ensure their survival.

One option is to relocate some individuals to new areas of Hainan Island. But this is risky: without buy-in from local people and the authorities, a new gibbon population would be threatened by hunting or deforestation – the same forces that caused their demise in the first place.

These fascinating apes are clearly capable of grabbing the public’s imagination. When an entirely new species, the Skywalker gibbon, was discovered recently, the announcement enjoyed international attention.

While the Hainan gibbon’s future may seem bleak, scientists across the world are working together to better understand the needs of extremely rare species. In the case of the Hainan gibbon, we just have to hope that this is enough to save them from being the first ape to go extinct.

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