Malaysia’s white-handed gibbons may be two subspecies, not one, study shows

The article has been significantly amended in light of challenges to both the research itself and Mongabay’s coverage of the journal article.

If you get yourself a map of Peninsular Malaysia and color in all the areas in which white-handed gibbons (Hylobates lar) live, you’ll find a blank spot across southern Kedah and northern Perak.

Here, in this stretch of land between the Muda and Perak rivers, an entirely different species dominates: black-handed gibbons (Hylobates agilis). Their presence keeps out white-handed gibbons, which are otherwise scattered across the whole peninsula.

Recent research from scientists sequencing the genes of white-handed gibbons has raised questions, and controversy, about whether this geographic interruption could have resulted in two distinct white-handed gibbon populations evolving in isolation.

The researchers, whose study was published in ZooKeys, sequenced a section of mitochondrial DNA each from 12 captive white-handed gibbons from Peninsular Malaysia’s National Wildlife Rescue Centre in a routine genetic assessment conducted before animals are released back into the wild.

For scientists looking to translocate and reintroduce captive gibbons into the forest, knowing the finer details like which subspecies and population a particular animal originated from can help reduce interbreeding and ensure the different populations stay healthy in the long run, co-author Jeffrine Rovie-Ryan, a researcher at the University of Malaysia, Sarawak, told Mongabay.

“Separate populations should be managed separately,” Rovie-Ryan said. “If you mix animals from different populations, that will contaminate the gene pool, making the animals more susceptible to diseases.”

The researchers had expected a straightforward result identifying the captive gibbons as belonging to the Hylobates lar lar subspecies, but instead discovered unusual mutations in their DNA.

Comparing their data against sequences of H. l. lar obtained from the genetic database GenBank, they found the genetic distance between the two to be on par with the genetic distance between H. l. lar and other subspecies.

Such a result hints at the existence of a separately evolving population in Peninsular Malaysia — in this case, one so genetically different it could potentially qualify as a distinct subspecies, Rovie-Ryan said.

In their paper, the scientists suggested the genetic distance between the captive gibbons’ sequences and GenBank data might have been a result of two geographically isolated and independently evolving H. l. lar populations in the north and south of Peninsular Malaysia, separated centuries ago by the presence of black-handed gibbons (Hylobates agilis) between the Muda and Perak rivers.

In particular, Rovie-Ryan said, the GenBank sequences could have come from the northern population, while the captive gibbons could have been part of the southern population.

But Lim Teckwyn, a forest ecologist who was not involved in the study, has called the researchers’ tentative conclusion of northern and southern populations of H. l. lar separated by black-handed gibbons “pure speculation.”

“There’s no evidence at all that [the GenBank sequences] come from the north,” he said. “They’ve got no individual that was caught in the north [that’s of the subspecies H. l. lar]. If they could get one from the north and look at its DNA, they would be able to answer the question,” Lim said.

“The discovery of the presumed/potential northern population should be approached as something that will invoke future extensive research to either prove or disprove the findings,” Rovie-Ryan wrote in a response email.

Rovie-Ryan said that confirming whether the gibbons should be classified as separate populations, or even subspecies, based on the mutations in their DNA would require more samples from wild individuals with known locations instead of only captive ones.

“We … need to look at whether the populations mix with each other at all,” he said. “Is there a gradient, or are they totally separate? If there’s a gradient, it’s possible they’re not different subspecies, but [exhibiting] variations within a big population.”

The fragment of DNA the scientists sequenced, known as the complete control region, was chosen for being the fastest-evolving segment in mitochondrial DNA for white-handed gibbons.

“It’s just a short fragment, but it is very powerful,” Rovie-Ryan said, adding that if the researchers had chosen a slower-evolving segment, it might not have been enough to differentiate between different species and subspecies — let alone something as specific as different populations within the same subspecies.

Sequencing the complete control region to differentiate between populations isn’t a method that works for all species of animals, however. “We did a few studies on Malayan pangolins [Manis javanica], but we couldn’t find any significant differences [in complete control regions] between two different populations,” Rovie-Ryan said. “Luckily, when we used this segment in gibbons, we could see a differentiation.”

In cases where it doesn’t work, researchers can resort to whole genome sequencing, where they map out the entire DNA sequence (including mitochondrial DNA) of a cell to search for genetic differences — though that requires more computational power, and tends to be costlier.

Citations:

Gani, M., Rovie-Ryan, J. J., Sitam, F. T., Kulaimi, N. A. M., Zheng, C. C., Atiqah, A. N. … Mohammed, A. A. (2021) Taxonomic and genetic assessment of captive white-handed gibbons (Hylobates lar) in Peninsular Malaysia with implications towards conservation translocation and reintroduction programmes. ZooKeys, 1076, 25-41. doi:10.3897/zookeys.1076.73262

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