The World from PRX

A gibbon with golden fur and a black face, resting its hand on its cheek while looking contemplatively to the side.

Survey of gibbons in Cambodia show signs of forest health

Scientists in Cambodia are using audio recordings to track endangered gibbons in the Central Cardamom Mountains. The World’s Host Carolyn Beeler speaks with scientist Ratha Sor about why the presence of these primates are an indicator of ecosystem health.

Environment
Updated:
HCHannah Chanatry

This Aug. 29, 2008 file photo shows a yellow-cheeked crested gibbon sitting in a cage at Cambodia’s Phnom Tamau Zoo in Takeo province, south of Phnom Penh, Cambodia.

In the forests of Cambodia’s central Cardamom Mountains, conservationists are listening for a very particular sound: the call-and-response song of endangered gibbons recorded by researchers who track their population in those mountains.

To go deeper into the topic, The World spoke to Ratha Sor. He is a biodiversity manager from Cambodia with Conservation International, and a research leader in the central Cardamom Mountains tracking endangered gibbons.

Carolyn Beeler: Ratha, I had to look up pictures of gibbons to remind myself
which primate they are. Can you refresh our memories about these creatures that you’re studying?
Ratha Sor: They are native to South and Southeast Asia, including Cambodia. And this kind of pileated gibbon, they almost live entirely on the forest canopy and they have very complex vocalizations. They are characterized by long arms. That specialized anatomy allows them to swing from one to another tree with remarkable speed.
A mother gibbon sits on grass while holding onto her baby, who is reaching out with one arm.
Nyami, a pileated gibbon, holds her baby E.T., a four-month-old and one of four recently born animals, at the Attica Zoological Park near Athens, Greece, Oct. 24, 2025. Thanassis Stavrakis/AP Photo
They live up in the canopy and swing among the trees. Are they the white furry ones?
The female one is a bit like a white-gray and the adult one for a male is a bit dark gray.
So, they are endangered. How many are thought to be left globally?
For Cambodia, the central Cardamom is harboring the largest population of the pileated gibbon. But [in] the other part of Cambodia in the central plain, in another wildlife sanctuary, they are estimated to be about 12,000 individuals. But their habitat and all degraded habitats really threaten their population.
So, they’re endangered because their habitat is being deforested or broken up by human development? Is that what you mean by degraded?
Yeah, in Southeast Asia and South Asia, [there is] a lot of development, infrastructure development and mining, illegal logging. So, illegal logging is one of the most driving factors that leads to the habitat loss. But this is one of [the reasons why] we really want to do the estimation of the  pileated gibbon [population] at our site. Because it is the last stronghold … of this pileated gibbon species.
Aerial view of a rainforest with a section of deforested land, where trees have been cut down and smoke is visible, surrounded by dense green foliage.
Illegal logging in the Cardamom Mountains, Koh Kong Province, Cambodia, on June 16, 2007.Paul Mason/ USAID/ File Photo
Can you take us now to the central Cardamom Mountains and set the scene for us? What does it look like there?
The Central Cardamom Mountains are located in southwestern Cambodia, and this habitat is characterized by a very diverse ecosystem, including evergreen and semi-arid forests, monsoon woodlands, mangroves and many, many river systems that drain to the Gulf of Thailand.
So, you set up microphones and recorders all over these mountains to record the gibbons. What did you discover?
We got more than 870 calls across the landscape. So, we deployed our acoustic meters at 40 locations. So, that really, really surprised me. We expect only like 10 or [fewer] than 10 calls at each location. So, with this survey, we just deployed in the forest without disturbing or [at least we did] very minimal [things] that [could] disturb the forest. And that is very good in particular with the animals that use the vocalization to communicate.
And so, does that tell you anything about the health of the population or the ecosystem in the Cardamom Mountains? What was your takeaway from this research?
The takeaway is that the sound of the pileated gibbon in this area is a sound of hope. Because where we find the  pileated gibbon, it is an indication of the forest health. It means there is very limited illegal logging or invasion by humans. So, when we hear the sound of our pileated gibbon across time, it helps us to take a message that the forest here is stable and it is really a strong hope for the community.
Aerial view of a winding river cutting through dense green jungle landscape.
An aerial shot of the Cardamom Rainforest that Wildlife Alliance rangers actively protect 1.4 million hectares of, on May 8, 2020.Andrew Maurice Ball/ Wikimedia Commons (CC BY-SA 4.0)
I’m curious about the content of the calls themselves. When there are two gibbons there, communicating, can you tell me what they’re saying to each other?
Yeah. The adult, pileated gibbons, they’re dueting. It means that they are communicating between the male and female.
What are they saying to each other? This is like a mating pair?
Yeah, they are a mating pair, they’re mating. Actually, in terms of what activity or what message they’re doing, there could be [a] possibility that they are calling for mating, so one possibility. And another one is the male, they make a very loud and long call, so it’s kind of a sign that this is their territory, something like that.
Ratha, you, yourself, are from Cambodia. I wonder if it was nice to learn that this species is doing well in your home country.
Yeah, yeah. Actually, I really loved when we went out to the forest and even if we’re staying in the camp, the area that we did not deploy the audiometers, we also heard the call of the gibbon and that really gave me joy and I understand that they’re doing great in our landscape.
I wanted to play one more piece of sound that you recorded before we let you go and ask you if you can tell us anything about what is happening here. Can you tell us what’s happening here?
Yeah, they are dueting. So like, one is calling and another one is replying, “Oh, I’m here.” “Hey, where are you?” “I’m here.” “Where are you?” “I’m here.” Yeah, this kind of call, they try to call, ‘Where is the family.'” So, if a smaller one is far away, they’re trying to call, as well.

Parts of this interview have been lightly edited for length and clarity.