Smaller population estimate underscores urgency of saving Cao-vit gibbon

The Cao-vit gibbon is one of the most critically endangered apes in the world, with its entire population living in a single patch of protected forest on the border between northern Vietnam and southern China.

Until recently, population estimates for the species stood at 120. Now, a new study published in the journal Nature, calculates that just 74 Cao-vit gibbons (Nomascus nasutus), also known as eastern black-crested gibbons, remain.

Researchers say it’s unlikely there was a dramatic population drop; instead, they say they’ve been able to come up with a more accurate estimate by mapping the gibbons’ locations and tracking individuals by identifying the unique song of each male.

Gibbons are the only primate, beside humans, known to vocalize or sing. These vocalizations are a core part of gibbon life, often done to alert potential mates or to delineate territory.

Previous studies assessed population size by triangulating groups from their songs. However, this method left room for double counting; gibbon groups generally sing in the morning, and could be counted more than once if they moved locations during the course of several monitoring days. The new study recorded and analyzed individual male songs, called “vocal fingerprinting” by the researchers, to differentiate all singing males recorded at 29 listening posts during a two-week period. The study also incorporated passive acoustic monitoring and a drone-mounted thermal camera, which aided in identifying the region’s 11 family groups. Based on observations during the survey and long-term monitoring data, the researchers arrived at a final population estimate of 74 individuals.

Driven to the edge of extinction

Currently, 19 of the 20 species of gibbons across Asia are listed as endangered or critically endangered. Hunting, forest loss, and the 20^th-century human population boom are primary reasons for the decline in gibbon numbers.

“Human population increase has driven habitat conversion through deforestation, habitat, fragmentation of expanding agriculture, increased hunting, mining, any kind of extractive resources,” says primatologist Paul A. Garber, emeritus professor at the University of Illinois, who wasn’t involved in the study. “Populations come in more from remote areas and then they hunt, they bring in roads. Primates are captured as pets or used in a lot of traditional medicine. And certainly, climate change moving forward will have a critical impact on primate population.”

Study lead author Oliver Wearn, of Fauna & Flora International, says the new findings, combined with past research, give conservationists and policymakers a broader understanding of what it may take to keep Cao-vit gibbons alive.

“The good news for the species is that we know what we need to do,” Wearn says. “We urgently need to restore the forest habitat where the Cao-vit gibbon lives, so that the gibbons can find the fruit and leaves they need, as well as the tall trees they need for traveling, sleeping, and singing from. This will allow dispersing individuals to find new territory, breed, and expand the population.”

Gibbons are slow to reproduce, Garber notes, with most females having just one infant every two to three years. “If you think about it from an evolutionary and a genetic perspective, this is not very good news,” Garber says. “Even if this population can survive for another 50 years or 100 years, it’s likely to become very inbred.”

Garber also points out that gibbons rely on fruit trees for survival, which are also slow to mature and regenerate.

But Wearn says he remains hopeful.

“Already the forest is in a much better state than it was in the 2000s, when the two protected areas — in Vietnam and China — were set up for the species,” he says. “Back then, the forest had been heavily impacted by decades of unsustainable timber extraction and charcoal production.”

Wearn says tree planting and assisted natural regeneration — cutting back grass species and limiting domestic animal grazing — are ongoing. “We just need more of the same, but faster and over a much-expanded area.”

Next steps for survival

In the long term, Wearn says, the current protected habitat “can’t support more than 200 gibbons even in the best case.” The next step, he says, it to establish protect forest corridors to connect potential habitats. “We’re working to rebuild connectivity with a large area of forest, albeit fragmented, to the west of the currently occupied site. We also need to begin to assess the feasibility of translocations, something which has only rarely been done for gibbons and something which involves a lot of risks, but also tremendous possible gains in terms of recovering the species.”

Prioritizing such measures requires political will, which Wearn and Garber both say is present in Vietnam and China, as the countries continue to protect areas where Cao-vit gibbons live.

The alternative, Garber says, would irrevocably change the environment, and humans’ place within it. “The reality is nonhuman primates are really the canary in the coal mine. If they go extinct, as their environments are changed such that they can’t live there, then it won’t be too much after that, that humans won’t be able to live in these environments as well,” he says.

For Wearn, protecting gibbons speaks to larger moral questions as well. “Gibbons are our closest living relatives in Vietnam and China, and I think we lose a bit of ourselves when we lose these species,” he says. “Besides the key ecological role that gibbons play in forests, by dispersing seeds and helping the next generation of trees come through, they are also beautiful, quiet, and peaceful animals. They do not harm people or their livelihoods. If we can’t even protect gibbons, and put aside some forests in which they can roam, then what does that say about us as a species?”

Banner image: A Cao-vit gibbon in a tree. Image © Oliver Wearn/ Fauna & Flora International.

Drones improve counts of rare Cao-vit gibbon, identify conservation priorities

Citations:

Wearn, O. R., Trinh-Dinh, H., Ma, C.-Y., Le, Q. K., Nguyen, P., Hoang, T. V., … Nguyen, T. D. (2024). Vocal fingerprinting reveals a substantially smaller global population of the critically endangered Cao vit gibbon (Nomascus nasutus) than previously thought. Scientific Reports, 14(1). doi:10.1038/s41598-023-50838-2

Wearn, O. R., Trinh-Dinh, H., Le, Q. K., & Nguyen, T. D. (2023). UAV-assisted counts of group size facilitate accurate population surveys of the critically endangered Cao vit gibbon Nomascus nasutus. Oryx, 1-4. doi:10.1017/s0030605323000017

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