YOGYAKARTA, Indonesia — Rahayu Oktaviani revered the work of Canadian primatologist Biruté Galdikas while a student in the mid-2000s, aspiring to join her hero in protecting the Bornean orangutan, which would be classified several years later as a critically endangered species. In the end, a lack of research funding kept the Borneo forest canopy out of reach for Ayu, as she’s better known, upon graduation.

“So, I turned to the Javan gibbon,” Ayu told Mongabay Indonesia.

An hour west of the Bogor Institute of Agriculture (IPB), her alma mater and Indonesia’s top forestry university, Ayu was instead drawn by the distinctive call of the silvery gibbon from Gunung Halimun-Salak National Park, named after the twin active volcanoes at its heart.

“I heard that voice every time I went into the forest,” Ayu said.

Last month, Ayu won the prestigious Whitley Award, which recognizes achievements in grassroots conservation, to advance her work on protection of Java’s silvery gibbon.

Ayu follows past winners like Amanda Vincent, today a world expert on seahorses, who won the inaugural Whitley Award in 1994 to study seahorses in the Philippines.

“Drawing on a decade of research into gibbon behaviour and habitat needs, Ayu and her team at Kiara (Konservasi Ekosistem Alam Nusantara) will work with community members, park rangers and managers to build capacity, monitor biodiversity and guide conservation strategies toward a shared vision,” the U.K.-based Whitley Fund for Nature wrote in a release.

Moloch, stock and barrel

Indonesia is home to nine species of gibbon, from the jet-black Kloss’s gibbon (Hylobates klossii) on the Mentawai Islands off western Sumatra to the white-bearded gibbon (Hylobates albibarbis) in the rainforests of southern Borneo.

The Javan gibbon (Hylobates moloch), also known as the silvery gibbon, is a shade of gray between the two, found only in fragmented pockets of the world’s most-populous island, where less than 10% of the land area today remains forested.

“Usually taken as babies, and sold illegally at markets across Indonesia, gibbons are highly sought as pets and also face high demand from the world’s black market,” according to the Aspinall Foundation, a British charity that has managed gibbon conservation work in West Java province.

A fuzz of silver fur lightens at the Javan primate’s ruff above a ring gymnast’s frame, which enables the gibbon to muscle up trees and swing along the canopy. The gibbon eats a diet of fruit and flowers. Javanese folklore contends its voice means rain is coming.

“It turned out that not many people knew about them,” Ayu said.

Population models indicate up to half of the world’s Javan gibbon population lives in Halimun-Salak, a 400-square-kilometer (150-square-mile) national park south of Jakarta, where Ayu focuses the work of Kiara, the organization she founded in 2020 in Citalahab, a traditional, ethnic Sundanese community on the border of the forest.

The Whitley Award means Kiara receives 50,000 British pounds ($67,000) to advance conservation and social development initiatives in more communities around Halimun-Salak, which research shows is the crucial arena for ensuring the Javan gibbon’s survival.

Silver screens

In the 1990s, a population survey concluded only a few hundred silvery gibbons remained in the wild on Java, evidence that led the International Union for Conservation of Nature (IUCN), the global wildlife conservation authority, to elevate the species’ status to critically endangered.

However, fears of imminent extinction cooled a decade later after new work indicated a few thousand more Javan gibbons had likely yet to be counted.

Improved population surveys combined with new data showing large populations in Central Java province, particularly in the Dieng mountain range, supported a new consensus that a larger number of individual populations existed.

Following peer review of these findings, the IUCN downlisted the Javan gibbon to endangered in 2006.

“The change in status from Critically Endangered to Endangered does not suggest that the threats have decreased,” emphasized the authors of a 2017 population analysis. “In fact, threats continue to increase but not yet to the level necessary to reclassify Javan gibbons as Critically Endangered.”

The most recent IUCN assessment indicated around 4,000-4,500 individuals remained in the wild.

By comparison, around 14,000 Sumatran orangutans (Pongo abelii) and no more than 50,000-100,000 Bornean orangutans (Pongo pygmaeus) live on their respective home islands, according to the latest IUCN assessment.

However, rapid habitat fragmentation caused by aggressive deforestation and land fires has caused an appalling decline in orangutan populations across both Borneo and Sumatra.

Orangutans are naturally incapacitated against these unnatural threats to population equilibrium: Bornean orangutans have the slowest interbirth period among mammals, an interval of eight years between single offspring.

Current IUCN assumptions show that the Bornean orangutan population decreased by 86.2% from 1950 to 2025, mainly due to the destruction of their habitat.

By comparison, researchers say Javan gibbon populations have remained more stable. The gibbons also have a relatively slow reproductive cycle, with monogamous pairs producing only single offspring, and sexual maturity reached around age 8. But adult gibbons reproduce throughout their 30-year lives, with a shorter interbirth gap of around three years.

Silver medal

A potentially pivotal factor is the fact that up to half of the silvery gibbon population is believed to live in the hills of the Halimun-Salak region of West Java, both in and around the reserve where Ayu and her team will work on the ground. The genetic diversity here is known to be sufficiently wide to support population equilibrium.

Kiara has designed a women’s initiative, Ambu Halimun, which holds eco-print workshops and financial literacy training for local women in conjunction with raising awareness of the local environment.

The project aspires to generate a sustainable income stream for local women, who mostly work as casual laborers on local tea plantations and farm rice.

“This award is a platform to make the Javan gibbon better known,” Rahayu said.

Kiara will increase local community involvement in mapping and development of a local conservation action plan.

The Whitley Award will enable Kiara to expand education and outreach to schools and communities reaching at least 100 households (nonprofits elsewhere in Indonesia have found teaching children in schools is an effective way of reaching the entire household).

Kiara will also support national park officials with provision of new field data.

“The main mission now is to disseminate data from long-term research in the Citalahab forest to the community and area managers,” Ayu told Mongabay Indonesia. “Especially in the key areas bordering settlements with the Javan gibbon habitat.”

Kiara is also an arena for scientific research conducted on the silvery gibbon.

A new study co-authored by Ayu, “Developmental Changes in Feeding Behavior and Maternal Influences in Wild Javan Gibbons,” published in the International Journal of Primatology in April, showed how maternal and environmental factors influence immature gibbons as they become self-sufficient feeders.

“It’s the most remarkable thing that we get to see the development of their lives, learn about their social bonding and their interactions,” Ayu said. “To get to see the birth of a Javan gibbon baby in nature, which is pretty rare, that’s also extraordinary — that motivates our team to continue working.”

Silver lining

Prospects for survivability of the Javan gibbon beyond a century are limited without the kind of action implemented by Ayu and Kiara, research indicates.

If hunting and habitat loss persist, a high probability exists that the Javan gibbon will go extinct within 100 years, according to a 2017 population model by researchers from Indonesia, Australia and the U.K.

Using a 1% annual deforestation rate and “fairly low” levels of hunting as a baseline assumption, the model saw very high probabilities of extinction within 100 years in Central Java’s Dieng region, as well as in Halimun-Salak and Ujung Kulon national parks in western Java.

By increasing the carrying capacity of these three sites within the baseline model, chances of long-term survivability increase, but prospects of a good outcome remain remote in Dieng and Ujung Kulon, with a respective 85% and 98% probability of local extinction over the next 100 years.

“The population in Halimun-Salak remains relatively more stable with a 44% chance of becoming extinct in the next 100 years,” the researchers concluded.

However, reducing the threats of habitat loss and poaching for the illegal wildlife trade presents a brighter future for Java’s silvery gibbon, the research showed.

“If these threats are eliminated the model shows each of the populations are large enough to persist in the long-term whilst maintaining high levels of current genetic diversity,” according to the estimate.

Last month, almost two decades after dreaming as a student to follow iconic primatologists Galdikas, Dian Fossey and Jane Goodall, Ayu received a Whitley Award from the U.K.’s Princess Anne, patron of the Whitley Fund for Nature. The award recognize the importance of Ayu and her organization’s work fostering conservation of Java’s silvery gibbon.

Ayu follows Indonesian conservationists such as Hotlin Ompusunggu and Wendi Tamariska, among others, in winning the Whitley Award for working with the archipelago’s threatened primates.

“Javan gibbons live in the shadows of these more charismatic species,” Ayu said. “I felt it was important we get to know that Indonesia has this little primate.”

Banner image: The Javan gibbon (Hylobates moloch), also known as the silvery gibbon is found only in fragmented pockets of the world’s most-populous island. Image by A.Baihaqi via Wikimedia Commons (CC BY-SA 4.0).

This story was first published here in Indonesian on May 6, 2025.

Winners of 2025 Whitley Awards, the ‘Green Oscars,’ are announced

Citations:

Smith, J. H., King, T., Campbell, C., Cheyne, S. M., & Nijman, V. (2018). Modelling population viability of three independent Javan gibbon (Hylobates moloch) populations on Java, Indonesia. Folia Primatologica, 88(6), 507-522. doi:10.1159/000484559

Lee, S., Oktaviani, R., Yi, Y., Choi, A., Mardiastuti, A., & Choe, J. C. (2025). Developmental changes in feeding behavior and maternal influences in wild Javan gibbons (Hylobates moloch). International Journal of Primatology. doi:10.1007/s10764-025-00493-3