The northern buffed-cheeked gibbon, Nomascus annamensis, was formally described in 2010 by Thinh and colleagues in the Vietnamese Journal of Primatology, on the basis of animals from the central Annamite range. Males are jet black with a pronounced crest and a splash of golden-orange on each cheek; females are a soft orange-beige with a paler chest, and lack the crest entirely. The species ranges across Vietnam, Laos and Cambodia, is listed as Endangered on the IUCN Red List, and appears on Appendix I of CITES, the strictest protection the treaty offers. It is a strange thing to consider: an ape loud enough to be surveyed by ear across an entire national park, and yet unrecognised as its own species until well into this century.

The 2019 work used a technique called acoustic spatial capture-recapture, which treats each identified singing group the way a field biologist would treat a tagged animal - located, logged, and re-encountered. The resulting density came out at 0.437 calling groups per square kilometre. Set that against the neighbouring Virachey National Park, where a comparable estimate ran to something like 5,750 groups, and the two protected areas together read as the global stronghold for the species. The gibbons share the canopy with red-shanked doucs, silvered langurs, macaques and pygmy slow lorises. It is one of the richest primate assemblages left in mainland Southeast Asia, and it is concentrated in forest that is still being cut.

The park was created on 9 May 2016 by Sub-decree No. 77, covering 57,469 hectares - about 575 square kilometres - straddling Ratanakiri and Stung Treng provinces. In July 2023, Sub-decree No. 183 expanded it to 280,359 hectares, close to 2,804 square kilometres, as part of a wider reclassification of biodiversity corridors into protected areas. On a map that is a spectacular gain, nearly five times the original. On the ground it is more complicated, because the new boundary was drawn over land that already had other people's claims on it: existing farms, and land concessions including more than 5,000 hectares of an economic land concession being used for large-scale logging. A line on a map does not by itself stop a chainsaw.

Illegal selective logging is widespread here. The same 2019 fieldwork that counted gibbons also recorded high-value trees being felled and chainsaw noise at multiple sites. Interviews in surrounding villages have documented hunting pressure on primates - for traditional medicine, and for the pet trade, which typically means killing a mother to take an infant. Forest clearing for agriculture, much of it by people who live at the park's edge and have few other options, drives deforestation in parts of the park too, and no honest account of the pressure here can leave that out. The response has been partly local: community wardens now conduct joint patrols alongside government rangers. It has also, at times, been costly. In 2024 an investigation into illegal deforestation near an economic land concession inside the park led to environmental activists being temporarily detained - the people documenting the clearing, rather than the people doing it.

The wager now being placed on Veun Sai-Siem Pang is that the gibbons are worth more singing than the timber is worth felled. A community-based ecotourism project run by Conservation International Cambodia with the TUI Care Foundation and the IUCN aims to strengthen ranger patrols while building village livelihoods that depend on the forest staying up. The climate sets the calendar for any of it: rain climbs sharply from May, stays heavy through October, and peaks in July and August, while January and February are the dry months when the tracks are passable and the listening is good. Whether the wager pays out will be measured the same way the gibbons were - by whether, in ten years, there is still something to hear at dawn.