Rare Himalayan Macaques Confirmed as Stable Residents of Xizang’s Forests

High in the forests of the eastern Himalayas, two of the world’s most recently discovered primates have finally been confirmed as genuine residents of China’s Xizang Autonomous Region. A systematic field survey conducted between April and September 2025 has documented stable, reproducing populations of both the Arunachal macaque (Macaca munzala) and the white-cheeked macaque (Macaca leucogenys), resolving a two-decade-old question about whether these elusive species truly persist within Chinese territory. The findings, published in the Biodiversity Data Journal, provide the first conclusive distribution records for the two macaques in Xizang and establish a scientific baseline for conservation planning in one of the planet’s most biodiverse mountain regions.

The Arunachal macaque was first described as a new species in 2005, distinguished by a pale yellow crown patch, centrally whorled black hair and dark stripes above the eyes. The white-cheeked macaque followed a decade later, formally named in 2015 after researchers captured photographs through direct observations and camera traps in Medog County, revealing its signature greyish-white to white pelage on the cheeks and chin. Both species were described from the eastern Himalayas, and their type localities lie within China. Yet despite these formal descriptions, the original literature offered only suggestions that stable, self-sustaining populations occurred in Xizang, and conclusive field evidence remained stubbornly absent.

This uncertainty had real consequences. Without reliable distribution data, IUCN Red List assessments lacked a solid foundation, and habitat conservation planning, transboundary population management and community-based conservation measures all proceeded without a scientific spatial framework. The new survey set out to change that by applying a rigorous operational definition of

The survey’s design reflects the particular challenges of studying primates in remote mountain terrain. Between April and September 2025, researchers established 120 camera-trap stations across Nyingchi City and the southern valley zone of Cona County in Shannan City, an area spanning roughly 91°57′ to 98°47′ east and 26°52′ to 30°40′ north. Each station operated for an average of 150 days after accounting for equipment malfunctions and battery or memory-card failures, producing a cumulative sampling effort of 18,000 effective camera-days. The cameras were mounted on tree trunks at heights of 1.5 to 2.0 metres, a placement chosen to reduce false triggers from low shrubs, splashed mud, water and passing small animals, while still capturing activity in the lower arboreal layer where macaques frequently move. Stations were spaced at least 500 metres apart to ensure statistical independence of records, and detections of the same species at the same station were counted as separate events only when separated by at least 30 minutes, a threshold drawn from widely used camera-trap data standards.

Site selection followed three explicit criteria designed to maximise detection probability while minimising bias. Sampling points covered the valley zones with the highest frequency of historical records of the target species, prioritised areas with high forest-landscape connectivity where the patch fragmentation index remained below 0.12, and deliberately included potential dispersal corridors formed by the lower Yarlung Zangbo River valley and southern low-mountain passages. This last criterion is particularly significant for conservation planning, because dispersal corridors determine whether populations on either side of a political boundary can be managed as a single connected unit. The study area sits at the tectonic bend near the eastern end of the Himalayas, where deep incision by the Yarlung Zangbo River has carved a massive canyon, creating extreme vertical relief and a complete zonation from alpine snow and ice down to tropical monsoon forest at lower elevations.

The ecological setting helps explain why these macaques remained so poorly documented. The region receives 800 to 1,400 millimetres of annual precipitation, concentrated between May and September under the influence of the southwest monsoon from the Indian Ocean. Mean annual temperature reaches 15.8°C in the valleys at 1,500 metres elevation but drops sharply on the ridges at 3,800 metres, where January means fall to −2.1°C. The lower and middle montane belts support monsoon evergreen broad-leaved forest dominated by Castanopsis hystrix, Machilus nanmu and Altingia chinensis, while higher belts transition into mixed coniferous–broad-leaved forest dominated by Tsuga chinensis, Picea likiangensis and Cyclobalanopsis glauca, with dense understorey thickets of Fargesia bamboo and Rhododendron. In primary forest, canopy closure exceeds 0.85, conditions that both shelter macaque troops and frustrate observers relying on visual detection alone.

The survey’s timing was strategically chosen as well as logistically convenient. April to September coincides with the post-parturition lactation and infant-following stages of Macaca species, when infants are readily identifiable and reproductive parameters can be recorded with greater confidence. Rising temperatures, abundant food, frequent macaque activity and good transect accessibility during these months all improve survey efficiency. This seasonal window proved decisive for the study’s central conclusion: the presence of infants among recorded troops of both species provided direct evidence of normal reproductive activity, which is the key criterion for classifying a population as stable rather than merely transient. Under the study’s operational definition, a stable distribution requires continuous or regular records of a species within a specific area together with populations capable of carrying out essential life activities such as reproduction and foraging.

The demographic data collected offer a preliminary portrait of social structure in both species. Arunachal macaque troops averaged approximately 14 individuals, while white-cheeked macaque troops averaged around 17. Troops of both species contained adults, subadults and infants, an age structure consistent with actively reproducing populations rather than senescing or sink populations dependent on immigration. These troop sizes fall within the range typical of macaque social groups generally, though multi-male multi-female grouping in macaques can vary considerably with habitat productivity and predation pressure. The elevational separation between the two species is also noteworthy: the Arunachal macaque troops were recorded in Himalayan fir forests and forest-edge habitats at 2,000 to 3,000 metres in Cona City, Shannan City, whereas the white-cheeked macaque occupied a broader span from 1,395 to 2,700 metres in Gedang Township and Damu Township of Medog County, ranging from evergreen broad-leaved forest into mixed coniferous–broad-leaved forest.

This elevational partitioning may reflect competitive exclusion, physiological tolerance limits, or simply the historical contingencies of colonisation, and disentangling these possibilities will require further study. What the data do establish is that the two species occupy largely distinct habitat bands within the same drainage system, which has implications for how habitat protection should be zoned. Protecting mid-elevation broad-leaved and mixed forest in Medog County would primarily serve the white-cheeked macaque, while conserving higher-elevation fir forest and forest edges in Cona County would be essential for the Arunachal macaque. Because both species’ ranges extend across the Himalayan frontier — the Arunachal macaque’s distribution is confirmed to reach into Bhutan — transboundary coordination will be necessary for any population-level management.

The methodological combination of camera trapping with walking transects addresses a known weakness of either approach used alone. Camera traps operate continuously around the clock and are insensitive to observer fatigue, but their effective detection radius at each station was only about 3 metres, and they systematically miss activity in the higher canopy. Transect surveys, conversely, cover broader visual fields and can detect arboreal activity that cameras miss, but they depend on favourable weather, daylight and observer skill. By running both methods concurrently across the same stratified sampling grid — with one fixed monitoring transect established at each 500-metre interval along the elevational gradient and sampling points georeferenced with hand-held GPS units accurate to within 5 metres — the researchers could cross-validate detections and reduce the risk that either method’s blind spots would produce false absences.

The broader significance of this work lies in the context of global primate decline. More than 60% of primate species worldwide are threatened by habitat loss, climate change and human activities, and accurate assessments of populations and distributions are a prerequisite for developing effective conservation strategies. Xizang’s enormous elevational range has produced a complete vertical vegetation zonation from subtropical montane forest to alpine meadow, supporting a diverse primate community that includes the black-and-white snub-nosed monkey, Tibetan macaque, Xizang rhesus macaque, Himalayan grey langur, Assamese macaque and Shortridge’s langur, the last of which was first recorded in the region only in 2025. These species differ markedly in ecological requirements and conservation status, meaning that habitat protections designed for one may not serve another. Baseline distribution data of the kind this survey provides are therefore the foundation upon which all subsequent spatial planning must rest.

For the two macaques at the centre of the study, the confirmation of stable reproducing populations removes the principal obstacle to formal threat assessment. IUCN Red List evaluations require evidence of extent of occurrence, area of occupancy, population size and trend — none of which could be credibly estimated while the species’ very presence in Xizang remained disputed. With 11 troops of Arunachal macaques and 10 troops of white-cheeked macaques now documented with precise localities, future assessments can proceed from an evidentiary footing rather than speculation. The researchers frame these results explicitly as baseline data for subsequent conservation assessments and management decisions, and the 2025 survey season provides a temporal benchmark against which future monitoring can measure change. Whether populations are expanding, contracting or shifting upslope in response to climate warming will only become detectable through repeated sampling using comparable methods, making the standardisation of this survey’s protocol — its camera-trap spacing, record definitions and transect intervals — as consequential for long-term monitoring as the distributional findings themselves.

Subject of Research: Distribution and population structure of the Arunachal macaque and white-cheeked macaque in Xizang, China

Article Title: The distribution and population structure of two macaque species in Xizang, China

Article References: The distribution and population structure of two macaque species in Xizang, China. (n.d.). https://doi.org/10.3897/BDJ.14.e199726

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e199726

Keywords: Arunachal macaque, white-cheeked macaque, Xizang, primates, camera traps, line-transect surveys, Himalayas, conservation, population structure, habitat, Medog County, biodiversity