A total of 1,002 independent records of at least 46 species, including 21 mammals, 24 birds and one reptile, were documented in our survey. For mammals, 19 taxa were identified to species level and two taxa were only identifiable to genus level (Table 1). Amongst the detected families, Phasianidae had the highest species richness (six species). The survey revealed the presence of three macaque species of the family Cercopithecidae, including the stump-tailed macaque (Macaca arctoides), the Assamese macaque (Macaca assamensis McClelland, 1840) and the rhesus macaque (Macaca mulatta Zimmermann, 1780). The species with the highest occurrence records were the ferret badger (Melogale I.Geoffroy Saint-Hilaire, 1831 spp.), the yellow-throated marten (Martes flavigula Boddaert, 1785) and the Asian palm civet (Paradoxurus hermaphroditus Pallas, 1777), with naïve occupancy of 0.765, 0.471 and 0.5, respectively (Table 1). Additionally, we recorded several rare species with high conservation value, including the Critically Endangered Sunda pangolin (Manis javanica Desmarest, 1822) (Challender et al. 2019), the Vulnerable mainland serow (Capricornis milneedwardsii David, 1869) and the Vulnerable stump-tailed macaque (Macaca arctoides) (Fig. 2).

Figure 2.  

Examples of records from our survey at Xuan Lien Nature Reserve, Thanh Hoa Province, Vietnam. From top left, clockwise: the Sunda pangolin (Manis javanica), the stump-tailed macaque (Macaca arctoides), the northern red muntjac (Muntiacus vaginalis), the large Indian civet (Viverra zibetha Linnaeus, 1758), the grey peacock-pheasant (Polyplectron bicalcaratum Linnaeus, 1758), the silver pheasant (Lophura nycthemera Linnaeus, 1758), the Asian water monitor (Varanus salvator Laurenti, 1768) and the Chinese serow (Capricornis milneedwardsii).

Interestingly, we documented a number of various small carnivore species, including the large Indian civet (Viverra zibetha Linnaeus, 1758), the masked palm civet (Paguma larvata C.E.H. Smith, 1827), the spotted linsang (Prionodon pardicolor Hodgson, 1841), the Asian palm civet (Paradoxurus hermaphroditus), the crab-eating mongoose (Herpestes urva Hodgson, 1836), the leopard cat (Prionailurus bengalensis Kerr, 1792) and the yellow-throated marten (Martes flavigula) (Table 1). Their relative commonness in Xuan Lien Nature Reserve highlights the Reserve's high diversity of small carnivores, especially civets. In contrast, we only obtained a single record of the northern red muntjac (Muntiacus vaginalis Boddaert, 1785) and we did not record the Roosevelt’s muntjac (Muntiacus rooseveltorum) during the survey. Furthermore, four species that had IUCN conservation status of near threatened and above recorded in our survey included the stump-tailed macaque (Macaca arctoides), the Assam macaque (Macaca assamensis), the Chinese serow (Capricornis milneedwardsii) and the Sunda pangolin (Manis javanica). All records of those species came from 14 camera stations in mostly undisturbed habitats of primary evergreen forests, with little to no sign of human activities nearby. The smallest distance from any of those sites to the nearest main roads or human settlements was at least 2.5 km. Four stations that documented the majority of the records (60.4%) for those species were all near or on the ridgeline of the mountain range. Hence, it is likely that remoteness and terrain difficulty play an important role in protecting the remaining threatened taxa in Xuan Lien.

The most recent field survey on mammal diversity for Xuan Lien Nature Reserve was published in 2013 (Phuong et al. 2013) using a combination of transect survey, night survey and traps. The study provided a list of 80 mammal species in Xuan Lien. A subsequent study using molecular approaches confirmed an additional mammal species, the small-toothed palm civet (Arctogalidia trivirgata) (Giang et al. 2016). Hence, the number of documented mammal species from our survey accounts for approximately 31% of the previously reported mammal species in Xuan Lien. Our survey did not document any new species, but validated previous mammal records via camera trap image data for the first time. Additionally, camera trap stations that record the greatest number of mammals were deployed in the south-western part of the Reserve, which is also the most remote region in Xuan Lien (Fig. 1).

Amongst our records, the Sunda pangolin is the least expected species, as Xuan Lien falls within the distribution range of the Chinese pangolin (Manis pentadactyla Linnaeus, 1758) (Phuong et al. 2013, Challender et al. 2019a, Challender et al. 2019b). Two pangolin experts rigorously verified the Sunda pangolin in our study. Nevertheless, its detection in our study might be a result of unwarranted releasing activities, as suggested by the Reserve staff. Furthermore, two muntjac species, including the northern red muntjac and the Roosevelt’s muntjac, are known to occur in Xuan Lien (Phuong et al. 2013, Le et al. 2014). As our camera stations were set in the elevation range of 150–1,300 m, within the known elevation range of the species (Tilker et al. 2020b, Nguyen et al. 2021) and our survey covered 77% of the Reserve, the lack of Roosevelt’s muntjac record suggests that, if the species still persists, it may occur at a very low density. Additionally, the fact that we only documented a single record of the northern red muntjac, which is naturally more resilient to hunting pressure and is more widely distributed across various forest habitats in Vietnam (Nguyen et al. 2021, Alexiou et al. 2022), raises serious concerns over potential overhunting and habitat loss threats in the Reserve.

During our survey, we documented several signs of ongoing human disturbances inside the Reserve, including four illegal logger camps, small mammal traps and snare lines. Some of those activities were spotted deep inside the Reserve and interviews with local people revealed that exploitation of forest products, while illegal, still happened occasionally in Xuan Lien. Accessibility to the Reserve from the dam and the road systems may facilitate illicit activities (Fig. 1). Therefore, regular patrols and law enforcement activities will play an important role in protecting the remaining species in Xuan Lien Nature Reserve.