Vol.:(0123456789) 1 3 Primates https://doi.org/10.1007/s10329-019-00737-w ORIGINAL ARTICLE Population abundance and distribution of the endangered golden langur (Trachypithecus geei, Khajuria 1956) in Bhutan Phuntsho Thinley 1,3  · Tshewang Norbu 1  · Rajanathan Rajaratnam 2  · Karl Vernes 3  · Kezang Wangchuk 1  · Karma Choki 4  · Jigme Tenzin 4  · Sangay Tenzin 4  · Kinley 5  · Shacha Dorji 5  · Tshering Wangchuk 5  · Karma Cheda 5  · Gempa 5 Received: 2 June 2019 / Accepted: 24 July 2019 © Japan Monkey Centre and Springer Japan KK, part of Springer Nature 2019 Abstract Reliable population estimates are lacking for many South Asian primate species, including the golden langur (Trachypithecus geei), which is endangered and restricted to Bhutan and northeast India. Although well studied in India, few studies exist on this species in Bhutan. In November 2017, we undertook a nationwide survey of golden langurs in Bhutan using double observers along trail-based transects in 17 blocks within its habitat, and modeled its distribution using MaxEnt. A total of 2439 golden langurs in 222 groups were collectively encountered by 17 teams of double observers, from which, an overall population of 2516 ± SE 363 individuals and 236 ± SE 9 groups were estimated. Group sizes varied from 2 to 35 individuals with a mean of 11 ± SD 0.38 individuals. A total of 468 adult males (19%), 924 adult females (38%), 649 juveniles (27%), and 398 infants (16%) were counted. Adult male-to-female sex ratio was 1:1.97 and adult female-to-infant ratio was 1:0.43. We determined 2848 km 2 of suitable area for golden langurs in Bhutan and estimated a density of 0.88 individuals/km 2 . Our population estimate of golden langurs in Bhutan is much lower than the current IUCN estimate of 4000 individuals for Bhutan, necessitating a reassessment of its current conservation status due to threats from road kills, electrocution, and development activities like road construction, hydropower, and electrical transmission lines. We further recommend our refned double-observer survey method to reliably estimate primate populations in rugged terrain. Keywords Bhutan · Endangered · Endemic · Eastern Himalayas · Golden langur Introduction Wildlife population monitoring is important for manage- ment, especially when species of concern share the same landscape with humans (Riley et al. 2015). As such, accurate assessment of population size and distribution is integral to species management and efective conservation (Kumara and Radhakrishna 2013; Matsuda et al. 2016). This is particu- larly true for 60% of the global primate species threatened with extinction on the IUCN Red List of Threatened Species (Estrada et al. 2018). Despite signifcant advances in primate census techniques (e.g., Buckland et al. 2010; Fashing and Cords 2000; Marshall et al. 2008; Peres 1999; Plumptre and Cox 2006), there are limited studies on South Asian pri- mate species (Khanal et al. 2018), notably in the Himalayan Kingdom of Bhutan. Bhutan is home to six primate species (Choudhury 2008; Wangchuk et al. 2004): golden langur (Trachypithecus geei Khajuria 1956), central Himalayan gray langur (Semnopithecus schistaceus Hodgson 1840), capped langur (Trachypithecus pileatus [Blyth 1843]), Assamese macaque (Macaca assamensis [M’Clelland 1840]), rhesus macaque (Macaca mulatta [Zimmermann 1780]), and Ben- gal slow loris (Nycticebus bengalensis [Lacépède 1800]). * Phuntsho Thinley chetsho78@gmail.com 1 Ugyen Wangchuck Institute for Conservation and Environment Research, Department of Forests and Park Services, Lamaigoenpa, Bumthang 32001, Bhutan 2 Geography and Planning, University of New England, Armidale, NSW 2351, Australia 3 Ecosystem Management, University of New England, Armidale, NSW 2351, Australia 4 Sarpang Forest Division, Department of Forests and Park Services, Sarpang 31002, Bhutan 5 Tsirang Forest Division, Department of Forests and Park Services, Tsirang 36001, Bhutan