Many ungulates live in social groups whose sizes vary both within and across species and are influenced by such factors as habitat structure and openness, vegetation productivity, predator risk, and life cycle. We studied changes in group size of sympatric Wild yak (Bos mutus), Tibetan wild ass (Equus kiang) and Tibetan antelope (Pantholops hodgsonii) in Arjin Shan National Nature Reserve (ANNR), Xinjiang of China in the summers of 2012 and 2013, and the winter of 2012 in relation to season, habitat structure, and vegetation productivity. The aim was to understand the extent to which their group sizes differed and responded to these factors. The small groups of 1e5 individuals were observed most frequently, with solitary animals being the most frequently observed. Wild ass had the largest median group size (6.0, ranging from 1 to 180), followed by wild yak (4.0, ranging from 1 to 118) and Tibetan antelope (3.0, ranging from 1 to 44) (including solitary animal groups). Median group size differed between summer and winter for wild yak only, but it was larger in eastern ANNR than in western ANNR for all three species possibly due to flat topography and higher vegetation productivity in the eastern part. Vegetation productivity had a positive effect on group size of Tibetan antelope only. Our results suggest that group sizes of the three ungulate species in ANNR are highly variable and respond to vegetation productivity differentially. Further studies are needed to investigate the species-specific factors that may influence ungulate group sizes. Because all three ungulate species form larger groups in the relatively flat and easily accessible eastern ANNR, measures need to be undertaken to prevent poachers from entering into and poaching large groups of the
ungulates in eastern ANNR.
