Sequential stable isotope analysis reveals differences in dietary history of three sympatric equid species in the Mongolian Gobi

1. Competition among sympatric wild herbivores is reduced by different physiological, morphological and behavioural traits resulting in different dietary niches. Wild equids are a rather uniform group of large herbivores which have dramatically declined in numbers and range. Correlative evidence suggests that pasture competition with livestock is one of the key factors for this decline, and the situation may be aggravated in areas where different equid species overlap. 2. The Dzungarian Gobi is currently the only place where two wild equid species coexist and share the range with the domesticated form of a third equid species. In the winter‐cold Gobi desert pasture productivity is low, highly seasonal, and wild equids additionally face increasing livestock densities. 3. We used stable isotope chronologies of tail hairs to draw inferences about multi‐year diet seasonality, isotopic dietary niches and physiological adaptations in the Asiatic wild ass (khulan), re‐introduced Przewalski's horse, and domestic horse in the Mongolian part of the Dzungarian Gobi. 4. Our results showed that even in the arid Gobi, both horse species are predominantly grazers, whereas khulan are highly seasonal, switching from being grazers in summer to mixed feeders in winter. The isotopic dietary niches of the two horse species were almost identical, did not vary with season as in khulan and were narrower than in the latter. Higher δ(15)N values point towards higher water use efficiency in khulan, which may be one reason why they can exploit pastures further away from water. 5. Synthesis and applications. The high degree of isotopic dietary niche overlap in the two horse species in the Mongolian Gobi points towards a high potential for pasture competition during the critical nutritional bottleneck in winter and highlights the need to severely restrict grazing of domestic horses on the range of the Przewalski's horses. The evolutionary more distant khulan are less constrained by water and seem more flexible in their choice of diet or less successful in exploiting grass‐dominated habitats in winter due to human presence. Providing additional water sources could increase the competition between khulan and livestock and should, therefore, only be done following careful consideration.