The tale of the horse´s tale- stable isotope analysis for comperative ecology of equids

All equid species are similar in size and body shape and seem to occupy very similar ecological niches. Overlap zones among species are small and little work has been done to understand resource use and physical adaptations that explains species distribution. In the Dzungarian Gobi of Mongolia three equid species, the Asiatic wild ass (Equus hemionus), the re-introduced Przewalski`s horse (Equus ferus przewalskii) and the domestic horse (Equus caballus) share the same habitat and thus provide a unique opportunity for comparative ecological studies. However, continuous observations of free-ranging equid species in the harsh environment and over the large expanse of their ranges are impossible to conduct, whereas physiological measurements either require highly invasive techniques and/or the confinement to a captive or semi-captive environment. Stable isotope analysis has become a powerful tool to study feeding ecology, water use or movement pattern in contemporary, historic and ancient species. Certain hair and teeth grow continuously and when sampled longitudinally can provide temporally explicit information on dietary regime and movement pattern. Wild equids were once abundant over a wide geographic range which made them ideal species for paleodietary and parleoclimatic reconstructions using isotope analysis. Results have been widely calibrated using isotopic signature variations from modern counterparts. As a consequence, there is an advanced understanding of the assimilation of dietary isotope composition, particularly carbon, into the horse tissue. In the proposed 3-year study we want to use isotope analysis of sequentially sampled tail hair from the three sympatric equid species as an indirect measure for seasonal feeding ecology, water use, movement pattern and metabolism. The Przewalski`s horse is rather special as it became extinct in the wild, but has been re-introduced back into its original habitat. Prior to extinction in the wild almost no ecological information had been collected and many aspects of their original ecology are unknown (e.g. migratory behaviour). Comparing samples of historic autochthonous Przewalski`s horses with those of present day re-introduced Przewalski`s horses as well as sympatric Asiatic wild asses and domestic horses can be expected to help re-construct some of the original ecology profile of the Przewalski`s horse.

All equid species are similar in size and body shape and seem to occupy very similar ecological niches. Overlap zones among species are small and little work has been done to understand resource use and physical adaptations that explains species distribution. In the Dzungarian Gobi of Mongolia three equid species, the Asiatic wild ass (Equus hemionus), the re-introduced Przewalskis horse (Equus przewalskii) and the domestic horse (Equus caballus) share the same habitat and thus provide a unique opportunity for comparative ecological studies. Stable isotope analysis has become a powerful tool to study feeding ecology, water use or movement pattern in contemporary, historic and ancient species. Tail hair in equids grows continuously and when sampled longitudinally can provide seasonal information on dietary regime and movement pattern. However, tail hair growth in the three equid species showed significant inter- and intra-specific variation, making temporal alignment problematic. For species comparisons, we thus developed a novel method making use of the strong environmental seasonality and its known interval length, reflected in the hydrogen isotopes, to temporally align the segmental stable isotope signature for each individual animal on a common timeline.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. This 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 Przewalskis horses. The evolutionary more distant khulan seem more flexible in their choice of diet or less successful in exploiting grass-dominated habitats in winter due to human presence. Their higher values of nitrogen isotopes point towards higher water use efficiency, which allows them to use pastures further away from water. Stable isotope chronologies of tail hair from museum samples of pre-extinction Przewalskis from the Dzungarian Gobi differed greatly from those of reintroduced horses. The direction of the dietary shift from being a mixed feeder in winter and a grazer in summer in the past, to a year-round grazer nowadays, is best explained by a release from human hunting pressure. A changed, positive societal attitude towards the species allows reintroduced Przewalskis horses to utilize the scarce, grass-dominated pastures of the Gobi alongside local people and their livestock whereas their historic conspecifics were forced into less productive habitats dominated by browse. Our results provide both important implications for species conservation and baseline information for global isotope patterns.