Summary 1.Historically, the overlap zones of wild equids were small in Africa but extensive for Przewalski's horses Equus ferus przewalskii and Asiatic wild asses Equus hemionus in Asia. Currently, the Great Gobi B Strictly Protected Area in south-western Mongolia is the only place where sympatric, free-ranging populations of these equids occur. This provides a unique opportunity to test the hypothesis that Przewalski's horses are primarily adapted to mesic steppes and Asiatic wild asses to arid desert steppes and semi-deserts. Understanding the spatial needs and habitat requirements of these little-studied species is a pre-requisite for setting aside and managing protected areas and planning future re-introductions. 2. From 2001 to 2005, we followed nine Przewalski's horses and seven Asiatic wild asses using satellite telemetry and direct observations to assess differences in their resource selection strategies and social organization. 3. Przewalski's horses had non-exclusive home ranges of 152-826 km 2 , selected for the most productive plant communities and formed stable harems groups. 4. Asiatic wild asses had non-exclusive home ranges of 4449-6835 km 2 , showed little preferences for any plant community and seemed to live in fission-fusion groups. 5. Synthesis and applications . Our results provide evidence for different resource selection strategies in two sympatric equid species. Our findings indicate that the Gobi areas provide an edge, rather than an optimal habitat for Przewalski's horses. Consequently, only small and isolated pockets of suitable habitat remain for future re-introductions. Asiatic wild asses, on the other hand, need access to large tracts of land to cope with the unpredictable resource distribution of the Gobi. Managers should be aware that protecting habitat where Asiatic wild asses occur does not necessarily benefit Przewalski's horse restoration, whereas setting aside habitat for the conservation of Przewalski's horses will only locally benefit Asiatic wild asses.

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RationaleIn recent years, segmental stable isotope analysis of hair has been a focus of research in animal dietary ecology and migration. To correctly assign tail hair segments to seasons or even Julian dates, information on tail hair growth rates is a key parameter, but is lacking for most species.MethodsWe (a) reviewed the literature on tail hair growth rates in mammals; b) made own measurements of three captive equid species; (c) measured δ2H, δ13C and δ15N values in sequentially cut tail hairs of three sympatric, free-ranging equids from the Mongolian Gobi, using isotope ratio mass spectrometry (IRMS); and (d) collected environmental background data on seasonal variation by measuring δ2H values in precipitation by IRMS and by compiling pasture productivity measured by remote sensing via the normalized difference vegetation index (NDVI).ResultsTail hair growth rates showed significant inter- and intra-specific variation making temporal alignment problematic. In the Mongolian Gobi, high seasonal variation of δ2H values in precipitation results in winter lows and summer highs of δ2H values of available water sources. In water-dependent equids, this seasonality is reflected in the isotope signatures of sequentially cut tails hairs.ConclusionsIn regions which are subject to strong seasonal patterns we suggest identifying key isotopes which show strong seasonal variation in the environment and can be expected to be reflected in the animal tissue. The known interval between the maxima and minima of these isotope values can then be used to correctly temporally align the segmental stable isotope signature for each individual animal. © 2015 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons Ltd.