From the middle Pleistocene to the early Holocene, there were two species of the Equid family present on the European continent. These were the European wild horse (Equus caballus ferus) and the European wild ass (Asinus hemionus hydruntinus). The former of these was of the same species as the living domestic horse (Equus caballus caballus) and the Asiatic wild horse (Equus caballus przewalski), while the latter was of the same species as the living Asiatic wild ass (Asinushemionus hemionus, hemippus†, khur, kiang, kulan, and onager). Both species would have been found over large portions of the continent, maintaining their separate niches through slight but significant differences in diet and habitat preference, much as living horses and asses do today in those few areas where they remain. In this first article, the European wild ass will be discussed.
The European ass, rather unfortunately, did not make it past the Iron Ages, after holding on to small refugia in areas of southern and eastern Europe, and in Asia Minor. Their persistence into this time is known from the subfossil record, but their relationship with humans is not well understood. It is likely that humans were responsible for their eventual extinction through a combination of hunting and habitat usage. For a time, the mysterious creature known as the “zebro” or “cebro” in Iberian legend was thought perhaps to have referred to a late-surviving population of the European ass, but the only reported skeleton turned out to be a donkey (Asinus asinus asinus), and other versions of the story claim it was actually a population of feral horses, commonly associated with the Sorraia breed.
For the past century or so, paleontologists have been undecided as to what sort of equid “Equus hydruntinus” was. Varying opinions placed it as a horse, an ass, a zebra, or even as a member of a completely distinct equid lineage only distantly related to the living varieties. Such conclusions are often unavoidable when dealing with the high scarcity and variability of bones, and the equally lofty egos of taxonomists. Genetic evidence has since placed the European wild ass quite neatly within the living hemione group, meaning that it is now most accurately described as a subspecies of the Asian wild ass, which might now more appropriately be called the Eurasian wild ass.
With its identity now firmly deduced, the question arises of how it can be replaced. The fact that, on a species level, it is not actually extinct, makes the answer to said question slightly easier. It is certainly not a new concept in ecological restoration that living subspecies can be used to fill the niche once occupied by their extinct conspecifics. If the habitat exists within Europe to support a population of Asinus hemionus, and if it is decided that this would be a positive move for much-needed biodiversity improvement, then it becomes a matter simply of importing individuals of an appropriate subspecies for a planned release.
What is the appropriate subspecies then? It just so happens that there are five to choose from. Among these is the Mongolian dzigettai (A. h. hemionus), the Indian khur (A. h. khur), the Turkmenian kulan (A. h. kulan), and the Persian onager (A. h. onager). Also included is the Tibetan kiang (A. h. kiang), which is considered by some to be a species in its own right, though this is not supported by genetic evidence. Not included of course is the European hydruntine (A. h. hydruntinus), nor the Syrian hemippe (A. h. hemippus), which went extinct in the nineteen-twenties.
The most appropriate subspecies would be one that was not necessarily the closest genetically to the European wild ass, but the one that resembled it most in habitat preference. While most would associate wild asses with deserts, scrublands, and steppes, these do not represent the whole spectrum of their original distribution. These more open and less human-friendly ecosystems were simply where the species remained after it had been driven from the wetter and more diverse areas of its range, which previously included forest-steppes, deltas, and coastal dunes. Such was also the case for the hydruntine which, during the Pleistocene, ranged over much of the European continent during both glacial and interglacial periods, but which by the early Holocene was confined to the Mediterranean shrublands of southern Iberia and Italy as well as the Anatolian and Pannonian steppes.
It is important to understand that subspecific designations are not always indicative of biological distinction. In some cases, such as that of the Eurasian wild ass, they are more accurately thought of as convenient names for geographically isolated populations. These populations were not nearly so closed off in prehistory, and there would have been frequent hybridization in the areas of overlap. Even today all of the hemione subspecies are completely cross-fertile. At least one hybrid population exists in the Negev desert of Israel, formed from a mixed founding herd consisting of five kulans and six onagers. This population serves as an ecological proxy for the hemippe, which would have occupied the area before its eventual extinction. The population has been successful, now numbering several hundred individuals, and has even started to expand its range north to the Negev highlands. Despite descending from such a low number of individuals, genetic diversity is maintained through social structure and from the broad genetic base inherited from their conspecific forebears. Such a strategy would possibly be advantageous for establishing new European populations as well.
It is also worth noting that different subspecies might be better suited to different potential release sites. After all, the hydruntine showed a high degree of morphological variation across the European continent, and were it alive today it might also be classified into separate taxonomic units, as has been done in for living hemiones. I would propose, for example, that the kulan, already beginning its re-establishment on the Ukrainian steppe, might be the ideal candidate for introduction to the steppic regions of Romania, Hungary, and adjacent regions. For the steppe and brush regions of Turkey and the Mediterranean however, the onager might be better suited, due to its smaller size and familiarity with warmer climates. These same traits were found in the hemippe, and it would be that subspecies that I would be recommending were it still with us. I would also use onagers for the coastal dune habitats of the western coast, though those would likely not be amongst the initial release sites. As with the Negev population, hybridization would be an option, if genetic diversity and/or adaptability was found to be an issue. If this were the case I would recommend introgression from the khur for the onager group, and from the dzigettai for the kulan group. Seeing as kulans and onagers are the only hemiones currently found in European zoos and these are likely the populations that new wild herds would be established from, it would first be advisable to import additional kulans and onagers to increase diversity before considering hybridization.
So ideally, we would be introducing two starting populations, from which we would later source new herds. One would be somewhere on the Pannonian steppe, while the other would be somewhere in the Mediterranean, perhaps one of the less populous areas of the Iberian peninsula. A third population could potentially be formed in Turkey, but perhaps it should wait until their political and environmental status improves. These populations would be allowed to breed until reaching the capacity of their prospective reserves, at which point they would then be used to start new populations, or to expand into new habitats on their own power. Population control would mostly be done through translocation at first, but once all available sites are at capacity, other options would have to be explored. Predation is one avenue worth considering, though predator diversity densities have significantly decreased since the onager was last present on the continent. Wolves would likely prey on European asses, as they do elsewhere, but due to the size of the animal and their various anti-predator behaviours, the number of individuals taken may not be comparable to the number of animals born. In the past, the predominant predators of wild asses were large cats, including leopards, cheetahs, tigers, and especially lions. The presence of lions (Panthera leo leo) in southern and eastern Europe is well supported for the early-mid Holocene, but their reintroduction to these areas, alongside leopards (Panthera pardus) would likely be far more controversial unfortunately. Dholes (Cuon alpinus) and hyenas (Crocuta and Hyaena) would also have been predators of the ass, but they are also now absent from these regions. Bears (Ursus arctos) and jackals (Canis aureus) might occasionally hunt foals or infirm adults, but this would be very occasional and would likely not affect the overall population. If wolf predation is inadequate, then culling for meat/leather or for sport might eventually be necessary.
That being said, a wild ass introduction could have many potential benefits for European rewilding areas. As a poorly-known and charismatic large mammal, it has the potential to support tourism and sustainable hunting industries. It will also be quite a useful species for reinvigorating arid and semi-arid environments in Europe, especially regions where, for whatever reason, horses cannot be used. Hemiones tend also to incorporate more browse in their diets than horses, meaning they will likely affect local biodiversity in a different way. Alongside other large mammals, such as bison and red deer, they will create sand-baths and wallows, creating microenvironments for reptiles and invertebrates. Their grazing will help prevent succession and promote more varied and diverse ecosystems. Their ability to eat poor-quality forage and clip down larger swards of grass allows them to create more usable habitat for other herbivores. Overall, I think there is a lot of potential for the introduction of Asinus hemionus to Europe. Already one of the most widely distributed herbivores in the world, though certainly not the least threatened, the Eurasian wild ass could eventually be found across the holarctic, from Spain to Siberia.
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