I have been asked by several people if crossbreeding with existing, extant wild cattle is a good idea for creating a bovine that resembles the aurochs and is suited to live in Europe’s wilderness. I have been wondering that myself already, and indeed it is tempting. As so many aspects in “breeding-back”, rewilding or “de-extinction”, this question is subjective. But I am going to try to list some of the pro’s and con’s and draw my personal conclusion.
The thoughts behind this idea are that domestic cattle have lost certain wild animal traits, what they undoubtedly did, and that wild bovines could bring them in, otherwise the cattle would have to redevelop them by dedomestication. And apart from that, they would be able to contribute morphological and external traits resembling the aurochs at least as good as primitive domestic cattle could, or even better. For example, Java banteng have a strong, well-marked sexual dimorphism and the tone of their coat colour is reminiscent to that of the aurochs (light brown in cows, black in bulls). Gaurs have an impressive size, bigger than any living cattle. Some wild yaks have horns that perfectly resemblethose of the aurochs, and they would contribute excellent cold tolerance. Yaks are closer to bison than cattle, and the males of the first generation have fertility problems. But that would disappear as the breeding progresses. The wisent would contribute aurochs-like proportions with a well-pronounced hump, but there are even more pre- and postzygotic isolation mechanisms, and wisent and aurochs were sympatric, so I would not use them. Actually, all of those wild bovines would add high processus spinosi in the shoulder area. The undesired traits they contribute, such as the overlong coat of the yak, the upright horns or white “socks” of banteng and gaur would be selected out just as any other undesired trait.
So using those wild bovines seems advantageous for optic traits and probably also cold resistance. But there are other aspects that must be considered. First of all, they are not just different varieties of one and the same species, but completely different species. They diverged more than two million years ago (the age of the oldest aurochs remains), so there are considerable differences not only in morphology but also genetics and ecology. The argumentation that an aurochs replica made by “breeding back” would be ecologically equivalent to the aurochs because domestic cattle are a domesticated variation of this species would be gone, and it is likely that the other species differ ecologically from the aurochs. The gaur for example was sympatric with the aurochs and prefers dense forest, live on higher altitudes and browse more than cattle. This distinguishes it from the aurochs in the same way as the wisent. The banteng on the other hand, whose range did not overlap with the aurochs but with the gaur, dwells in dryer and open landscapes and is therefore comparable to the aurochs. Yaks definitely are grazers. According to Wikipedia, they need less food intake than robust cattle and survive a couple of days without water and food, and they cope with way colder temperatures than cattle usually do.
While I don’t know of fertility problems between the closely related Bos members gaur, banteng and aurochs, cattle and yak do have some as stated above. Yak are closer to bison, which might be problematic – it could increase the chance of hybrid cattle interbreeding with wisents. But this is just an assumption, I do not know if yak and bison readily interbreed at all.
From the ecological point of view, yak and banteng shouldn’t be that problematic. The yak would probably even be beneficial. Morphologically/optically, they would be advantageous as well: banteng contribute size, high shoulder spines and a well-marked sexual dimorphism, and (wild) yaks add perfectly aurochs-like horns. So why not using them, at least in small doses?
There are people who are opposed to the usage of breeds with visible zebuine influence (like Watussi) because zebuine and taurine cattle diverged in the late Pleistocene, but still are B. primigenius, so they would totally reject the usage of extant wild bovines in the first place. The argument that dedomesticated, aurochs-like cattle have a legitimation as native wild animals in Europe because they belong to the same species as the aurochs would lose its credibility since such a population would consist of hybrids, at least to a certain degree. This would be problematic for their academic and public acceptance. Considering the controversies around releasing endangered animals even with subtle introgression of another subspecies, I would be surprised if the majority, or even a third, of serious scientists would be in favour of releasing a population consisting of domestic stand-ins for an extinct species mixed with species that are certainly not native – which is understandable. And I can imagine headlines in crappy tabloids like “Hybrid monster cattle to be released into nature!”. Of course I am aware that hybridization is a part of evolution and takes place anywhere closely related species neighbour each other, and regular readers of my blog will know that I am not averse against bison introgression into controlled wisent herds as diversity donors. But primarily I think that man should try to conserve or reconstruct nature as it is or was, and not to try playing evolution if not necessary. Regardless of how strong the hybrid influence would be, the population will probably display a mosaic of not only morphology but also ecology and behaviour. I have not mentioned yet that yaks also live on higher altitudes than cattle, and banteng have a different combat behaviour. They have a higher profile and display it to their opponent, raising their head to look as large as possible. They do not fight nearly as often as cattle do, what explains their upright horn shape. Who knows what kind of effect it will have on the behaviour of the population.
So there are arguments for and against using wild cattle to breed an aurochs substitute. My personal opinion is that I probably would not do it, because of the scientific and public response and a weaker argumentation basis as outlined above. Nevertheless it would be interesting to see hybrids between Yak, Banteng and primitive cattle. A “test herd” could be an idea.