I think that all of the current “breeding-back” projects and breeds are progressing very nicely. It is always exciting for me to see how well they are doing. However, one important point on the “aurochs checklist” seems nearly unreachable: a truly aurochs-like bodily morphology.
With this I am referring to everything concerning the body of the animal: skeletal proportions, musculature, intestinum size, athletic appearance. All “breeding-back” cattle so far do not quite fit concerning proportions and do not really have the body of a wild bovine. Why is that?
The explanation might be that this is generally the case in (taurine) cattle. Look at Sayaguesa – they are one of the best-suited breeds for “breeding-back”, yet even they have a somewhat domestic morphology. Lidia might be the only exception, at least concerning the less-derived herds (there are also some rather massive Lidia bulls), but that breed is highly problematic to handle due to their aggressiveness. And interestingly, the Lidia crosses I have seen so far are not nearly as athletic as Lidia themselves and it has not been possible yet to conserve the Lidia-morphology in the crossbreeding process.
In order to achieve a truly aurochs-like morphology (that is, a slender waist, a well-developed front with a large hump, a comparably small intestinum and long legs on a trunk that is only slightly longer than the shoulder is high, plus a large head), I see the following possibilities:
1. Using dedomestication for the cattle to re-develop the morphology of a wild bovine
Dedomestication has the potential to change the morphology of “breeding-back” cattle, as has been demonstrated in Heck cattle at Oostvaardersplassen. Just look at that lightly coloured bull at 0:33 here. Its morphology is very aurochs-like, perhaps one of the most aurochs-like morphologies I have seen so far in a taurine bull. Since the ancestral Heck population was way more domestic in morphology, it can be assumed that natural selection at least played a role in this change in morphology in single individuals. However, it took more than 30 years for such a morphology to appear, and only in a single individual (at least judging from what I have seen so far). That means that we would have to wait for a quite long time until “breeding-back” cattle in the wild have the morphology that makes them best-adapted for a life in the wild, namely that of the aurochs.
2. Crossing in a suitable breed
Crossing in a suitable breed might work faster. However, which breed should be used for that? Lidia do sometimes have a very wild-like body shape and musculature but not the proportions, as their legs are usually notably shorter. Some zebu breeds, such as Deoni, have a remarkably short trunk with long legs. There are Criollo cattle bred from crossing Lidia with zebu in Colombia, which mostly look like Lidia but with zebu proportions, and those cattle would be very useful for “breeding-back”. It would be nearly impossible to get them to Europe alive because of legal restrictions, but perhaps semen could be imported. But there is still the aggression issue.
Looking at European breeds, Pajuna and Camargue stick out because of their deer-like elegance, but their proportions are not always reliably aurochs-like and they are quite small breeds too. Crossing in such small breeds could revert the breeding work of the last three decades to get “breeding-back” cattle large.
3. Hybridization with wild bovines
Hybridization with wild bovines of different species is surely a polarizing issue. Some are categorically against it, others consider it the only option to get “breeding-back” cattle fit for nature. My stance has shifted from considering it a no-go to something that should be considered an option, at least in an experimental herd.
In particular, I am thinking of Java banteng and wild yaks. The former have the short trunk with a slender waist and high hump, the latter have a high hump too and both are useful for several other aspects (f.e. their body size, the colour dimorphism of the Java banteng, the horns of the wild yak). And they have the body shape of wild bovines because they are wild bovines. Small doses of hybridization followed by wise and strict selection with a large number of animals could result in something that is more aurochs-like than anything we ever imagined.
But of course I am aware of the controversy that could arise from using a different species for the breeding. However, after eight generations of backcrossing with taurine “breeding-back” cattle, the influence from those species would be – genealogically – be a mere 0,3% percent. Which would be virtually nil except for those traits that we want the cattle to preserve.
Which of these options do I prefer? It would be a mix of 2 and 3. Zebu have been found to share aurochs genes with the European aurochs which taurine cattle have not, so having them in the mix is legitimate (however, we do not have yet the capacities to check if the crosses actually have those aurochs alleles that would be inherited by zebu). And Java banteng and wild yak would speed up the breeding process considerably. I think using those three (Java banteng, wild yak and suitable zebu) is the only option to achieve a bovine that is actually indistinguishable from the aurochs in almost any respect. And that is the best we can have until one of the de-extinction organizations give the aurochs a try using gene editing.