The previous three posts (here, here, here, here and here) have explored the borders of “breeding-back”. I give a quick summary. The real differences between aurochs and cattle do not only concern size, coat colour and horns but actually the whole organism: its developmental biology, its endocrinology, its neurology, its behaviour, its morphology, its genome. And all those factors are interconnected and interact in a complicated way. It was probably selection on tameness alone that dramatically changed the organism and the typical traits of a domestic animal (“domestication syndrome”) was more of a by-product. Additionally to that, some new morphologic mutations and artificial selection on certain traits produced the typical domestic phenotype that we see in cattle. The involved number of loci are probably hundreds or more likely thousands. It is very likely that cattle are uniform for these key genes that are responsible for the typical domestic traits that are universal among them, and consequently a number of important wildtype alleles was probably lost in the process. “Genetic breeding-back” is probably not feasible because of the high number of loci involved, the loss of a lot of wildtype alleles during domestication and because the necessary research work (identification of the involved loci and tracing down the wildtype alleles in cattle) has not been done yet. So we have to stick with conventional breeding. The Tamaskan is an example for exactly the same kind of “breeding-back”, but with a species that still has an extant wildtype. It shows that while a high optic resemblance can be achieved, the result is still a domestic animal with all the symptoms of the domestication syndrome. I give another example now, this time with “breeding-back” cattle itself:
This is the Taurus bull Lamarck. I chose him as an example because I think this bull is among the best “breeding-back” results produced so far. Let us examine this animal in detail. It might be about 160-165cm in size so its size does match Holocene European aurochs bulls. Its phenotypic colour is as far as we can tell flawless. The dewlap is short, leg length is ok, horn curvature is ok and the hump is present. Overall, only some Corriente and Lidia bulls are be as aurochs-like. Yet this animal is totally domestic. The trunk is still a bit too long or the limb and skull size is reduced, the brain volume is probably reduced, the hump size is reduced. The muscling is reduced and the belly/intestines enlarged. Those are classic symptoms of the domestication syndrome and probably a result of an altered corticosteroid level and thyroid hormone activity. The shortened skull and horns are probably a result of developmental delay, as is the reduced sexual dimorphism in the population. Also, the seasonality in the reproductive circle of the population is reduced as well. Lamarck is a very relaxed and comparably tame good boy – you could never approach an aurochs or any wild animal the way you can approach and handle him, which is probably a result of the altered hormone level as well and perhaps selection on a few neurological genes.
These symptoms of the domestication syndrome have a millennia-long history and are true of all individuals of any “breeding-back” project and probably true for any cattle breeds that are used or at least considered for “breeding-back” (even grown Lidia bulls grow very heavy and have unambiguous domestication syndromes). And therefore this will be true for any “breeding-back” result. You can fix the horn curvature as Maronesa shows, you can probably achieve even better proportions and body shape (see some Chianina) and also achieve a more aurochs-like skull (see Lidia or Hereford and Holstein) but it will still have the basic domestic traits.
You cannot simply reverse domestication and revive the aurochs by selective breeding with domestic cattle, at least not in the conventional way. The result will always be a domestic animal, it will always have symptoms of the domestication syndrome. The best we can achieve with domestic cattle in the conventional way, or achieve in any case, is a bovine Tamaskan. But would that be that bad? I do not think that. Actually I think that is pretty much considering the purpose we are doing the whole work for (that is, filling the niche of the aurochs with something that is very aurochs-like). And in a sense a bovine Tamaskan would be more like the aurochs than it sounds at first.
The similarities between aurochs and cattle
The latest few posts and also this post until now focused a lot on the differences between aurochs and cattle, so it is also time to remember the similarities between both animals. And there are actually a lot, which is not surprising as cattle are just modified designer aurochs.
Similarities in ecology.First of all, it is very likely that aurochs and cattle fulfil the same ecological niche. Not much is known of the food choice of aurochs, but it seems to have been the same hypsodont grazer as cattle. According to isotope analyses, it dwelled more wet habitat than cattle, but that might be an artefact of habitat limitation caused by humans (who, ironically, needed grazing areas for their cattle). Cattle are intermediary in their food choice between horses (which are almost full-grazers) and the wisent, which is intermediary between a grazer and a browser. This suggests niche partitioning and that an aurochs would not be different. Surely most domestic cattle are probably less good in digesting and seizing nutrients (see here) and consequently the spectrum many derived breeds can feed on is perhaps smaller than in the aurochs, but hardy landraces might be pretty good in this respect. It is also possible that the requirements of domestication slightly shifted the dietary focus, considering that dogs can digest carbohydrates more efficiently than wolves. But I still assume the overlap of food choice between aurochs and cattle to be very large. And however large the niche overlap between aurochs and cattle truly is, European cattle seem to work very well in European nature systems.
Similarities in behaviour. When I write that the behaviour of aurochs and cattle differs greatly, I am mainly talking about the fight/flight reaction towards humans, docility, agreeableness, trainability and everything else that is relevant when handling these animals. But of course “behaviour” encompasses a lot of other aspects as well, such as social behaviour. What we know of the social behaviour of the aurochs and cattle is absolutely congruent, and aspects of that are not documented for the aurochs are match between cattle and other wild bovines, so it must have been the same in the aurochs. For a little excurse see this article. For example, a cattle herd forms a defensive circle around calves when threatened by predators – all wild bovines do that, and so must have aurochs as well. Cattle form herds up to thirty individuals. The report by Schneeberger in Gesner 1620 also suggests that aurochs herds were not larger either. One could continue this list of matches and similarities in behaviour of aurochs, cattle and living wild bovines and also extend it to display behaviour, combat, vocalization and many other behavioural aspects. So probably the behavioural repertoire displayed by cattle under natural circumstances would be very similar to that of the aurochs, if not identical. This is not surprising as the requirements of cattle domestication probably did not affect these factors much.
Self-sustainment/survival capacity. You might assume that domestic animals lost the ability to sustain their individual lives and reproducing populations in the wild after millennia of living in human husbandry. But this is not the case, at least not in cattle. There are numerous examples of self-sustaining populations of feral cattle that descend from farm cattle that ran wild or were released into nature. Often these populations did not descend from hardy landraces but rather common derived breeds. Cattle still do have the necessary instincts and physiological requirements (climate resistance, disease resistance, wound healing, food exploitation et cetera) in order to survive and reproduce successfully in nature – not only in Europe but actually a variety of different habitats around the world.
Summary: what “breeding-back” can achieve
The outcome of “breeding-back” projects will always remain domestic animals with typical symptoms of the domestication syndrome concerning morphology and behaviour. However, these animals would still bear a striking overall similarity to the aurochs in many respects and they would probably show a very similar if not identical behavioural repertoire under natural circumstances. More importantly, they would fulfil a – again – very similar if not identical ecological niche and would be able to completely sustain themselves and their population in nature.
All in all, these cattle would be very much like the aurochs and more than sufficient in order to replace it ecologically in a way that satisfies extinct animal enthusiasts. And after enough generations in the wild, dedomestication would eradicate the typical symptoms of the domestication syndrome. These animals might be hardly distinguishable from aurochs. Actually, only a genetically reconstructed aurochs would be better because it is the original, and it would be available immediately while breeding the “perfect” breeding-back result might take some more decades (but honestly, the best that are available today are already almost sufficient to me) and a true dedomestication would probably take even longer.
To put it in a nutshell, the latest posts underlining the real differences between aurochs and cattle and showing the limits of “breeding-back” are no reason to be demotivated. “Breeding-back” can still achieve very, very much in terms of replacing the aurochs authentically. And if we indeed can one day genetically reconstruct true aurochs, even better.
 Axelsson: The genomic signature of dog domestication reveals adaptation to a starch-rich diet. 2013.