I already did a post on the question if it would be wise to use existing wild bovines for crossbreeding in “breeding-back” a few years ago. I came to the conclusion that it is not recommendable to use hybridization with other wild bovines for “breeding-back” for a number of reasons. For once, while cattle are probably ecologically and ethologically identical with the aurochs and are derivations of the same species, other bovine species differ in behaviour (f.e. banteng fight less than domestic cattle, yak have a different display behaviour by using their tail tuft) and also ecology (banteng are a tropical species and eat more tree fruits than cattle do, yaks are more adapted mountainous habitat while the aurochs was a lowland species that preferred plain habitat). Therefore, the argument that “breeding-back” results are ethologically and ecologically identical with the aurochs as they are derivations of the same species would be gone. Furthermore, the acceptance for a species hybrid in the European wilderness would be lower than for “just” feral cattle.
However, I am starting to believe that using careful, small doses of hybridization with wild yaks could be worth a try.
I am mainly thinking about this because of the horns of wild yaks. If you want to see an animal that has horns exactly looking like those of an aurochs, look at wild yaks. It is fascinating how the horns are really identical in both species. See here, here or here. The curvature matches that of the aurochs’ horns as much as the relative and absolute size. Horn tips that truly curl inwards as much as in the aurochs is rare in “breeding-back”, and also rare in primitive taurine cattle, which is why I consider this trait one of the challenges for “breeding-back”. Strict selection would be necessary to produce this kind of horn shape, which would take its time and selection also has to care about other traits as well, while crossbreeding with wild yaks could produce that horn shape quite fast.
Another reason is the morphology of the wild yak. A hairless wild yak superficially resembles an aurochs. They have a large hump in both sexes and usually a very long snout. Also, they have a comparably short trunk in both sexes. As the right horn shape, these traits are comparably rare in “breeding-back” and primitive taurine cattle. Most “breeding-back” cattle have a hump that is smaller than in the aurochs, and a truly large hump is mainly restricted to Spanish fighting cattle. The very long snouts of many aurochs bulls has not yet been reproduced by “breeding-back”. Wild yaks would thus also be beneficial for morphological traits.
Also, the body size fits European aurochs. Wild yak bulls reach 170 to 205 cm withers height [1,2], which is the upper size class of European aurochs bulls. This very large size would be very useful for breeding for large size, especially as it compensates the small sizes of breeds used like Heck cattle (in Hungarian Taurus cattle), Watussi (in Auerrind and Hungarian Taurus cattle), Pajuna (Tauros cattle and Auerrind) or Highland (Tauros cattle).
Thus, using wild yaks would speed up the process of achieving aurochs horns, body size and morphology considerably. Another advantage is that wild yaks are very cold-adapted and have a very dense undercoat. This would compensate the use of Watussi, which is adapted to a subtropical climate. Also, the yak is predominantly a grass eater, and thus ecologically overlaps with cattle. Crossbreeding with wild yaks would also lead to an increase in physiological fitness: wild yaks have a higher endurance than domestic yaks (and possibly domestic cattle), a more efficient respiratory metabolism as domestic yaks (and possibly domestic cattle), a more efficient digestion (the weight of wild yaks increases during winter, whereas that of domestic yaks and cattle decreases) and a lower death rate during winter [3,4].
To sum up the benefits of crossbreeding with wild yaks in “breeding-back”:
- Wild yaks have horns identical to those of the European aurochs.
- Wild yaks have large humps, a short trunk and a long snout
- The body size of wild yaks matches that of large aurochs and would compensate the use of small breeds
- Wild yaks are very cold-adapted with a dense undercoat what would compensate the use of Watussi
- The wild yak has a higher physiological fitness than domestic yaks and possibly domestic cattle
However, there are some problems. Of course typical yak traits, behavioural (f.e. presenting their tail tuft when threatened) and optical (f.e. the long hair on the ventral body side), would have to be selected out just as all other undesired traits. But some traits are not visible in the living animal:
- Yaks have 14-15 rib pairs, while cattle/aurochs have 13
- Yaks have a larger lung than cattle, an adaption for living in high altitudes
- Yaks are adapted to a mountainous habitat, while cattle/aurochs prefer flat lowlands
- Yak have less and barely functional sweat glands as an adaption to cold, which could be problematic in hot regions
- Yak have a slightly different skull anatomy: in cattle/aurochs, the premaxilla reaches the nasal bone, while in yaks the premaxilla only reaches the maxilla and not the nasal bone
As these traits are not visible, they can barely be actively eradicated by breeding. One possibility to get rid of them in the population is diluting the yak influence as greatly as possible, while at the same time taking care that the horns, snout length, body morphology and size remain in the population.
I would take a herd of aurochs-like “breeding-back” cows and inseminate them with wild yak semen (obtaining a wild yak bull for breeding would be very difficult as they are barely kept in zoos). The F1 bulls would be infertile. Thus, I would take an aurochs-like “breeding-back” bull to cover all the F1 cows. The resulting B1 bulls might be fertile, but reduced. If they are not able to breed, I would cross the B1 cows with the “breeding-back” bull again. The resulting B2 bulls would be fertile in any case, and would be 12,5% wild yak. The B2 bull could be bred to the 25% wild yak cows, and then a F2 of that combination by crossbreeding the results could be produced. When crossbreeding, one must pay attention to only breed with the hybrids that have the desired horn shape, size and body shape. Hopefully this results in a bull that is about one third yak (strictly genealogically) but only has the horns, body size and large hump that reveals its wild yak ancestry, while obvious yak traits like the long ventral body hair would be gone. This bull, being an F2 individual of the new combination, might be stable for at least some of its traits and can be used for backcrossing with the pure “breeding-back” cows. I would remove the other yak hybrids then. This way, the resulting animals would be taurine cattle with merely introgression from wild yaks and the yak influence would be further diluted in future generations while the aurochs-like horns, body size and morphology would remain in the population.
This plan would take five generations to produce the aurochs-like result using careful wild yak introgression. Very strict selective breeding using the best of the best “breeding-back” cattle could be successful in five generations too, on the other hand.
I think executing this plan in an experimental herd would do no harm. Unfortunately this will probably never happen, as wild yaks are very difficult to obtain. They are rarely kept in zoos. As far as I know, the only wild yaks in captivity are found in two Chinese zoos and one in Chicago.
I think it would be vital for the preservation of the wild yak to build up a solid population in captivity, ideally with a herd book in order to secure the purity of the animals (there is the danger of hybridization with domestic yaks).
 Castello: Bovids of the World. 2016.
 Jianlin et al.: Ecology, Evolution and Behaviour of Wild cattle: Implications for conservation. 2014.
 Zhonglin: Development of a new yak breed through utilization of wild yak genetic resource – serial technologies of the development of the Datong yak breed. 2004.
 Lanzhou Institute of husbandry and pharmaceutical sciences: the 5th international conference on yak. 2015.