Hybridization as a perspective for breeding-back?

Sometimes commenters on my blog ask me on what I think about the idea of using extant, wild cattle species for breeding an accurate aurochs substitute. One could even go further and try to replace the aurochs with other, related bovine species instead of “breeding-back” results and/or dedomesticated cattle. For other exterminated species that did not leave domestic descendants this option is considered (such as using extant water buffalo for Bubalus murrensis). However, domestic cattle are probably way closer to the aurochs in behaviour and ecology than extant wild bovines (see here). Nevertheless these could be used in order to obtain wild traits that domestic cattle lack altogether or to increase the fitness for survival in nature instead of waiting for those traits to re-develop via dedomestication. There is a good chance that the aurochs shared some wild traits or even the same alleles with its closely related species that domestic cattle have lost, and they could re-enter the gene pool via hybridization. This would make it easier for new “breeding-back” populations to establish in harsh regions and perhaps also a higher morphological resemblance could be achieved.

Looking at extant wild bovines, I see three species which could qualify as useful for hybridization: wisent, wild yak and the Java banteng. I am going to explain which traits make these species useful; of course, being completely different species, they also have a lot of undesired traits that would occasionally reappear in the hybrids such as the beard of a bison or the white “socks” of banteng. But those could be bred out just as any undesired domestic trait such as white spots on the belly or deviant horn shapes as well, one could argue. With this post, I want to go over the pro’s and con’s of using hybridization with wild bovines for “breeding-back” in detail (I already did one in 2015, but this post will be more comprehensive).

Wisent

Bison bonasus

The wisent has, of all extant wild bovines, the most aurochs-like proportions and body shape

As other wild bovines, wisents have a strongly marked sexual dimorphism in size. The wisent is not only as large as large aurochs (bulls reaching 180cm at the shoulders) but also has bodily proportions rather similar to those of the aurochs, with a squarely built trunk, the hump is high but not as high as in American bison and they have a neck bulge similar to that of taurine cattle (that the aurochs probably also had). These similarities in body shape are not coincidental. In fact, mitochondrial and nuclear DNA have confirmed the wisent as an aurochs hybrid that evolved at the end of the Pleistocene and it is estimated that about 10% of the nuclear genome of the wisent is of aurochs legacy (see here). Wisent and domestic cattle have been hybridized repeatedly, the results are called Zubron in Poland. Although first generation calves have to be born by caesarean section, female F1 hybrids are fully fertile and males from the second generation onwards. Some zubrons in the internet show interesting morphs, such as this bull with his high-legged athletic build or this cow with its large hump and aurochs-like colour. Both these zubrons were probably bred using ordinary domestic cattle, therefore hybridization between aurochs-like cattle and wisent could produce even better results. One quarter wisent could boost the morphology of “breeding-back” results quite dramatically, and concerning the aurochs legacy of Bison bonasus, why not?

The problem is that aurochs/cattle and wisent are sympatric species, what means that they theoretically share the same ecosystems. Therefore, their niches differ (an ecological principle that is called competitive exclusion). Wisents are more adapted to dryer habitats on higher altitudes and aurochs/cattle to more humid habitats on lower altitude. Furthermore, wisents are so-called semi-intermediary feeders, which means that their food choice includes more browsing than in cattle, which are more grazers. Also, wisents also slightly differ in behaviour, including vocalization. Therefore, when using wisents for “breeding-back”, you would not only have to select out animals based on their external appearance but you also have to watch food choice, habitat choice, behaviour and even vocalization, otherwise many biological aspects of the aurochs substitute would be messed up. Furthermore, wisent and cattle differ in their number of spine elements, which cannot be observed in living individuals without an X-ray. 

The next problem that is a result of both species being sympatric comes to play when both these “breeding-back” results with wisent introgression and wisents would be released in the same reserve. Wisent and aurochs shared the ecosystem and naturally probably did not or extremely rarely interbreed. If the “breeding-back” results are wisent hybrids it could increase the frequency of interbreeding and reduce fertility barriers. This would be a danger for the biologic integrity of the endangered wisent. And in fact, the danger of hybridization between cattle and wisent is sometimes used as an argument not to release both species in one reserve although the danger is virtually nonexistent₁.

Wild yak

Poephagus* mutus

The horns of wild yaks are as good as identical to those of aurochs

* Yes, I have the yak in its own “genus” here. It is phylogenetically actually closer to bison than cattle (I explain why I write “genus” under quotation marks in an upcoming post).

The wild yak is a large bovine of the right size (bulls having a shoulder height of 1,6-2m) and sexual dimorphism (cows being 30% smaller in linear dimensions according to Wikipedia) that also has a large shoulder hump. But what is most striking is that wild yaks tend to have horns that match those of the aurochs exactly in size, diameter and curvature. Almost identical, one could dare to say. Considering the fact that finding breeds that have strongly inwards-curving horns in both sexes is rather difficult for “breeding-back”, and that most “breeding-back” cattle are deficient in this respect, using wild yaks would be a great aid for achieving this trait. I did this sketch of what a wild yak without hair would look like, and it resembles the morphology of an aurochs pretty well, so the benefit of crossbreeding with aurochs-like breeds would be obvious.

What is more striking is the yak’s adaptions to harsh climate. Yaks need less food intake than domestic cattle due to their larger rumen and survive longer without food and water. These traits could be beneficial for establishing “breeding-back” populations in harsh areas. For example, the Puszta of Hortobagy is not ideal cattle habitat but the best area available for large-scale natural grazing, which is why Taurus cattle had problems during winter there during the first couple of years. Yak introgression would help the cattle to deal with the conditions of the Puszta.

Fertility is a problem in yak hybrids as well. As it is often the case in hybrids, the heterogametic sex (males in the case of mammals) is infertile (Haldan’s rule), but in the case of the yak not only in the first generation but also in the second generation.

Wild yaks are adapted to way colder temperatures than cattle. They get overheat problems above 15°C due to a higher amount of body fat and fewer sweat glands. Yaks are furthermore more suited to higher altitudes than cattle.

They also differ in behaviour. Yaks, when they feel threatened, erect their tufted tail in order to appear larger – a display behaviour that cattle do not do. So when selecting hybrids, one would have to pay attention to that too.

Java banteng

Bos javanicus javanicus

Java banteng have the same colour dimorphism as the aurochs

The Java banteng is way more closely related to the aurochs and domestic cattle than both yak and bison are. As far as I know, they can interbreed with cattle without fertility problems and there is a hybrid breed that was bred by intermixing with zebus, Madura cattle. Java banteng have the right size and body with a large shoulder hump and long legs, and the sexual dimorphism in size is considerable as in all wild bovines. But the biggest advantage would be their sexual dimorphism in colour: bulls are very dark brown to black while cows have an orange-reddish brown colour, which is very reminiscent of the aurochs. This sharp contrast is present universally in the subspecies, while even in those domestic cattle breed with a good sexual dichromatism (such as Maronesa) it is reduced and tends to vary from individual to individual. The colour of Java banteng differs in having white “socks” and buttocks, but they can be bred out. The historical range of aurochs and banteng did not overlap, and there is a chance that they might have been ecologically equivalent. Banteng and cattle/aurochs differ slightly in combat behaviour, though. Banteng rely more on display than actual fights, which is why they have a higher profile with higher shoulder spines, a longer dewlap (thermoregulation might play a role here as well) and upright horns.

So there are pro’s and con’s for all those three species. Theoretically you could even build a look-alike by hybridizing these three species alone taking the horns of a yak, aurochs-like colour of banteng and aurochs-like proportions of wisent, but that resemblance would be only vague and superficial, and it also would not be an ideal ecological substitute for the aurochs. In order to reach the goal that is replacing the aurochs as authentically as possible in morphological, behavioural and ecological respect, hybridization should only be used in small doses otherwise it would be a step away from the goal. But apart from the fact that introducing other species into the gene pool makes the selection criteria far more comprehensive and complicated, the biggest problem would be that hybrids are stigmatized. Although hybrids were a part of speciation in many cases (for example, sheep, bovines including the wisent, canids, geese and even humans), happen in nature in many tetrapod groups all the time (within genera such as Bombina, Pelophylax and Canis just to name a few; more on hybridization in evolution in a future post), hybrids are still regarded as Frankenstein creations done by humans for no particular good. Especially in conservation hybridization is often considered “genetic pollution”, often with a good reason but in some cases this purity fanaticism can actually be harmful and do more harm than good, such as in the case of the wisent population in the Caucasus. “Breeding-back” results with introgression from other cattle breeds would loose their legitimation as native species and there will surely be tabloids with screaming headlines like “hybrid monster cattle to be released into European nature”. Both will not help the academic and public acceptance of these “breeding-back” results.

For these reasons, I would not recommend using existing wild bovine species for “breeding-back”. I think nothing speaks against an experimental project using hybrids of aurochs-like cattle and one of these bovine species (although I do not recommend using wisent because both species are sympatric) and in order to see how it works out, although I think the danger of the hybrids becoming a behavioural and ecological mosaic is given.

₁ Frans Vera: Do European bison and cattle cross spontaneously? 2002. Vakblad Natuurbeheer.