The last aurochs hybridized with cattle in the wild

Domestic animals and their wild counterparts are usually able to interbreed freely and produce fertile hybrids. Thus, it is always likely that everywhere they share their habitat, they might interbreed and thus mutually influence their populations. Domestic animals always might escape, and wildtype animals always might leave a track in domestic stock by occasional mating. 

When discussing whether aurochs and cattle interbred in Europe, it mostly concerned the question if local aurochs left a genetic trace in cattle populations. The other way round, domestic cattle influencing local aurochs, was not examined yet. However, I have always considered it very likely that escaped domestic cattle left a trace in European aurochs. It happens everywhere where wildtype and domestic type are neighbouring – you see that in wolves (some colour variants, such as black in wolves, are believed to have been inherited from domestic dogs), in wild boar displaying domestic colour, and it has also been proven for late European wild horses that inherited the emutation from domestic stock (see Pruvost et al. 2011). I see no reason why it should not have happened that escaped domestic cattle interbred with aurochs and left a detectable trace in wild populations. 

This interesting question has now been examined by Bro-Jorgensen 2018. A number of ornamented drinking horns from medieval times or shortly after that are suspected to stem from aurochs because of their shape and size have been genetically analysed for mitochondrial haplotypes and sex. The sample also includes the horn of the last aurochs bull that died in Jaktorow, Poland, in 1620. 

Medieval aurochs drinking horn - large, thick and evenly curved

Horn of the last aurochs bull - smaller, thin and not that curved

All of the horns tested turned out to be from males – considering their shape I would be surprised if turned out otherwise – and most of them have the aurochs haplotypes P. The horn of the last bull and two other drinking horns, however, surprisingly carry the haplotype T, which is widespread among taurine cattle. While the origin of the two drinking horns might not be that clear, it is very likely that the claimed horn of the last aurochs bull is indeed from this population and individual. It is also very likely that the population in Jaktorow was not simply a feral cattle population because of historic reports. Thus, the most likely conclusion is that aurochs of the latest centuries, or at least the remnant population at Jaktorow, was genetically influenced by escaped domestic cattle. As this influence is found on mtDNA, which is maternally inherited, the influence must be from a domestic cow at least. Influence from domestic bulls is of course also possible, but was not examined and domestic bulls probably had a hard time competing with wild aurochs bulls anyway. 

Natural selection would probably eradicate most of the domestic influence, except for factors with little selective pressure on them, such as these mitochondrial haplotypes. Perhaps, if the domestic influence was widespread towards the end of the existence of the aurochs (when space became increasingly limited and thus they often neighboured domestic stocks) there also were aurochs populations displaying domestic colour variants, although no such cases have been reported in historic texts. Domestic cattle influence also shows in American bison, where it is particularly visible in horn shape and size (see here, for example). The horn sheath of the last aurochs bull was comparably small (only 45 in length, which is considerably smaller than the bony cores of earlier aurochs males). Even if it was a young individual at the time of death, most likely the last aurochs population had comparably meagre horns as a consequence of limited resources and trophy hunting, but domestic cattle intermixing might be a further reason. This would also explain why the curvature is not nearly as intense and even as in the older drinking horns (cattle intermixing affects horn shape in wild bovines as the bison example shows). 

This discovery is interesting and totally what I expected and predicted. Questions that are particularly intriguing are how widespread and intense that domestic introgression into local aurochs population was, and how large the influence on the visible phenotype of these aurochs population was. 

Literature 

Bro-Jorgensen et al.: Ancient DNA analysis of Scandinavian medieval drinking horns and the horn  of the last aurochs bull. 2018.

Pruvost et al.: Genotypes of predomestic horses match phenotypes painted in Paleolithic works of cave art. 2011.