The “standard aurochs colour scheme” is as it follows: bulls being more or less completely black except a dorsal stripe and a muzzle ring of a light colour (whether there were also bulls with lightly coloured forelocks is unknown). For cows, various shades from completely reddish-brown, dark brown or black with a reddish-brown back/colour saddle and also “bull colour” are supported by evidence.
There is not one good reason to assume that European aurochs bulls had a colour saddle, but at least some North African aurochs (I tend to think it was universal in this subspecies) had one, as outlined in my post on Bos primigenius africanus. For the Indian aurochs, nothing is known of its life appearance except for what goes beyond osteologic information, but we can speculate (emphasis on speculate) that it might have had a very similar colour to that of the European aurochs, but it could also had some differences as I outlined in the posts on Bos primigenius namadicus (the most recent and most comprehensive one here).
But apart from what is either proven or at least not implausible based on the evidence, could there have been some more colour variants? The aurochs’ original range was quite large, and genetic drift and habitat differences could have produced some local variants, or some that were limited in time, or perhaps such that were so rare and unremarkable that they were not noticed. I made some thoughts on that, and also illustrated them and would like to share them with you in this post. I am going to start with the hypothetical colour variants that I consider most likely and end with the least likely one.
1: Completely black aurochs
The degree to which the muzzle ring and the dorsal stripe were expressed may have varied. Usually, as wildtype coloured cattle age, the muzzle ring gets reduced and may even almost disappear except for the chin (mostly in bulls). I would not be surprised if some or many wild aurochs bulls also showed only reduced muzzle rings or none at all, which might be the reason why there are no contemporaneous literature references to this trait. In Bantengs and Gaurs, we have the same situation – some individuals show them, others do not. Some aurochs bulls may have even lacked a dorsal stripe. But that is not what I am thinking about here. I am talking about the possibility that the Ed allele was actually present in wild aurochs populations before domestication and is actually a second wild type allele of the Extension locus. In this case, some aurochs would have been completely black in both sexes without any light markings, as we see it in many breeds (f.e. Angus, many fighting bulls et cetera). Usually, wild animal populations are quite uniform regarding coat colours, but the phenotypic difference between E+ and Ed, especially in bulls, would be so marginal that I hardly believe natural selection would have purged it out again in a few millennia after aurochs spread to Europe from the Middle East.
We cannot say there is direct evidence for this colour variant to have been present in Aurochs. Surely a lot of sources just describe the aurochs simply as “black” (f.e. Plinius), but that would also be the case if someone would describe an E+ bull without making the effort of making an extra remark on the light markings.
This is what aurochs with an Ed phenotype would look like:
The only way to test if there were Ed aurochs would be to do a genetic test for coat colour alleles in aDNA of fossil and subfossil aurochs material, as it has been done for wild horses in recent years.
I think that would be worth examining it, as I consider the possibility of aurochs with the Ed allele absolutely plausible. If it was found in Holocene or historic aurochs only, it may also be possible that the black allele would still be of domestic origin and found its way into the wild population by domestic introgression. Domestic animals escape all the time all over the world and can leave a mark in wild populations, especially in the form of colour variants as they are most neutral to selection. This is evident in some wolf and wild boar populations, and the same happened in wild horses in Europe, where the e mutation (sorrel) got into wild populations in historical times (Pruvost et al. 2011). So it might have happened in Holocene aurochs as well, and the black mutation would maybe not have been discernable for eyewitnesses.
2: Red aurochs in far Eastern Europe
Many of you know this chart showing the maximum range of the aurochs. The way I understand it, it shows the sum of all ranges the aurochs originally had, and not where it once ranged all at the same time. It seems that the aurochs was not an animal of the steppes, it would probably not do well in the cold and dry Eurasian steppe of the Baikal area. There are bone findings from this area, but from a time when climate was warmer and allowed Bos primigenius primigenius, adapted to the temperate European climate, to live there. That is why the Holocene range of the aurochs ended in the west of Russia in the transition zone from the European temperate biome to the Eurasian steppe biome. What is interesting is that van Vuure notes that Russian and Romanian tales tell of “red aurochs”, while most Central and European literature refers to “black aurochs” or mention sexual dimorphism. Could that mean that the aurochs of far eastern Europe in the semi-steppe lacked sexual dimorphism and that their bulls were of a red colour, perhaps caused by genetic drift? This would add another colour variant to the list, and quite frankly, it would be very interesting.
But I consider the evidence for that too weak. First of all, folk tales are not all too precise. Furthermore, cattle usually form herds of cows with calves and young bulls, where most individuals would be of a red colour, and bulls form either small groups of youngsters or wander around solitarily. So the chance is good that when people thought of big herds of aurochs most individuals would have been red because they consisted of cows, calves and young bulls. Again, genetics could resolve this question, but examining the amount of sexual dichromatism would probably go less quick than just a test for colour alleles.
3: Aurochs with the White Park pattern
The cave paintings at Lascaux are from the Paleolithic and about 17.000 years old. It includes black bulls, red cows, and line drawings showing bulls. What is peculiar about these line drawings is that they show small, irregular black spots on the neck, face and shoulder area, distributed in the same kind of pattern we find in the British breeds White Park and Chillingham cattle. This colour variant is caused by the homozygous presence of the Colour sided allele Cs. Could it be that some, perhaps only Pleistocene Southern European aurochs showed the so-called White Park pattern? This is not entirely implausible, and I revealed this idea in 2013 already. Cave paintings also show spotted horses long before the emergence of domestic horses, and a study by Pruvost et al. 2011 showed that such spotted wild horses probably did exist and where no invention of Pleistocene artists. So why should not be the same possible here?
It seems that these line drawings at Lascaux would be the only evidence supportive of this idea. There are no artistic or literary references that ever mention white or very faintly coloured aurochs, not even mystery tales. But this colour variant must have survived in the aurochs population until at least 8.500 years ago when the first aurochs where domesticated, otherwise it would not be found among domestic cattle. Interestingly, the heterozygous state Cs/cs+ results in a spotted colour called “colour sided”, found in many Texas Longhorn, for example. So if aurochs with the White Park pattern would have mated with typically coloured aurochs, “colour sided” aurochs would be the result – such a piebald colour is rather untypical for wild animals and probably of selective disadvantage (camouflage, especially for calves). However, a disadvantageous heterozygous state is not impossible for a wildtype allele. An aurochs with a “colour sided” pattern would have probably been considered a hybrid by eyewitnesses, but there are no contemporaneous notions of (alleged) aurochs-cattle hybrids running around in the wild that I am aware of.
In any case, the line drawings from Lascaux are the only evidence that would support a White Park pattern in aurochs, but the small black spots can also be interpreted differently. Perhaps the artist wanted to indicate curly hair. Many domestic bulls, especially those of populations/breeds where the bulls fight on regular basis (Chillingham, Betizu, Eringer, Lidia, some Heck cattle at Oostvaardersplassen), often have rather curly hair on head, neck, face and shoulder area what – I hypothesize – might protect their skin in a fight. Some, or even all, wild aurochs bulls might have had this trait as well (the curly hair between the horns is proven in any case). See this post.
So the evidence for this colour variant is very weak, but it could be worth to test aDNA from the suspected population (Southern European aurochs of the Pleistocene) for the Cs allele.
Note that I am neither saying that I “believe” these colours were present in wild aurochs, nor do I say that I consider that likely. I am just speaking of possibilities. To test these ideas, genetic tests of historic and prehistoric aurochs aDNA would be necessary. The same work has been done with wild horses, and revealed surprises. It is likely that testing aurochs would result in a confirmation of status quo, but it would be worth examining. However, the problem is that there is way fewer interest in cattle as animals than in horses and many geneticists might consider genetically examining the colour of wild aurochs too trivial.