Were European wild horses black or black dun?

Coat colour is the only aspect of the life appearance of the Holocene European wild horse that can be determined with a high degree of certainty thanks to studies examining the coat colour genotype of aDNA samples from wild horses. 

A 2017 study found that the a allele on the Agouti locus, which originated in Iberia in the late Pleistocene and which causes a black phenotype on an E⁺/E⁺ genotype in non-dun horses, became increasingly common among European wild horses in the Holocene until it became the prevalent allele in the later Holocene [1]. This alone does not tell us the phenotype of the animals, because they could have either been black or black dun (also called grullo, mouse dun or blue dun). The question now is: was the late Holocene European wild horse black or black dun?

The Dun locus was resolved in 2015 [2]. It was found that there are three alleles on this locus in horses: dun D, which is wildtype and basal for all living Equus, wildtype non-dun d1 which dates back well into the Pleistocene, and the domestic non-dun d2. 

The black allele was restricted to Europe, it was not found in Siberia so far [1]. The wildtype non-dun allele, on the other hand, was so far found only in Siberia [2]. This could lead us to conclude that there were no horses of a genotype a/a d1/d1, therefore being black. However, it must be considered that the Dun locus was so far only tested for Siberian wild horses, and not for European wild horses. I think it is well possible that there were horses with an a/ad1/d1 genotype, because genetics suggest that during the Pleistocene there was one large panmictic population of wild horses from the Pyrenees to Siberia [3], what makes it possible at least that the d1 allele was also found in European Pleistocene wild horses, which were the predecessors of European Holocene wild horses [4]. Furthermore, some cave paintings might depict non-dun wild horses, such as those at Lascaux, which clearly show black and blackish brown or dark brown horses among a large yellowish-brown horse. This must be viewed with caution, however, because the exact shade of a cave painting is always dependent on the pigments available to the artists and may not necessarily reflect the true colour of the animals. But so far, cave paintings proved to be rather accurate on horse colours. F.e. the leopard spotted horses illustrated at Pech Merle were found to be based on horses having that colour [5], and Ekain also shows black dun among bay dun horses. Therefore, I think it is well-possible that there were black wild horses at least during the Pleistocene. 

This might have depended on another allele as well. Sponenberg & Bellone (2017) state that seal brown, which is a colour found in Exmoor ponies, can be caused by a black phenotype being diluted by the dominant pangare allele Pa⁺[6]. Pangare is a basal equine allele because all living wild equines have it, so it must have been the ancestral state in European wild horses as well and some cave paintings illustrate it very clearly. If seal brown is indeed caused by a black phenotype being diluted by pangare, the existence of black wild horses would depend on if the non-pangare allele pa^np was present in wild horses or not. So far, this has not been tested. I used to think that black suppresses pangare, also because black dun horses always have a dark head while they sometimes may have faint countershading on the body (and as foals sometimes have a white muzzle). The genetic background of seal brown should be tested to be sure. 

Another clue could be historic evidence. There are several records describing free-ranging horses in Europe, and until recently it was unclear whether they were truly wild horses, feral horses or hybrids. A recent study confirmed that they were most likely hybrids of feral domestic horses and the native European wild horses. Go here for a summary of those records on these hybrid populations. What is striking is that black dun horses seem to dominate these reports. Black horses are mentioned only very rarely. Of course, the phenotype of hybrids is not very strong evidence, only a hint, as the domestic horse introgression likely has changed the frequency of the colour phenotypes present in the populations. But black dun is not very frequent among domestic horse breeds. Also, dun in general is less frequent than both non-dun alleles in domestic horses, so that it is unlikely that introgression from dun-coloured domestic horses turned an originally black wild population into a black dun hybrid population. Considering that non-dun is more frequent than dun in domestic horses, I do not think that introgression increased the frequency of dun in the wild population and instead assume that the population was originally dun in most of the individuals. If the original wild population was black, this colour probably would be mentioned much more frequently than black dun in those reports on the hybrids. 

 

All in all, I think the evidence for late Holocene European wild horses being mostly black dun outweighs that for them being black. I think it is well possible that there were black wild horses, at least in the Pleistocene, but more research would have to be done. For example, Holocene European wild horse remains could be tested for the dun and non-dun alleles as much as for the pangare and non-pangare alleles, and the genetic background of seal brown should finally be confirmed. 

 

Literature

 

[1] Sandoval-Castellanos et al.: Coat colour adaptation of post-glacial horses to increasing forest vegetation. 2017.

[2] Imsland et al.: Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. 2015. 

[3] Cieslak et al.: Origin and history of mitochondrial DNA lineages in domestic horses. 2011. 

[4] Fages et al.: Tracking five millennia of horse management with extensive ancient genome time series. 2019.

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

[6] Sponenberg & Bellone: Equine color genetics. 2017. 

 

 

 

 

Taxonomy: how to classify domestic animals?

Back in the time of Linnaeus when modern taxonomy was started in 1758, some domestic animals were classified as the members of the species they were derived from, others as distinct species. Taxonomists have been trying to find a universal standard to handle the naming of domestic animals. The main question is: should they be regarded as members of the species they were derived from, or as distinct species? 

Till today, there is no consensus on how to answer this question. Both sides have good arguments for domestic animals either being subspecies of the wild species they derived from or being distinct species. 

 

Arguments for domestic animals being distinct species 

 

Domestic animals have undergone a unique evolutionary path because their evolution is mainly controlled by selective pressures determined by another species, Homo sapiens. Domestic animals have, in contrast to their wild counterparts, experienced a kind of coevolution with humans, and the abiotic and biotic factors of the “natural” (not human-influenced) ecosystem played either only a minor role in the evolution of domestic animals or absolutely none. This is a drastic difference between wild and domestic. Therefore, evolutionary, wild and domestic animals are radically different because the evolution of wild animals is not controlled (influenced, in some cases certainly, but not totally controlled) by humans while that of domestic animals is to a very large extent.

Also, domestic animals differ from their wild counterparts in structure of the genome. The absence of natural selection, the artificial selection executed by humans as well as inbreeding usually leads to a mutation accumulation (often of deleterious alleles) and decrease of genetic diversity, what is called the “costs of domestication”. Thus, domestic animals also differ genetically from their wildtypes. 

Morphologically, domestic animals are distinct from their wild ancestors, albeit the degree differs from breed to breed. For example, a Chihuahua differs from a wolf much more dramatically than a German shepherd dog. But general morphological differences between wildtype and domestic are found in any domestic form. In most cases, the difference in morphology between wild and domestic exceed the differences found between wild subspecies of a wild species. 

One of the most obvious differences between wild and domestic animals is in the behaviour towards humans. These differences are caused by modifications of the endocrinology and neurology, which probably also cause many of the morphological differences between wild and domestic (for details, see the Dedomestication series). 

Due to the absence of natural selection and the more or less intense artificial selection, domestic animals also differ from their wildtypes in physiological aspects. These aspects are directly related to the evolutionary fitness of the animals. I outlined the physiological differences between wild and domestic yaks in this post. A similar reduction of physiological fitness is to be expected in other domestic animals. 

 

Therefore, there are drastic differences between wild and domestic in their evolutionary history and presence, genetics, morphology and physiology. These differences definitely exceed the degree of variation found in wild species that have not been domesticated. Therefore, a status of domestic animals as a distinct species would be justified from an evolutionary, genetic, morphologic and physiologic standpoint. 

 

Arguments for domestic animals being members of the species they were derived from 

 

Domestic animals and their wildtypes are usually able to interbreed without fertility barriers, i.e., they can produce fully fertile offspring. According to Mayer’s species definition, they would be one species because of that. However, Mayer’s species definition does not work universally (for example, wolves, coyotes and golden jackals would be one species because they can interbreed without fertility barriers). 

Another argument for domestic animals being members of the wild species they were derived from is that the social behaviour of domestic animals does not differ from that of their wildtype in most cases when given the chance to life under natural circumstances. 

Also ecologically, most domestic animals are much like their ancestors in habitat preference, food choice and ecologic niche when living under natural circumstances. 

 

It appears that there are good arguments for both sides. It simply is the case that domestic animals are very different from their wildtype on some aspects, and very similar on other aspects. Because of that, I do not think that there will ever be a consensus on how to classify domestic animals. 

Another problem is different domestic animals probably require different solutions. Domestic dogs for example, do not have the same social structure as wolves (as far as I know). Also, ecologically they are not identical. Feral dogs often live as commensals to humans, while wolves do not. So, it might be justified to classify dogs as a distinct species, while many other domesticated animals might be classified as subspecies of their ancestral wildtype. But when not finding a universal standard for all domesticated animals, taxonomy becomes even more arbitrary than it already is. Another problem is that not all domestic breeds/populations are domesticated to the same extent. The differences from the wildtype, be it genomic, ecologic, behavioural, morphologic or physiologic, might be more intense in some breeds than in others. Extreme in some cases, not extreme in others. 

 

This leads to the question if domestic animals should be regarded as taxa at all. Domestic animals are highly heterogeneous, they differ in the extent to which they are domesticated, and often experienced secondary introgression from their wildtype and even other species during their domestication, often only certain populations. In some cases, it even is questionable if all members of a domesticated form can reproduce with each other under natural circumstances due to physical barriers (such as in the case of Chihuahuas and Irish Wolfhound). Maybe it would be better to give domestic animals no taxonomic status at all, but rather regard them as inhomogeneous, artificially created populations of certain wild animals that have been domesticated to a varying extent – varying when comparing different domestic forms to each other (f.e. horses versus dogs) as much as within the domestic forms (f.e. Spanish fighting cattle versus Fleckvieh). 

 

Who's better: Taurus cattle or Tauros cattle?

Sometimes, Taurus and Tauros cattle get confused. This is not surprising, considering that their name is almost identical. But I think it is important to differentiate between both breeds, because of their different history and also different quality as a “breeding-back” result. This post is going to examinate which breed has so far been more successful in approximating the goal of “breeding-back”.  

 

Some may say it is unfair to compare the two breeds because Taurus cattle were created in 1997 while the TaurOs Project started in 2009. However, I am only comparing individuals of the same crossbreed generation in order to be make it comparable. The youngest Tauros cattle may be of the fourth or fifth generation, the youngest adult ones (only adult cattle can be compared as the traits are not full expressed in juvenile and subadult individuals) may be of the second and third generation. Therefore, I will compare only animals of the first, second and third crossbreed generation of both projects. 

I compare them based on the animals that are available to me. The largest Tauros cattle herd is in Keent, Netherlands, but that herd is not very present on the web, only single individuals. The Kettingdijk herd has been documented very well by Geer vanne Smeed (go here for the flickR stream) and there are also many photos and videos of the Milovice herd in the Czech Republic. For a collection of recent photos of several Tauros cattle go here. Regarding Taurus cattle, I only use the Lippeaue and Hortobagyi herds for comparison, because they can be considered the only “true” Taurus cattle herds in the strict sense. Most of the other herds are basically Heck cattle herds that included single Taurus individuals, sometimes as sires (f.e. the Schmidtenhöhe herd, the Cuxhavener Küstenheiden herd). There is a continuum between Heck and Taurus cattle. I only use the “source” of Taurus cattle, Lippeaue and Hortobagyi, for the comparison. Taurus cattle are often labelled as “improved Heck cattle”, but considering that in the Lippeaue there was little to no backcrossing with Heck but they relied heavily on backcrossing with Sayaguesa, and that the portion of Sayaguesa makes up to 50% of the genetic composition of the animals there, the Taurus cattle in the Lippeaue are actually more “improved Sayaguesa” than “improved Heck cattle”. In Hortobagyi, the situation is more diverse as they have more herds and more breeds. 

 

Size 

 

Several Taurus cattle individuals have been measured. The Sayaguesa x Heck bull Lucio was between 160 and 165 cm tall at the withers, the Sayaguesa x (Heck x Chianina) bull “Laokoon’s brother” is about 172 cm tall at the withers. Three cows have been measured, and they ranged between 153 and 155 cm at the withers. Considering that they use Sayaguesa, which can reach up to 170 cm at the withers and Chianina, which even surpass that height, the size of Taurus cattle is what is to be expected. 

For Tauros cattle, no measurements have been published so far. But based on the photos and videos available on the web, they cannot be very large. I saw a video that is not online anymore of the Maremmana x Pajuna bull Manolo Uno and he was only insignificantly taller than the Highland crossbreeds (and Highland is a rather small breed). There is also a photo of the Maashorst bull (probably of the same breed combination) which I cannot find anymore that shows it next to a person, and it did not look large either. If Tauros cattle are not that large (not reaching 160 cm, the lower limit of European aurochs bulls), I would not be surprised considering that many of their founding breeds (Pajuna, Highland, Maronesa) are not large. For the size, the Tauros Programme relies on Maremmana, which also may reach 170 cm at least occasionally, but their Maremmana bull at Keent was not that large, it was about the size as the Pajuna bull. We simply need measurements from at least a few bulls to get an idea of Tauros cattle. 

 

Colour 

 

Taurus cattle mostly have the right colour with no domestic colour mutations, except for the recessive dilution allele(s) contributed by Chianina that cause a diluted coat colour in some individuals. Considering that most of the Taurus cattle at the Lippeaue are part Chianina, the allele(s) might be widespread in the population. Small white spots occur rarely in the Lippeaue, and in some Holstein-influenced individuals in Hortobagyi. 

Tauros cattle show a wider variation spectrum than Taurus cattle concerning colour. The populations have the recessive dilution allele contributed by Maremmana and Tudanca that removes the red pigment in the coat, and also Simmental dilution contributed by Highland. Several individuals also have the dominant brindle allele, contributed by Highland. White spots seem to be a little bit more common than in Taurus cattle. 

 

Sexual dichromatism 

 

I did a post on the sexual dichromatism found in the Lippeaue for the year 2015. It occurs that more than 80% of the individuals display the right colour for their sex. It is, of course, possible that some black bulls inherit black cows and that some cow-coloured cows inherit bulls with a saddle. But all in all, I think the sexual dichromatism in the Lippeaue is rather good. The dichromatism in Hortobagyi is slightly less good, as dark cows are more common there than in the Lippeaue. 

The sexual dichromatism in Tauros cattle is less clear. Bulls with a saddle are not a rare sight, as well as pretty dark cows. 

 

Morphology

 

The morphology of Taurus cattle is variable. Most individuals are long-legged, but also have a trunk that is longer than in the aurochs (a problem found in most taurine cattle). Most individuals have a comparably long snout, although probably not to the same extent as in the aurochs. A hump is always present. Some individuals are more massive than others, but the body morphology of single individuals such as Lamarck or Lisette is quite good. 

Tauros cattle are even more variable concerning body morphology, skull morphology and proportions. Especially the Highland influenced individuals can be rather short-legged and massive (go here). Some bulls resemble Heck cattle in build. Some individuals are comparable to Taurus cattle regarding morphology. What is nice is that some Tauros cattle bulls have rather large humps, but there are also those with a small hump. The skull length varies greatly, some individuals have the same skull shape as Taurus cattle, others can be rather short-faced. 

 

Horn size 

 

The horn size of Taurus cattle is variable. Some individuals in the Lippeaue, such as the bull Lamarck, can have horns that are within the variation range we find in the European aurochs, others have smaller horns than what is average for the European aurochs. Some individuals in the Lippeaue have horns only slightly larger than in Chianina. In Hortobagyi, the average horn volume of Taurus cattle is larger due to the influence of Watussi and Grey cattle. The horn dimensions match those of the European aurochs quite often in Hortobagyi. 

Tauros cattle are variable regarding horn size as well. Some individuals can be small-horned too, but on average the horn size in Taurus cattle is larger than in the Lippeaue Taurus cattle, but not larger than in the Hortobagyi Taurus cattle. 

 

Horn curvature 

 

The horn curvature of Taurus cattle is variable, but the horns always face forwards in an aurochs-like angle (the angle should be between 50 and 80°). Some individuals, like Lamarck, Lerida, Loxia and Lisette, have inwards-facing horn tips, sometimes to the same extent as in the aurochs. Other individuals may have outwards-facing horn tips, particularly the cows. 

The horn curvature of Tauros cattle is variable as well, more so than in Taurus cattle. The horns of many bulls do not curve inwards, and several cows have lyre-like horn shapes due to influence from Maremmana and Highland. In some bulls, probably the Maronesa-influenced ones, the horn tips face inwards. The horns of some bulls face forwards in an aurochs-like manner, others have too upright horns. Some bulls have horns very reminiscent of those of some Heck or Cachena bulls (go here, for example). 

 

All in all, I think that it would be most fair to conclude that Tauros cattle are somewhere between Heck cattle and Taurus cattle in quality as a “breeding-back” result. Taurus cattle are larger, the horn shape matches the aurochs more often than in Tauros cattle, their sexual dichromatism is clearer, and the Taurus cattle population does not include extremely short-legged chubby individuals that are found in the Tauros cattle population, at least not to the extent found in the Highland crossbreeds. Tauros cattle seem to be more variable on each aspect than Taurus cattle. 

So far, Tauros cattle have not reached the quality of Taurus cattle. In order to catch up, the TaurOs Project would need strict selection. Some individuals certainly have potential, that is undoubtedly the case. But based on what the herds currently look like (f.e. their inhomogeneous colours and morphologies), as much as the fact that they have several bulls per herd instead of one quality sire, I wonder if they select their animals at all. Perhaps their plan was simply to crossbreed a number of breeds and then let them breed for themselves rather than strategic selective breeding over several years or decades. 

Photos of Tauros cattle

Browsing for Tauros cattle photos, I found some recent photos of Tauros cattle bulls and also cows from the Netherlands. The breed combinations of the animals is top secret as usual (or perhaps unknown since there seems to be no herd book and several bulls per herd), but in some cases the looks of the animals suggests influence from certain breeds. 

Photo 1  This bull is likely Maronesa-influenced, perhaps it even is a pure Maronesa. 

Photo 2 The body shape of this bull is rather good, I love the large hump. The legs could be longer though.

Photo 3 Rather Heck cattle-like horns, perhaps Maremmana ancestry. 

Photo 4 Maybe Pajuna influence. 

Photo 5 Certainly Highland-influenced, possibly also Maremmana because of the horns. The body is very short-legged and massive, also a very paedomorphic skull shape. I would not continue to breed with this animal.

Photo 6 Very Heck cattle-like horns, very likely Maremmana-influence. 

Photo 7 The same individual as on photo 6. 

Photo 8 Perhaps a pure Limia bull. 

Photo 9 Bull from Maashorst, very likely Maremmana-influence. 

Photo 10 Heck cattle-like head and horns, but a superb hump. 

Photo 11 Resembles a Taurus bull, no idea on the breed background. 

Photo 12 Quite possibly a Maronesa x Maremmana cow. 

The photos don't tell much about the size of the animals, but some of the cattle certainly have potential. Especially the bull on photo 2, if it had the horns of the bull on photo 1 it would be a great success for the project. Maremmana seems to have had a big influence in Taurus cattle, if they would increase the influence of Maronesa now by using Maronesa-influenced breeding bulls they would probably increase the quality of the animals, at least in terms of horn shape. 

What the Near Eastern aurochs looked like

Taurine cattle in Europe descend from both the local European aurochs populations as well as the Near Eastern aurochs. Initially, the aurochs was domesticated in the Near East about 11.000 years ago. There was probably no domestication of aurochs in Europe, but there was subsequent introgression from local populations in Europe (go here). Nevertheless, the Near Eastern aurochs (which was still a member of the primigenius subspecies) is the main ancestor of the taurine cattle on this world. Therefore, it would be interesting to know what the Near Eastern aurochs looked like. 

 

The problem is that there are no historic descriptions describing the life appearance of the Near Eastern aurochs and also the skeletal remains have not been described in detail in the literature yet. However, we know that the African aurochs and the aurochs of Europe were morphologically more or less identical (see van Vuure, 2005), so that it is highly likely that the Near Eastern aurochs had the same morphology. The question is, in this regard, how large the aurochs in the Near East were. The giants of 200 cm withers height were probably limited to Europe (only “probably” because the very old remains of an aurochs from Tunisia of 700.000 years ago were very large as well), so it is not likely that the aurochs in the Near East reached the same size. But it is also unlikely that the bulls were smaller than 160 cm, because there are no aurochs remains of either Bos primigenius primigenius or Bos primigenius africanus that indicate bulls smaller than a withers height of 160 cm. 

Regarding the coat colour, it is questionable if there were any differences to the aurochs of Europe. So far, the evidence for colour saddles in aurochs bulls is limited to Africa, while there is only evidence for black bulls in the primigenius subspecies. 

A difference between the European primigenius aurochs and domestic cattle is the colour of the dorsal stripe. While historic evidence suggest that the dorsal stripe in bulls of at least the Central and Eastern European aurochs was whitish grey (according to Sigismund von Herberstein it was composed of white hair mixed with black hair, see van Vuure 2005), the dorsal stripe of wildtype coloured domestic bulls is grey only in breeds that have the mutation that removes the red pigment, such as Podolian cattle or Tudanca, while it is yellowish, reddish or reddish brown in all wildtype coloured bulls that have red pigment in their coat. Now the question is if the dorsal stripe of all aurochs populations, or at least of all B. p. primigenius populations, was whitish grey or if the dorsal stripe of the Near Eastern aurochs, which are the main ancestors of domestic cattle, displayed the colour palette from yellowish to reddish brown. As there is no evidence telling us anything about the colour of the Near Eastern aurochs, we will never know. However, we know that Southern European aurochs share the mitochondrial T haplotype with cattle, and it is very likely that the Near Eastern aurochs had the T haplotype as well. Central and Northern European aurochs had the P haplotype. Therefore, as Northern and Southern primigenius aurochs differed genetically, it is not impossible that there were subtle differences in coat colour characteristics such as the dorsal stripe. If the Near Eastern aurochs had a whitish grey dorsal stripe like those in Europe, the colour of the dorsal stripe is a general difference between the wildtype (B. p. primigenius) and the domestic form. 

 

Literature

 

Cis van Vuure: Retracing the aurochs – history, morphology and ecology of an extinct wild ox. 2005. 

Wild horses: bay, wildtype bay, seal brown, brown?

I repeatedly wrote on my blog that western wild horses had, according to genetic studies, several colours: black, black dun, bay, bay dun and leopard spotted. 

The basis for the presence of bay in wild horses is that genetic tests in predomestic DNA samples have identified the presence of both the allele a and A on the Agouti locus in wild horses [1]. But it is, unfortunately, not as simple as it seems at first. 

A bay European wild horse. All rights reserved

There are two types of bay colour: “normal” bay and wildtype bay (or wild bay). The wildtype bay is similar to bay but lighter in colour, and only the toes are dark-coloured while in bays the whole legs are dark. Wildtype bay is hypothesized to be caused by the hypothetical allele A⁺ [2]. Personally, I do not believe that wildtype bay is actually the wildtype version of bay. The dark areas on the legs in Przewalski’s horses cover almost the entire leg and not only the toes, as in horses of a normal bay colour, what suggests that they share the allele which would then very likely be the wildtype allele. So, we have bay and a form of bay which is called wildtype but is not necessarily wildtype. 

And then there is seal brown, a colour found for example in Exmoor ponies. The genetic background of seal brown seems to be unclear. It is speculated that it is caused by the hypothetical allele A^t on the Agouti locus. There was a genetic test for this allele, which is now considered inadequate, and the sequence of the allele has not been published [2]. 

Druml et al have a different hypothesis on the genetic background of seal brown. They postulate that this phenotype is caused by the genotype A/a, therefore being the result of heterozygosity of the allele causing black in homozygous individuals (a) and the allele causing bay in homozygous individuals (A). What is striking is that they consider bay itself to be the result of a heterozygous genotype, namely A/A E/e [3]. The e allele is the domestic allele on the Extensionlocus that causes a chestnut colour in homozygous individuals. Therefore, bay would be a domestic colour, and the wildtype E/E A/A colour would be brown, which, according to them, is caused by that genotype. I have to say that I doubt the purported genotypes for these colours, for once because they differ from what is usually considered to be the genotype for those colours, and because 1) if seal brown was the result of A/a, there would be not only bay and seal brown Exmoor ponies but also black Exmoor ponies, which is not the case in the current population. Also, if bay was the result of the genotype E/e, there would be chestnut (e/e) Exmoor ponies, which is not the case and 2) if bay was the result of a heterozygous state, it would be impossible to breed a breed that is exclusively bay. However, there is at least one such a breed, the Cleveland Bay horse [2]. Therefore, bay cannot be the result of a heterozygous state. Brown, is, as defined by Sponenberg and Bellone (2017), sooty + bay. The genetic background of sooty is unclear [2].

One could assume that since, according to the literature, both the alleles A and a but not A⁺ and A^t have been found in European wild horses the case is clear that seal brown and wildtype bay are not wildtype colours. But the problem is that the common genetic test for the alleles A and a can only confirm the presence of a, and the presence of A is deduced by the absence of a [2]. Therefore, the test cannot discriminate between A, A⁺ and A^t (if the latter two exist at all). If the authors of the papers proposing the presence of A in wild horses used that test, the presence of the hypothetical alleles A⁺ and A^t in wild horses cannot be ruled out currently. 

To further complicate the subject, Sponenberg and Bellone (2017) state that seal brown can also be caused by the pangare allele Pa⁺ diluting a black phenotype [2]. This would have consequences for the phenotype of wild horses. If that is correct, it would mean that this form of seal brown is a wildtype colour and that the non-pangare allele Pa^npwould have to have been present in wild horses besides the pangare allele, otherwise black and other non-pangare phenotypes would be domestic colours. The pangare allele has been identified [2], therefore I hope that it will be tested for extinct wild horse samples one day. 

Another factor that plays in the game is the Dun locus. Since 2015 we know that there are three alleles on this locus: dun (a wildtype colour identified in all living wild equines and Pleistocene wild horse samples), non-dun 1 d1 which is wildtype as well, and non-dun 2 d2 which is domestic [2,4]. Wildtype non-dun and domestic non-dun look different; the former have a clearly visible dorsal stripe and the surrounding colour is lighter than in the latter, the latter are darker all over the body [4]. Wildtype non-dun bay horses, such as some Gotland ponies, almost look like bay dun horses, only slightly darker in shade. 

 

So, which colour did the wild horses have that were neither leopard spotted, black, black dun or any other form of dun? I think that this cannot be said with a 100% certainty with the current knowledge on horse colour genetics and the current genetic tests we have. Very likely the horses had the A allele since it is the basal allele of wild equines, but the presence of other alleles on the Agouti locus cannot be ruled out yet. The genetic background of sooty and seal brown would have to be identified and tested for wild horses. Also, it would be interesting to know if wild horses had the non-pangare allele. The colour of Exmoor ponies, however, is domestic in any case because so far this breed has been found to exclusively carry the domestic non-dun 2 allele d2. It would be interesting to know what seal brown combined with a d1/d1 phenotype would look like. 

 

Literature

 

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

[2] Sponenberg & Bellone: Equine color genetics. 2017.  

[3] Druml et al.: Discriminant analysis of colour measurements reveal allele dosage effect of ASIP/MC1R in bay horses. 2018. 

[4] Imsland et al.: Regulatory mutations in TBX3 disrupt asymmetric hair pigmentation that underlies Dun camouflage color in horses. 2015.