Friday, 7 February 2014

Is the Exmoor less unique than we used to think?

A lot of Exmoor pony fanciers and also some people involved in rewilding consider the Exmoor pony the horse breed that is closest to the original European wild horse or even a surviving wild, undomesticated population [1]. And in my last post on this breed, I praised the Exmoor as the only virtually pure member of a feral pony type I called the “British primitive horse” that allegedly was the ancestor of many other native British horse breeds. A very interesting and well-sourced document I came across thanks to the Blogger user “Unknown” provides information that the Exmoor pony may be not as special as I and a lot of other people used to think.

Let’s start right from the beginning: the far-fetched claim that the Exmoor pony is a surviving wild horse has to be discarded because of paleontological and genetic data. Wild horses died out on the British isle at the end of the Mesolithic, and no equines are present in the archaezoological record of the island until domestic horses were introduced by the Celts [1,2,3]. Exmoor supporters claim that this is the case because the wild horses retreated into mountainous areas where fossilization is less likely, so that they might not have left a trace in the record [1]. But this is a mere speculation.
Genetically, the Exmoor pony is seemingly not set apart from other domestic horses. I am going to simply quote myself now (references below) so that I don’t have to repeat myself:

Inclusion of wild mares into domestic stock all over Europe is very probable for whole Europe based on the genetic evidence [2,3,4] and the fact that wild stallions are virtually intractable is probably the reason why only very few of them were used in domestication and therefore why domestic horses have such few Y chromosome variants. An Y haplotype or a complete Y chromosome not present in any other horse lineages would therefore be a very strong indication of local introgression in a breed (and a very high Y diversity in a domestic horse population would thus imply very strong introgression or even descendant from local wild horses). A 2004 study [4] showed that a large number of breeds have the same few Y chromosomes, including the Exmoor (and therefore ruling out that it descends from British wild horses or that it experienced much, not sex-biased local introgression)

Even more interestingly, Cieslak et al. 2010 examined mitochondrial haplotypes of predomestic and domestic horses. 87 haplotypes were recognized, 30% of them were found to be still present in modern domestic horses [5]. And the distribution of these haplotypes within the Eurasian horse population is not surprising assuming that all or most horses were domesticated in the Eurasian steppe. In the east, the number of retained predomestic haplotypes are much higher (Arabian 18, Akhal Teke 13) than of those in the far west of the continent (Fjord 5, Exmoor and Welsh 2). This is what is to be expected if the population was continuously inflated and fragmented during the expansion westwards [5]. The very low number of predomestic haplotypes detected in the Exmoor and its relative is especially revealing. Although it is possible that this was the result severe bottlenecks [2], anthropogenic or not, a conclusive indication for a genetic relationship of the Exmoor with western European wild horses special among other European horse breeds is still lacking.

The fact that all contemporary Exmoors display various shades of bay and brown and no other colours, that most of them have very prominent countershading and mealy mouth, and that only very few of them have white spots, is interpreted as evidence that the Exmoor is a wild horse-like animal with little intermixing with domestic horses by advocates because most wild animal populations have a stable phenotype [1]. But in fact there is no conclusive evidence that the Exmoors had a uniform phenotype prior to the bottleneck events and artificial selection – actually there are hints for the contrary [2]. Apparently the horses always were small, but there are no helpful references on the colours within the population over the time prior to the 18th century. Between 1805 and 1809, 81 Exmoor ponies were sold from the moor (a sample which might be of sufficient size) and their colours were documented. 33 were black, 19 were grey, 17 were bay, 9 were dun, 2 were chestnut and one was piebald [2]. It is argued that animals with non-Exmoor influence were captured from the more to maintain the alleged purity of the population, but there is no evidence for such an intention and since there is no earlier evidence for the colour of the horses/ponies at Exmoor, it simply cannot be said whether individuals displaying these colours were “pure” or “non-pure” [2]. Two illustrations in the Illustrated London News from 1835 show the capture of free-ranging Exmoors; these clearly show horses with long manes, their colour is either brown with a white blaze and a white “sock” or grey (one portrayed horse might be black) [2]. A chestnut stallion is mentioned as well. At this time, crosses with the Thoroughbred and Arab horse were present in the moor, so the portrayed horses could indeed be the results of intermixing [2]. However, the portrayed colours are in accordance with those described from the sales between 1805 and 1809, and they are not a support for the contemporary colour of the Exmoor pony having been present homogeneously in the population prior to the 20th century either. The notion made by Worthley Axe in the year 1906 is also interesting: “…the majority of so-called Exmoors are simply mongrels” [2]. 1860 some the larger members of the horse population at Exmoor were captured to be used as packhorses [2], I interpret that this means there also was size variation in the Exmoor pony back this time. The Acland herd, which made a considerable contribution to the modern Exmoor population, also had a number of greys in the herd in the year 1900. There is a record that suggests that they started to select out black Exmoors because they lacked the mealy mouth, indicating that this herd was artificially selected for this trait [2].
Dartmoor ponies. Some of the older Exmoor pony herds might have looked like this.
Postcranial bone material of Pleistocene horses has been compared with the Exmoor pony, and good matches have been found [1]. The problem with these comparisons, apart from the limited samples, is that the variation within predomestic and domestic horses is that large that they can hardly be distinguished only on the basis of their bones [2]. Furthermore, since the Exmoor is one of the “primitive breeds”, it is not surprising that an osteologic match has been found, except for the not examined skull material.
There are colour matches between the modern Exmoor population and wild horses. Countershading and mealy mouth are an ancestral trait, as is the bay base colour [6]. However, the presence of the allele for sooty bay/brown has yet to be confirmed in wild horses, and the lack of dun might not be a primitive character in European wild horses (see here), but there are cave paintings that look like they portray non-dun horses (see here and here), and the dun factor was not yet tested in predomestic horse populations [6], so I cannot rule it out and won’t use it as an argument against the Exmoor. But a colour match between a horse breed and their wild counterparts no way indicates either a close relationship nor direct descent or even that this breed is predomestic, neither do the other phenotypic matches regarding bodily proportions or the short mane. Have a look at how many dog, sheep or goat breeds show the colour of their wild type, and look at their resemblance in other optical features (examples here, here [ wildtype colour, yet clearly domestic] and here). Also the hardiness and wild behaviour under natural conditions does not necessarily endorse the view that the Exmoor is closer to the wild European horse than other breeds; ecologic and behavioural resembling wild animals is, not surprisingly, a feature of many landraces and can either be retained traits or secondarily developed traits because of their exposure to natural selection. Furthermore, mind that those primitive, less-derived features of the Exmoor are not unique to this breed (but that is not an argument against its purported primitive status of course).

You might remember my earlier post in which I adopted Sue Bakers proposal [1] that the Exmoor ponies represent the only virtually pure members of a primitive feral horse type that is ancestral to all other sturdy British landraces. This article made me rethinking this assumption; if the Exmoor always had a more or less heterogeneous phenotype prior to the 20th century, and since related landraces like the New Forest pony, Dartmoor Pony and others are heterogeneous today and there is no evidence either that they were homogeneously brown/bay with white muzzle before their evident intermixture with other breeds, it is more parsimonious that the Exmoor probably never was homogeneous in the past and nor that the original stock introduced by the Celts was.

Another possibility is they originally were indeed homogeneous but intermixture with other domestic horses had taken place before the documenting of their colours started to take place. Furthermore, intermixture with derived horse breeds like the Arab and the Thoroughbred evidently took place at in the late 19th century [2] and the modern homogeneous phenotype of the Exmoor apparently is a product of artificial selection. First of all, we know that the Exmoor went through several human-caused bottleneck events [1,2,3], and that it was selected for its bay/brown coat with countershading and mealy mouth right from the late 19th century till today [2]. It might surprise you that black and grey actually were permitted colours at the beginning of systematic Exmoor pony breeding with the formation of the Exmoor Pony Society in 1921 [2]. This shows that there must have been animals with such colours appearing in the population at that time, and that artificial selection purged them out. Additionally, white markings must have been present too, because they still appear today, albeit only rarely.

Iberian breeds that bear a certain similarity to the Exmoor, like the Garrano, Asturcon or Pottoka could be used an argument for the hypothesis that both those pony groups descend from a western European wild horse type that was separated when the English channel emerged and that looked like the British and Iberian ponies. However, this hypothesis is far-fetched when considering the aforementioned gap of equines in subfossil record of Great Britain. The mainland ponies must have been the base for the ponies the Celts introduced to the British isle, and there was exchange of culture and goods between Great Britain and Iberia during the bronze age and antiquity. Therefore it is in my opinion not surprising that the ponies of Iberia and Great Britain look alike, and no hypothesis of local ancestry from wild horses that is not supported, even contradicted by genetic data is necessary. Furthermore, it is important to know that the use of British ponies such as the Exmoor on breeds like the Pottoka during the 20th century is well-documented [7], although I think it must not be the case in all Exmoor-like northern Iberian landraces.

The Exmoor pony advocates involved in Rewilding will hate me probably for this post, but note that I am not biased against the Exmoor. I love the Exmoor pony. But the hypothesis that the Exmoor pony is strongly influenced by western European wild horses or even a surviving wild horse population seems to be zoologically untenable. And this is the reason why the Exmoor is “just a breed” from zoological perspective and not listed as a wild representative of Equus ferus as the Przewalski by zoologists – not because they are “conservative” or have preconceptions.
It seems, or is at least likely based on the historic and genetic evidence, that the Exmoor is a mix of local ponies of Celtic origins, and probably did not have a uniform appearance prior to artificial selection in the 20th century. Intermixture with more-derived horses probably was always there and is documented for the late 19th century. This is, in my view, actually similar to the nature of the Konik, although the Konik/Panje horse has no known feral/semi-feral history. Intermixture now is a problem for the pedigree of the horses in the moor again because non-Exmoor horses somehow got into the moor and interbreed freely with them [2].

That all is not to say that the Exmoor is not useful for ecologically substituting the wild horse in Europe, far from it. The Exmoor, with its sturdy and strong body, thick head and short mane, still has a “functional phenotype” resembling what we assume for the European wild horse (although the lack of dun, as outlined above, might not be an ancestral trait). Furthermore, its feral or semi-feral history made it retain or redevelop useful behavioural traits such as shyness towards humans and effective defensive behaviour against predators and a very hardy nature (not unique to the Exmoor, though). Therefore it is good that many rewilding projects use the Exmoor, although I think that a more diverse set of breeds should be used in whole Europe, what apparently is the case.

The article is really refreshing as it re-investigates the merely anecdotic evidence regarding wide-held opinions on this breed and its relations to the predomestic western European wild horse and provides a new approach to this breed. It shows that the study of extinct animals keeps being exciting when new data emerges or considerate revisions are done.


Literature

[1] Baker, Sue, 2008: Exmoor Ponies: Survival of the Fittest – A natural history.
Baker
[2] Peter Green, 2013: The free-living ponies within the Exmoor National Park: their status, welfare and future. A report to the Exmoor moorland landscape partnership.
[3] Bunzel-Drüke, Finck, Kämmer, Luick, Reisinger, Riecken, Riedl, Scharf & Zimball: „Wilde Weiden: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung“. 2010
 [4] Lindgren et al.: Limited number of patrilines in horse domestication. 2004
Lindgren
[5] Cieslak M, Pruvost M, Beneke N, Hofreiter M, Morales A, Reissmann M and Ludwig A
(2010) Origin and history of mitochondrial DNA lineages in domestic horses. PLoS ONE
[6] Pruvost et al. 2011: Genotypes of predomestic horses match phenotypes painted in paleolithic works of cave art
[7] L. J. ROYO, I. A ́ LVAREZ, A. BEJA-PEREIRA, A. MOLINA, I. FERNA ́ NDEZ, J. JORDANA, E. GOń MEZ, J. P. GUTIE ́RREZ, AND F. GOYACHE: The Origins of Iberian Horses Assessed via Mitochondrial DNA. 2005.

  

3 comments:

  1. Nice one. Great read.
    The paper you referred to was pretty comprehensive. It would be interesting to hear what Exmoor pony enthusiasts have to say in their defence.
    So far there is silence....
    Sam.

    ReplyDelete
  2. Losino horse, an ancient breed of iberian horse. http://zaragozasalvaje.blogspot.com/2014/01/caballos-losinos-al-matadero.html

    ReplyDelete
  3. Hi there,
    There is strong evidence that the Iberian Exmoor type horses have Celtic origin or at least strong Celtic influence, the Celtic culture was predominant in all Western Iberia until the first centuries of our present time surviving in the NW all the period of Roman domination. One of many examples is the word Garrano (type of small horse from NW of Iberia), this word has Celtic origin as many others in the region and one can still find similar or equivalent words in the modern Celtic languages like the Irish "gearr" that means short or small :)

    ReplyDelete