Saturday, 18 November 2017

Wild horses pt. IV: The Przewalski's horse - is it still a wild horse?

This title is surely a bit provocative – it is of course zoological consensus that the Asiatic Przewalski’s horse, Equus ferus przewalskii, is the last living genuine wild horse that is extant today after the last western wild horses disappeared. However, advocates of a number of horse breeds that they purport as living remnants of the European wild horse or at least being strongly influenced by original wild horses, sometimes put the status of the Przewalski’s horse as a genuine wild animal into question. They argue that decades of breeding in captivity and introgression from domestic horses has altered the nature of the Przewalski’s horse, and claim that the situation is comparable to what has happened to their favoured “wild” breed: an original wild population has been influenced by domestic horses and artificial selection. As a consequence, they argue that if the Przewalski’s horse still deserves status as a genuine wild horse, which is zoological consensus, then so does their breed of choice. However, the often purported background stories for those breeds being “near-wild horses” are not tenable after objective examination for a number of reasons (see here), but what about the arguments against the original, wild status of the Przewalski’s horse?

The lineage of the Przewalski’s horse separated from that of domestic horses several millennia ago. The exact point of separation varies from study to study, depending on the molecular chronometer and its calibration. The maximum I found was 160.000 [1] and the minimum 38.000 [2] years ago. This comparably long reproductive separation resulted in a different karyotype, the Przewalski’s horse having 23 pairs of chromosomes and the domestic horse 24 due to a fission or fusion (depending on what is the plesiomorphic state), but they intermix readily and without fertility problems. In the millennia of living side by side in the Eurasian steppe, the Przewalski’s horse contributed genetically to the domestic stock (which is not only genetically [3] but also optically apparent, f.e. see some Mongolian horses), and vice versa. The Przewalski’s horse gene pool was introgressed by domestic horses, especially in the 20th century. Photographs of wild herds from 1954 showed individuals of divergent colours (Wikipedia), indicating admixture. The whole modern population descends from 13 founding individuals, one of them was a domestic Mongolian stallion [4]. Does this mean that the original, genuine Przewalski’s horse is lost and the modern population is an altered result of intermixture?
Orlando et al. 2015 made a genomic study compromised of a large sample of Przewalski’s horses, post and prior to the bottleneck (including the holotype specimen), domestic horses plus a late Pleistocene wild horse as outgroup. The result is that although there are genetic traces of intermixture, also including such having an effect on the phenotype such as an allele associated with increased withers height, there are still lineages in the population that are virtually free of admixture[3]. Also, height is a highly multifactorial trait, therefore it cannot be claimed that Przewalski’s horses are taller now due to admixture because of one allele – the average withers height is still between 122-142cm according to English Wikipedia, between 120-146 according to German Wikipedia (note that there is also sexual dimorphism in size). Przewalski’s horses are still uniform in their typical colour, sturdy build, robust head shape, erect and short mane, short-haired tail basis, a very lightly coloured almost white winter coat, and other typical morphologic differences to domestic horses such as thicker hooves (Wikipedia). I would even say that domestic cattle left a bigger trace on modern American bison than domestic horses did on the Przewalski’s horse, yet nobody is questioning the bison’s status as a wild animal. Also I found no source stating that Przewalski’s horses with a domestic karyotype have been observed.

Yes, the Przewalski’s horse seemingly intermixed with domestic horses continuously after their point of separation, but I see no compelling evidence that this fact altered the genetic integrity of this wild subspecies. Furthermore, domestic animals introgressed the gene pool of their wild counterparts everywhere they shared the habitat – this evident in European wild boar that show deviant, domestic colours and there is also the hypothesis that American wolves inherited black and other colour variants from domestic dog introgression several millennia ago (this might also explain blue-eyed wolves). Yet nobody is calling their wild animal status into question.

If Przewalski’s horses indeed lost part of their wild animal nature due to domestic introgression and being bred in captivity for a number of decades, it might be helpful to look at a checklist of aspects typical signs of domestication:

- Morphological paedomorphy
- Behavioural paedomorphy
- Reduced brain volume
- novel morphological/optical traits (very typical: colour variants, particularly white spots)
- Earlier maturity and increased litter size (the latter aspect is not true of domestic horses either, so let us ignore it for now)

Przewalski’s horses do not show any signs of morphological paedomorphy, not even if you compare photos of the early 1900s to modern individuals. Przewalski’s horses still always have the robust, donkey-like skull with small eyes and their proportions do not seem to be altered as well. I have not found any remarks in the literature stating that Przewalski’s horses as a whole lost brain volume; domestic horses have about 14% less brain volume than Przewalski’s horses [5]. Captive Przewalski’s horses also have 14% less brain capacity than wild counterparts [5]. Since there are no separate genetic lineage between wild and captive Przewalski’s horses, this should be applicable to phenotypic plasticity. According to Wikipedia, earlier maturity in captive Przewalski’s horses has been reported, but explained with better nutritional conditions in zoos than in the wilderness and as far as I know the same phenomenon can be observed in other zoo animals. The behaviour of Przewalski’s horses and domestic horses is well comparable, but Przewalski’s horses have a way higher aggression potential than domestic horses, especially the stallions. This is universal for this subspecies and evident in zoos as well as grazing projects. I once was told that zookeepers are more afraid of Przewalski’s horses than lions. Przewalski’s horses can be tamed and ridden to a certain degree, but this is also true for zebras, including the quagga.
There are not any novel traits found in any Przewalski’s horse, such as a new colour variant, or fur modifications. There are occasionally individuals showing a white streak along the face or white socks, which is applicable to introgression from domestic horses.
Looking at some deer populations which have been kept in game parks for many generations, we see incipient signs of domestications, such as new colour variants or typical domestic spotted patterns, or beginning paedomorphic skull shapes – you can find this in some roe deer, red deer and fallow deer in European game parks and this is what I would call an early state of slow domestication. But we do not see that at all in Przewalski’s horses.
All in all I think there is not one compelling reason to claim that the original Przewalski’s horse is gone, that it has been altered by man and hybridization, or that it is on the edge of domestication. I see nothing that calls their status as a genuine wild animal seriously into question, especially when we look at other wild animals. And even if the critics were right, it would not make any of the domestic horse breeds praised as near-wild horses “wilder” than they are (or not are, actually).

Literature

[1] Ryder et al.: A massively parallel sequencing approach uncovers ancient origins and high genetic variability of endangered Przewalski’s horses. 2011.
[2] Orlando et al.: Recalibrating Equus evolution using the genome sequence of an early Middle Pleistocene horse. 2013.
[3] Orlando et al.: Evolutionary genomics and preservation of the endangered Przewalski’s horse. 2015.
[4] Bunzel-Drüke, Finck, Kämmer, Luick, Reisinger, Riecken, Riedl, Scharf & Zimball: „Wilde Weiden: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung“. 2010
[5] Röhrs, Ebinger: Are zoo Przewalski’s horses domesticated horses? 1998.



8 comments:

  1. Thank you again for an interesting post, Daniël. I also do like the consistent extensive literature-links in your posts. One thing in the post was not clear to me: the holotype specimen used by Orlando... (if that is clear in the study do not bother, i downloaded it already but no time to read yet). Vriendelijke groet, Frank.

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    1. It means that they included the taxonomic holotypus specimen for Equus ferus przewalskii among other individuals of the subspecies.

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  2. Thank you Daniël, but does that mean the taxonomic holotypus is one individual specimen ?

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    1. Holotype specimen are always individual specimen.

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  3. This comment has been removed by the author.

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  4. Thank you, partner. This is a really interesting article on the Mongolian Wild horse and whether it should be called domesticated or not due to cross-breeding with domesticated horses. Speaking of cross-breeding with the said wild horse, I have a hypothetical question for you and you might be able to give me a good answer on this (it's for both my own scientific interest and a potential future post on my blog):

    You know about the Yukon horse (Equus lambei)? I have read that it is the oldest genome that has been able to be sequenced. And I've wondered if that means that if it could be cloned? I know that it would be a stretch, but if it was possible, would it be a good animal to use in both giving more genetic diversity to the wild horse (if the yukon horse was cloned via oocyte from a Przewalski's horse, so it could have the mitochrondrial DNA of the said horse) and used to resurrect the Tarpan? Any other effects could the Yukon horse clone(s) could have on this idea?

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    1. Concerning the Yukon horse, I doubt it can be cloned as viable cells are needed for that. But it could be genetically resurrected via genome editing using CRISPR-Cas9 as long as the full genome is known as far as I understand, but I am not a geneticist.

      As to the "Tarpan" (I assume you are talking about the predomestic wild type of the western wild horse subspecies, I avoid using that ambiguous term), this animal cannot be resurrected by selective breeding, just as the aurochs cannot be resurrected by selective breeding. You'd need a full genome for that as well, and use the same technique (genome editing). Crossing a resurrected Yukon wild horse with robust European landraces for releasing them into nature is not something I would do.

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  5. New study online: Coat colour adaptation of post-glacial horses to increasing forest vegetation

    https://www.nature.com/articles/s41559-017-0358-5

    german summary: http://www.izw-berlin.de/pressemitteilung/ueberlebt-dank-farbanpassung.html

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