The post-domestic wildtype: Is the Przewalski's horse wild or feral?

EDIT: A 2021 study found that the Botai horses were not domestic, therefore the Przewalski's horse is still a predomestic, never domesticated wild horse. That explains why it has no domestic traits. 

The Przewalski's horse - wild or feral?

The Przewalski’s horse was long considered the last strain of wild horses that remained undomesticated, while all other populations have been wiped out by man. But as most of my readers will know, this view has been challenged in recent years. A study by Gaunitz et al. 2018 found that the modern Przewalski’s horse descends from the domesticated horses of the Botai culture, the earliest domesticated horses known so far [5]. As it seems, these domesticated horses were not of the caballus/ferus subspecies, but of the przewalskii subspecies. Assuming this is correct, the modern Przewalski’s horse has to be considered a feral form of horse, and not a genuine wild horse, according to the authors of the study. I, however, think that there is still a way to consider the Przewalski’s horse a wild animal. 

 

Domestic animals all show similar traits as a result of domestication. This is called the “domestication syndrome”, which I covered here several times on my blog (go here or here). Classic symptoms of the domestication syndrome are a spotted/piebald coat colour, paedomorphy (the retention of juvenile traits into adulthood), less intense fight-or-flight reaction (i.e. the animals become more tame and agreeable, even trainable), reduced sexual dimorphism, earlier maturity, loss of seasonal adaptions and a reduced brain volume [1,2]. You find these traits in all currently domesticated mammals, be it dogs, cats, sheep, goats, domesticated silver foxes or domestic horses of the ferus/caballus subspecies. Even feral domestic horses, such as mustangs in North America or brumbies in Australia, show vestiges of domestication (f.e. piebald coat colour). Does the Przewalski’s horse show any of these typically domestic traits, being a once-domesticated horse? 

Although the coat colour variation of the Przewalski’s horse was greater before the genetic bottleneck event (f.e. more different nuances of brown), piebald individuals have never been documented. All known individuals seem and seemed to be of the bay dun base colour, which is also found in other equine species such as the kiang or onager. 

Skull of a Pleistocene wild horse from Denmark (left), a Przewalski's horse (middle) and an Exmoor pony (right)

Also, the Przewalski’s horse cannot be considered paedomorphic. A comparison between the skull of a Przewalski’s horse, a wild Pleistocene horse that surely was undomesticated and a robust domestic breed, the Exmoor pony, shows that the Pleistocene skull and the Przewalski’s horse’s skull look very similar while the Exmoor pony skull clearly has a shorter snout, larger eyes and a less massive lower jaw. The Exmoor pony’s skull has to be considered paedomorphic, while the Przewalski’s horse’s skull shows no signs of paedomorphy (it has smaller eyes, a longer snout, a massive lower jaw). 

Do Przewalski’s horses have a reduced brain volume? The brain volume of Przewalski’s horses has never been compared to that of an older wild horse, but domestic horses have 14% less brain case capacity and 16% less brain weight than wild-living Przewalski’s horses [3]. The brains of Przewalski’s horses therefore are larger than those of domesticated horses (it has to be noted that Przewalski’s horses living in zoos have smaller brains though [3]). The brain volume of wild-living Przewalski’s horses are therefore probably not smaller than those of predomestic wild horses, or at least not significantly otherwise it would be comparable to that of the domestic horse. 

Przewalski’s horses reach maturity later than domestic horses. While domestic stallions reach maturity between the 12. and 20. month, mares reach it after 12 to 18 month. Przewalski’s horses reach maturity after two years (Wikipedia). This is not that much of a difference, but it is later. It is not known how long it took predomestic horses to reach maturity. According to Wikipedia, other wild equines such as the onager also reach maturity at the age of two years. 

Sexual dimorphism is not as considerable in equines as in, for example, wild bovines. In all species and subspecies of extant equines, including both the Przewalski’s horse and the domestic horse, the males are slightly larger than the females. 

Both the Przewalski’s horse and the domestic horse have retained some degree of seasonality in their reproductive circle, most foals are born in spring. Therefore it cannot be claimed that the Przewalski’s horse lost seasonal reproductive adaptions as many domestic animals did. 

Regarding the behaviour of the Przewalski’s horse, it can be said that there are noticeable differences to the domestic horse in temperament and trainability (while the social behaviour is more or less identical). Przewalski’s horses are considerably more aggressive, especially the stallions. These often fight very harshly in a herd, sometimes to death [4]. They react very aggressively to other equines in their range and may even attack their mares, and humans (while free-ranging domestic horses do not attack humans) [4]. While free-ranging domestic horses can be transported without sedating, Przewalski’s horses have to be sedated [4]. Because of their aggressive and mostly untameable behaviour, Przewalski’s horses are much more difficult to handle than feral domestic horses. Nothing in their behaviour would suggest that the Przewalski’s horse is not a wild but a feral horse. Bunzel-Drüke et al. write in 2011: “Keeping Przewalski’s horses should be left to specialists. These animals are not just another horse breed, but a genuine wild animal, which is to be handled totally different than feral domestic horses due to the aggressiveness of the stallions”[4].  In rare cases, Przewalski’s mares have been trained, such as the individual Vaska, which was trained to be ridden. This can also be done with zebras (go here for a video), which are undomesticated, so it is possible to do this with a wild equine and hence the (limited) trainability of single Przewalski’s horses is not an argument for a domestic nature. As outlined above, the behaviour of Przewalski’s horses differs from that of feral domestic horses by the high level of aggression and the difficulty to handle it.

 

All in all, when taking a close look at the Przewalski’s horses’ biology, it becomes evident that it does not possess any of the typically domestic traits found in domesticated animals. On the basis of behaviour, the Przewalski’s horse is clearly distinguishable from feral domestic horses. The Przewalski’s horse was, ever since its discovery, always viewed as the wildtype of horse and nobody had the suspicion it might be feral instead of genuinely wild before the Gaunitz et al. study came up. There is only one morphologic difference to the domestic Botai horses the modern Przewalski’s horses are supposed to descend from: the limb elements seemingly became less robust than in their ancestors. This is not necessarily a typically domestic trait and could also be the result of genetic drift. The modern Przewalski’s horses may descend from a domesticated population, but this domestication process obviously could not have been very intense, otherwise the modern Przewalski’s horse would have clear domestic traits. And not to forget, the Botai horses were from 5500 years ago. These ancestors must have run wild again at some point since the Przewalski’s horse was exclusively known in its free-ranging state. Assuming this happened not long ago after the domestication event, the modern Przewalski’s horse was exposed to natural selection for – at maximum – 5500 years. It seems that the Przewalski’s horse became fully dedomesticated during these millennia, after being in a domesticated stage that was not intensely domestic in the first place. 

How to categorize the Przewalski’s horse, then? It seemingly never was very intensely domesticated, it must have become fully dedomesticated after millennia in the wild and consequently does not possess any domestic traits today. Yet, it is not predomestic as it has a domestic history. 

In my Dedomestication series, I coined the term “post-domestic” (as opposed to predomestic) for a fully dedomesticated animal population. It would be a wildtype, but a new, secondary wildtype – shaped by nature, just as the predomestic wildtype. My concept of the post-domestic wildtype was theoretical, as I did not have any unambiguous examples for this state – feral cattle, feral pigs or feral domestic horses living today are not there yet, and it is questionable if the dingo can be truly considered a post-domestic wildtype or just a feral dog. But now that we have the Przewalski’s horse as probably the only once-domesticated animal that became fully dedomesticated during millennia of natural selection, there is a very good candidate for a post-domestic wildtype. 

Regardless of whether the concept of a dedomesticated, post-domestic wildtype will ever become established in the zoological community, I consider the Przewalski’s horse a wild animal. Not a predomestic, but a post-domestic one. The establishing of these terms is only paperwork, dedomestication is undoubtedly a real evolutionary process that is sadly under-studied. 

 

References

 

[1] Trut, 1999: Early Canid Domestication: The Farm fox experiment. 

[2] Dobney & Larson, 2005: Genetics and animal domestication: new windows on an elusive process.

[3] Röhrs & Ebinger, 1998: Are zoo Przewalski horses domesticated horses? 

[4] Bunzel-Drüke, Finck, Kämmer, Luick, Reisinger, Riecken, Riedl, Scharf & Zimball: „Wilde Weiden: Praxisleitfaden für Ganzjahresbeweidung in Naturschutz und Landschaftsentwicklung“. 2011 

[5] Gaunitz et al.: Ancient genomes revisit the ancestry of domestic and Przewalski's horses. 2018.