Wild horses: getting the taxonomy right

I used to refer to the European wild horse as Equus ferus ferus, which may be wrong. This post deals with the issue of the taxonomy of the European wild horse(s), the domestic horse and the Przewalski’s horse. 

 

Some consider the domestic horse and the Przewalski’s horse different species, mainly because of the different chromosome number and morphological differences. However, we find differences in the karyotype also in other species, such as the Banteng (the Cambodian banteng has a chromosome number different from the other subspecies), and morphological differences are also found between subspecies, and both horse types interbreed without fertility problems. Thus, it is not unjustified to consider the domestic horse and the Przewalski’s horse members of the same species. The western subspecies of the Eurasian wild horse, which was the predecessor of the domestic horse, was definitely part of this species too, for once as many consider wildtypes and their domestic derivates members of the same species, and because they most likely could interbreed without fertility problems too and the morphological differences were likely smaller than between domestic horses and Przewalski’s horses. How to name this species, then? Wikipedia, and I too in the past, uses Equus ferus ferus for the western wild horse subspecies, Equus ferus przewalskii for the Przewalski’s horse and Equus ferus caballus for the domestic horse. I consider this problematic. 

At first we have to consider the priority rule of the ICZN. Equus caballus was described in 1758, Equus ferus in 1784 and Equus przewalskii in 1881. Following the priority rule, Equus caballus would definitely be the senior synonym and thus the name that should be used for the species. However, in 2003 a number of names of wildtypes that are synonymous with their domestic derivates, have been conserved by the ICZN, including Equusferus (and also Bos primigenius, by the way). Thus, the name to be used would be Equus ferus. But the main problem is: what type of horse is Equus ferus based on?

The original description of Equus ferus is a short description by Boddaert from 1784, lacking a holotype, but referring to free-ranging horses of the Russian steppe. According to most recent research as well as the fact that these horse populations included many individuals that might have been feral domestic horses or hybrids, these free-ranging horses (called “tarpan” by other contemporaneous authors), were most likely hybrids between native wild horses and domestic horses. Thus, Equus ferus is not based on the predomestic western Eurasian wild horses, but on hybrids with domestic horses. Hence, it does not describe the wildtype. Therefore, the reason to preserve this nomen in the ICZN falls apart. Also, the description lacks a holotype and is based on hybrids of two subspecies, thus the legitimacy of the taxon is to be questioned. 

Considering this, I no longer use Equus ferus as the name for the species of the western Eurasian wild horse, the eastern Eurasian wild horse (Przewalski’s horse) and the domestic horse. Rather, Equus caballus should be used: Equus caballus caballus for the domestic horse, Equus caballus przewalskii for the Przewalski’s horse and the western subspecies of the Eurasian wild horse is yet not scientifically described. 

The western Eurasian or European wild horse thus needs a proper description in the scientific literature. Someone should describe this subspecies, using a type specimen that is definitely a member of the Holocene western wild horse subspecies, f.e. bone remains predating domestic horses (so that they are predomestic for sure) found in Europe. I know of no such specimen that are mounted and on display, but there are remains that have been found. 

Another important aspect is that the Iberian wild horses probably deserve a subspecies status on their own. They are a genetically independent lineage that is less closely related to the domestic horse (and their predecessors) than the Przewalski’s horse and Equus (caballus) lenensis, respectively [1], and thus should not be regarded as a part of the western subspecies that gave rise to domestic horses and was found on the rest of Europe, because that subspecies is younger than the Iberian wild horse. As the European wild horse, the Iberian wild horse subspecies needs a proper description with a reliable type specimen. 

 

Literature 

 

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