The wild horse (Equus ferus)
The wild horse (Equus ferus) is a species of the genus Equus, which includes as subspecies the modern domesticated horse (Equus ferus caballus) as well as the endangered Przewalski's horse (Equus ferus przewalskii, sometimes treated as a separate species i.e. Equus przewalskii). The European wild horse, also known as the tarpan, that went extinct in the late 19th or early 20th century has previously been treated as the nominate subspecies of wild horse, Equus ferus ferus, but more recent studies have cast doubt on whether tarpans were truly wild or if they actually were feral horses or hybrids.
Other subspecies of Equus ferus may have existed and could have been the stock from which domesticated horses are descended. Przewalski's horse had reached the brink of extinction, but was reintroduced successfully into the wild. The tarpan became extinct in the 19th century, but is theorized to have been present on the steppes of Eurasia at the time of domestication. Since the extinction of the tarpan, attempts have been made to reconstruct its phenotype using domestic horses, resulting in horse breeds such as the Heck horse. However, the genetic makeup and foundation bloodstock of those breeds is substantially derived from domesticated horses, so these breeds possess domesticated traits.
The term "wild horse" is also used colloquially in reference to free-roaming herds of feral horses; for example, the mustang in the United States, and the brumby in Australia. These feral horses are untamed members of the domestic horse (Equus caballus), not to be confused with the truly "wild" horse subspecies extant into modern times.
Description
Equus ferus caballus like its extinct wild Eurasian ancestor (Equus ferus ferus ferus), known as "tarpan", as well as Przewalski's horse (Equus ferus przewalskii), a wild taxon still living in the steppes of central Asia.
Equus ferus is a herbivorous mammal with unpaired hooves, a species of the Equidae family (Equus), ancestor of the modern horse. It is one of the species of the genus Equus, which includes as subspecies the domesticated horse, as well as the tarpan and, according to some taxonomists, Equus ferus przewalskii.
Distribution
Evidence supports E. ferus as having evolved in North America about 1.1 - 1.2 million years ago. Around 800,000 - 900,000 years ago, E. ferus migrated west to Eurasia via the Bering Land Bridge, and south to South America via the Isthmus of Panama as part of the Great American Interchange. By the mid-late Pleistocene, it had an extremely large range across the Americas, Eurasia, and North Africa, across which it was abundant. There have been several fossil horse taxa from throughout this range, such as Equus lambei and Amerhippus, that were formerly considered distinct species, but genetic and morphological analysis supports them as being conspecific with E. ferus.
By the latest Pleistocene or early Holocene, American populations had disappeared as part of the Quaternary extinction event, leaving only the Old World populations. It remained widespread there and was ultimately also domesticated around 3600 B.C., but wild populations continued to decline. The last completely wild populations of the tarpan went extinct in Eastern Europe and the southern parts of Russia around the late 19th century, and Przewalski's horse of Central Asia became extinct in the wild in 1969. However, over the past few centuries feral horses have been introduced to all continents except Antarctica, and Przewalski's horses have been reintroduced to their former habitats in Mongolia.
Ecology
In general, wild horses are grazers that prefer to inhabit open areas, such as steppes and grasslands. They may have seasonal food preferences, as seen in the Przewalski's subspecies. Horses may fall prey to native predators including wolves, cougars, and spotted hyenas.
Subspecies and their history
E. ferus has had several subspecies, those of which survived into modern times are:
- The domesticated horse (Equus ferus caballus).
- The Eurasian wild horse (Equus ferus ferus), incorrectly listed as Equus caballus ferus in MSW 3; originally considered synonymous with the tarpan, though recent research has cast doubt on this. Horses identified as tarpans were found in Europe and western Asia before the last surviving animals —possibly hybrids by that time — became effectively extinct in the late 19th century. The last specimen died in 1909 whilst in captivity in an estate in Poltava Governorate, Russian Empire.
- Przewalski's horse (Equus ferus przewalskii), incorrectly listed as Equus caballus przewalskii in MSW 3; also known as the Mongolian wild horse or takhi, it is native to Central Asia and the Gobi Desert. It is sometimes considered its own species, Equus przewalskii.
- The latter two are the only never-domesticated "wild" groups that survived into historic times. However, other subspecies of Equus ferus may have existed.
- In the Late Pleistocene epoch, there were several other subspecies of E. ferus which have all since gone extinct. The exact categorization of Equus remains into species or subspecies is a complex matter and the subject of ongoing work.
History of Equus ferus
At the end of the last ice age, 10 000 years ago, millions of horses were grazing in Europe and also in northern and central Asia. They all belonged to the same species: the wild horse. These herds roamed the steppes, migrating hundreds of kilometres each year.
Climate change and the replacement of the steppes by forests have greatly reduced the number of horses: they did not have enough pasture. Wild horses were also exterminated by primitive hunters. The wild horse became rare in Europe about 4,000 years ago. In the early 20th century, two subspecies of wild horse were found, the Tarpan in Russia and Equus ferus przewalskii in Mongolia. By comparing the genome of the ancient horse whose remains were found in the Yukon with those of the zebra, the donkey, Equus ferus przewalskii, the 43,000-year-old Equus ferus and several modern horse breeds, scientists concluded that they all had a common ancestor that lived about four million years ago. This means that wild horses appeared two million years earlier than previously thought. Today, Equus ferus przewalskii is the only extant descendant of Equus ferus, and its genome shows no traces of admixture with the domestic breeds studied; the separation of the two lines of horses occurred 40-70 thousand years ago.
Przewalski's horse
Przewalski's horse occupied the eastern Eurasian Steppes, perhaps from the Urals to Mongolia, although the ancient border between tarpan and Przewalski's distributions has not been clearly defined. Przewalski's horse was limited to Dzungaria and western Mongolia in the same period, and became extinct in the wild during the 1960s, but was reintroduced in the late 1980s to two preserves in Mongolia. Although earlier researchers such as Marija Gimbutas theorized that the horses of the Chalcolithic period were Przewalski's, a 2003 study indicated that the Przewalski's horse is not an ancestor to modern domesticated horses. A 2015 study determined that the Przewalski and domesticated horse lineages diverged from a common ancestor about 45,000 years ago.
In 2018, a DNA study revealed that the horses found associated with the Botai culture were Przewalski's horses, raising the question of whether these animals were an isolated population, if extant Przewalski horses today represent feral descendants, or if the domestication attempt at Botai failed. A 2021 study noted that arrowheads were found in conjunction with some Botai horse remains, suggesting these horses were hunted, rather than domesticated, and thus the question remains unresolved.
Przewalski's horse is still found today, though it is an endangered species and for a time was considered extinct in the wild. Roughly 2000 Przewalski's horses are in zoos around the world. A small breeding population has been reintroduced in Mongolia. As of 2005, a cooperative venture between the Zoological Society of London and Mongolian scientists has resulted in a population of 248 animals in the wild.
Przewalski's horse has some biological differences from the domestic horse; unlike domesticated horses and the tarpan, which both have 64 chromosomes, Przewalski's horse has 66 chromosomes due to a Robertsonian translocation. However, the offspring of Przewalski and domestic horses are fertile, possessing 65 chromosomes.
