Equus () is a genus of mammals in the perissodactyl family Equidae, which includes horses, asses, and zebras. Within the Equidae, Equus is the only recognized extant genus, comprising seven living species. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. The genus originated in North America and dispersed into the Old World and South America during the Early and Middle Pleistocene. Equines are odd-toed ungulates with slender legs, long heads, relatively long necks, manes (erect in most subspecies), and long tails. All species are herbivorous, and mostly grazers, with simpler digestive systems than ruminants but able to subsist on lower-quality vegetation.
While the domestic horse and donkey (along with their feral descendants) exist worldwide, wild equine populations are limited to Africa and Asia. Wild equine social systems are in two forms; a harem system with tight-knit groups consisting of one adult male or stallion/jackass, several females or mares/jennets, and their young or foals; and a territorial system where males establish territories with resources that attract females, which associate very fluidly. In both systems, females take care of their offspring, but males may play a role as well. Equines communicate with each other both visually and vocally. Human activities have threatened wild equine populations.
Etymology The word is Latin for "horse" and is cognate with the Greek (, "horse") and Mycenaean Greek , the earliest attested variant of the Greek word, written in Linear B syllabic script. Compare the alternative development of the Proto-Greek labiovelar in Ionic ().
Taxonomic and evolutionary history
The genus Equus was first described by Carl Linnaeus in 1758. It is the only recognized extant genus in the family Equidae. The first equids were small, dog-sized mammals (e.g. Eohippus) adapted for browsing on shrubs during the Eocene, around 54 million years ago (Mya). These animals had three toes on the hind feet and four on the front feet with small hooves in place of claws, but also had soft pads. Equids developed into larger, three-toed animals (e.g. Mesohippus) during the Oligocene and Miocene. From there, the side toes became progressively smaller through the Pleistocene until the emergence of the single-toed Equus.
The genus Equus, which includes all extant equines, is believed to have evolved from Dinohippus, via the intermediate form Plesippus. One of the oldest species is Equus simplicidens, described as zebra-like with a donkey-like head shape. The oldest material to date was found in Idaho, USA. The genus appears to have spread quickly into the Old World, with the similarly aged E. livenzovensis documented from western Europe and Russia. Molecular phylogenies indicate that the most recent common ancestor of all modern equines (members of the genus Equus) lived ~5.6 (3.9-7.8) Mya. Direct paleogenomic sequencing of a 700,000-year-old middle Pleistocene horse metapodial bone from Canada implies a more recent 4.07 Mya for the most recent common ancestor within the range of 4.0 to 4.5 Mya.
Mitochondrial evidence supports the division of Equus species into noncaballoid (which includes zebras and asses) and caballoids or "true horses" (which includes E. caballus and E. ferus przewalskii, alternatively E. przewalskii). Of the extant equine species, the lineage of the asses may have diverged first, possibly as soon as Equus reached the Old World. Zebras appear to be monophyletic and differentiated in Africa, where they are endemic. Members of the subgenus Sussemionus were abundant during the Early and Middle Pleistocene of North America and Afro-Eurasia, but only a single species, Equus ovodovi survived into the Late Pleistocene and Holocene in south Siberia and China, with the youngest remains from China dating to around 3500 BP (1500 BC), during the Shang dynasty. Genetic data from E. ovodovi has placed the Sussemionus lineage as closer to zebras and asses than to caballine horses.
Molecular dating indicates the caballoid lineage diverged from the noncaballoids 4 Mya. Genetic results suggest that all North American fossils of caballine equines, as well as South American fossils traditionally placed in the subgenus E. (Amerhippus), belong to E. ferus. Remains attributed to a variety of species and lumped together as New World stilt-legged horses (including E. francisci, E. tau, and E. quinni) probably all belong to a second species that was endemic to North America. This was confirmed in a genetic study done in 2017, which subsumed all the specimens into the species E. francisci which was placed outside all extant horse species in the new genus Haringtonhippus, although its placement as a separate genus was subsequently questioned. A separate genus of horse, Hippidion existed in South America. The possible causes of the extinction of horses in the Americas (about 12,000 years ago) have been a matter of debate. Hypotheses include climatic change and overexploitation by newly arrived humans. Horses only returned to the American mainland with the arrival of the conquistadores in 1519.
Extant species Subgenus Image Scientific name Common name Distribution Equus(Horses) Equus ferus przewalskii / E. przewalskii and Equus caballus (Przewalski's horse and domesticated horse) Eurasia Asinus(Asses) Equus africanus African wild ass (includes domesticated donkey) Horn of Africa, in Eritrea, Ethiopia and Somalia Equus hemionus Onager, hemione, or Asiatic wild ass Iran, Pakistan, India, and Mongolia, including in Central Asian hot and cold deserts of Kazakhstan, Uzbekistan, Turkmenistan, and China Equus kiang Kiang Tibetan Plateau Hippotigris(Zebras) Equus grevyi Grévy's zebra Kenya and Ethiopia Equus quagga Plains zebra south of Ethiopia through East Africa to as far south as Botswana and eastern South Africa Equus zebra Mountain zebra south-western Angola, Namibia and South Africa.
Prehistoric species Many extinct prehistoric species of Equus have been described. The validity of some of these species is questionable and a matter of debate. For example, Equus niobrarensis is likely synonymous with Equus scotti, while Equus alaskae is most likely the same species as Equus lambei, which itself may be a North American form of the living Equus przewalskii.
DNA studies on American horse remains found frozen into permafrost have shown that several of the supposed American species, as well as the European Equus ferus, are actually a single highly-variable widespread species, as if the evolutionary process of speciation was persistently being frustrated by large herds moving long distances and mixing.https://www.youtube.com/watch?v=Tg3lt5zYO_k Youtube video Ice Age Horses of the American West
American species Equus alaskae – Alaskan horse Equus conversidens – Mexican horse Equus fraternus Equus francisi – now placed in a separate genus, Haringtonhippus Equus giganteus – Giant horse Equus lambei – Yukon horse Equus neogeus – often placed in a separate genus, Amerhippus Equus niobrarensis – Niobrara horse Equus occidentalis – Western horse Equus scotti – Scott's horse Equus semiplicatus Equus simplicidens – Hagerman horse
Eurasian species Equus altidens Equus major Equus suessenbornensis Equus livenzovensis Equus senezensis Equus dalianensis Equus lenensis – Siberian horse Equus latipes Equus hydruntinus – European wild ass Equus namadicus Equus ovodovi Equus yunnanensis Equus stenonis – Stenon zebra Equus sivalensis - Sivalik horse
African species Equus algericus Equus capensis – Giant zebra Equus mauritanicus – Saharan zebra
Domestic species Equus caballus – Domestic horse Equus asinus – Domestic donkey
Hybrids
Equine species can crossbreed with each other. The most common hybrid is the mule, a cross between a male donkey and a female horse. With rare exceptions, these hybrids are sterile and cannot reproduce. A related hybrid, a hinny, is a cross between a male horse and a female donkey. Other hybrids include the zorse, a cross between a zebra and a horse and a zonkey or zedonk, a hybrid of a zebra and a donkey. In areas where Grévy's zebras are sympatric with plains zebras, fertile hybrids do occur. Ancient DNA identifies the Bronze Age kunga as a cross between the Syrian wild ass and the donkey.
Biology
Physical characteristics
Equines have significant differences in size, though all are characterized by long heads and necks. Their slender legs support their weight on one digit (which evolved from the middle digits). Grévy's zebra is the largest wild species, standing up to and weighing up to . Domesticated horses have a wider range of sizes. Heavy or draft horses are usually at least high and can be as tall as and weigh from about . Some miniature horses are no taller than in adulthood. Sexual dimorphism is limited in equines. The penis of the male is vascular and lacks a bone (baculum). Equines are adapted for running and traveling over long distances. Their dentition is adapted for grazing; they have large incisors that clip grass blades and highly crowned, ridged molars well suited for grinding. Males have spade-shaped canines ("tushes"), which can be used as weapons in fighting. Equines have fairly good senses, particularly their eyesight. Their moderately long, erect ears are movable and can locate the source of a sound.
A dun-colored coat with primitive markings that include a dorsal stripe and often leg striping and transverse shoulder stripes reflect the wildtype coat and are observed in most wild extant equine species. Only the mountain zebra lacks a dorsal stripe. In domestic horses, dun color and primitive markings exist in some animals across many breeds. The purpose of the bold black-and-white striping of zebras has been a subject of debate among biologists for over a century, but 2014 evidence supports the theory that they are a form of protection from biting flies. These insects appear to be less attracted to striped coats, and compared to other wild equines, zebras live in areas with the highest fly activity. With the exception of the domestic horses, which have long manes that lay over the neck and long tail hair growing from the top of the tailhead or dock, most equines have erect manes and long tails ending in a tuft of hair. The coats of some equine species undergo shedding in certain parts of their range and are thick in the winter.
Ecology and daily activities
Extant wild equines have scattered ranges across Africa and Asia. The plains zebra lives in lush grasslands and savannas of Eastern and Southern Africa, while the mountain zebra inhabits mountainous areas of southwest Africa. The other equine species tend to occupy more arid environments with more scattered vegetation. Grévy's zebra is found in thorny scrubland of East Africa, while the African wild ass inhabits rocky deserts of North Africa. The two Asian wild ass species live in the dry deserts of the Near East and Central Asia and Przwelski's wild horse's habitat is the deserts of Mongolia. Only the range of the plains and Grévy's zebras overlap. In addition to wild populations, domesticated horses and donkeys are widespread due to humans. In certain parts of the world, populations of feral horses and feral donkeys exist, which are descended from domesticated animals that were released or escaped into the wild.