Equus (genus)

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. Equids developed into larger, three-toed animals (e.g. Mesohippus) during the Oligocene and Miocene. 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, 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. 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 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. • 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 • 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. Equines are monogastric hindgut fermenters. They prefer to eat grasses and sedges, but they, particularly asses, may also consume bark, leaves, buds, fruits, and roots if their favored foods are scarce. Compared to ruminants, they have a simpler and less efficient digestive system. Nevertheless, they can subsist on lower-quality vegetation. After food is passed through the stomach, it enters the sac-like cecum, where cellulose is broken down by micro-organisms. Fermentation is quicker in equines than in ruminants—30–45 hours for a horse compared to 70–100 hours for cattle. Equines may spend 60–80% of their time feeding, depending on the availability and quality of vegetation. Wild equines may spend seven hours a day sleeping. During the day, they sleep standing up, while at night they lie down. They regularly rub against trees, rocks, and other objects and roll around in dust for protection against flies and irritation. Except the mountain zebra, wild equines can roll over completely. Communication s interacting When meeting for the first time or after they have separated, individuals may greet each other by rubbing and sniffing their noses followed by rubbing their cheeks, moving their noses along their bodies and sniffing each other's genitals. They then may rub and press their shoulders against each other and rest their heads on one another. This greeting is usually performed among harem or territorial males or among bachelor males playing. Estrus in female equines lasts 5–10 days; physical signs include frequent urination, flowing muscus, and swollen, everted labia. In addition, estrous females will stand with their hind legs spread and raise their tails when in the presence of a male. Males assess the female's reproductive state with the flehmen response and the female will solicit mating by backing in. Length of gestation varies by species; it is roughly 11–13 months, and most mares come into estrus again within a few days after foaling, depending on conditions. Among harem-holding species, foals are cared for mostly by their mothers, but if threatened by predators, the entire group works together to protect all the young. The group forms a protective front with the foals in the center and the stallion will rush at predators that come too close. In territory-holding species, mothers may gather into small groups and leave their young in "kindergartens" under the guard of a territorial male while searching for water. A Grévy's zebra stallion may look after a foal in his territory to ensure that the mother stays, though it may not be his. ==Human relations==

pottery depicting horse and chariot The earliest archaeological evidence for the domestication of the horse comes from sites in Ukraine and Kazakhstan, dating to around 4000–3500 BCE. By 3000 BCE, the horse was completely domesticated, and by 2000 BCE, a sharp increase occurred in the number of horse bones found in human settlements in northwestern Europe, indicating the spread of domesticated horses throughout the continent. The most recent, but most irrefutable, evidence of domestication comes from sites where horse remains were buried with chariots in graves of the Sintashta and Petrovka cultures c. 2100 BCE. Studies of variation in genetic material shows that a very few wild stallions, possibly all from a single haplotype, contributed to the domestic horse, mating with many mares in early domesticated herds. Przewalski's horse has been conclusively shown not to be an ancestor of the domestic horse, though the two can hybridize and produce fertile offspring. The split between Przewalskii's horse and E. caballus is estimated to have occurred 120,000–240,000 years ago, long before domestication. Of the caballine equines of E. ferus, E. f. ferus, also known as the European wild horse or "tarpan", shares ancestry with the modern domestic horse. In addition, tarpans that lived into modern times may have been hybridized with domestic horses. Archaeological, biogeographical, and linguistic evidence suggests that the donkey was first domesticated by nomadic pastoral people in North Africa over 5,000 years ago. The animals were used to help cope with the increased aridity of the Sahara and the Horn of Africa. Genetic evidence finds that the donkey was domesticated twice based on two distinct mitochondrial DNA haplogroups. It also points to a single ancestor, the Nubian wild ass. Attempts to domesticate zebras were largely unsuccessful, though Walter Rothschild trained some to draw a carriage in England. Conservation issues Humans have had a great impact on the populations of wild equines. Threats to wild equines include habitat destruction and conflicts with local people and livestock. Since the 20th century, wild equines have been decimated over many of their former ranges and their populations scattered. In recent centuries, two subspecies, the quagga and the tarpan, became extinct. Przewalski's horse was considered to be extinct in the wild from the 1960s to 1996. However, following successful captive breeding, it has been reintroduced in Mongolia. In the United States, feral horses and burros are generally considered an introduced species because they are descendants from domestic horses brought to the Americas from Europe. While they are viewed as pests by many livestock producers, conversely, a view also exists that E. caballus is a reintroduced once-native species returned to the Americas that should be granted endangered species protection. At present, certain free-roaming horses and burros have federal protection as "living symbols of the historic and pioneer spirit of the West" under the Wild and Free-Roaming Horses and Burros Act of 1971, and in Kleppe v. New Mexico, the United States Supreme Court ruled that the animals so designated were, as a matter of law, wildlife. == References ==