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Temporal range: 1.8–0Ma Early Pleistocene to Recent
E. africanus - African Wild Ass
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Equus is a genus of animals in the family Equidae, which includes horses, donkeys, and zebras. Within Equidae, Equus is the only extant genus. Like Equidae more broadly, Equus has numerous extinct species known only from fossils. This article deals primarily with the extant species. The term equine refers to any member of this genus, including horses.
The word comes from Latin equus, "horse", cognate with Greek "ἵππος" (hippos), "horse", and Mycenaean Greek i-qo /ikkʷos/ (cf. the alternative development of the Proto-Greek labiovelar in Ionic "ἴκκος" ikkos), the earliest attested variant of the Greek word, written in Linear B syllabic script.
Equines are medium to large mammals, with long heads and necks with a mane. Their legs are slender and end in a single, unguligrade toe, protected by a horny hoof. They have long, slender tails, either ending in a tuft, or entirely covered in flowing hair. They are adapted to generally open terrain, from plains and savannas, to mountains or deserts.
The pinnae (outer ears) of equines are mobile, enabling them to easily localise the origin of sounds. They have two-color, or dichromatic vision. Their eyes are set back far on the head, giving them a wide angle of view, without entirely losing binocular vision. Equines also have a vomeronasal organ, which allows males to use the flehmen, or 'lip-curling' response, to assess the sexual state of potential mates. Equines are one of only two mammals (the other is the human) capable of producing copious sweat perspiration for thermoregulatory cooling, enabling fast running over long distances.
Equines are herbivores, and feed predominantly on tough, fibrous food, such as grasses and sedges. When in need, they will also eat other vegetable matter, such as leaves, fruits, or bark, but are normally grazers, not browsers. Unlike ruminants, with their complex stomachs, equines break down cellulose in the "hindgut" or caecum, a part of the colon. Their dentition is almost complete, with cutting incisors to crop food, and grinding molars set well back behind a diastema. The dental formula for equines is:
Equines are social animals, living in herds or bands. Horses, along with Plains and Mountain Zebras, have permanent herds generally consisting of a single male and a band of females, with the remaining males forming small "bachelor" herds. The remaining species have temporary herds, lasting only a few months, which may be either single-sexed or mixed. In either case, there are clear hierarchies established among the individuals, usually with a dominant female controlling access to food and water resources and the lead male controlling mating opportunities.
Females, usually called mares in horses and zebras, or, in the case of asses and donkeys, jennies, usually bear a single foal, after a gestation period of approximately 11 months. Young equines are able to walk within an hour of birth, and are weaned after four to thirteen months (animals living in the wild naturally wean foals at a later date than those under domestication). Depending on species, living conditions and other factors, females in the wild may give birth every year or every other year.
Equines that are not in foal generally have a seasonal estrous cycle, from early spring into autumn. Most females enter an anestrus period during the winter and thus do not cycle in this period. The reproductive cycle is controlled by the photoperiod (length of the day), with estrus triggered when the days begin to lengthen. Anestrus prevents the female from conceiving in the winter months, as that would result in her foaling during the harshest part of the year, a time when it would be more difficult for the foal to survive. However, equines who live near the equator, where there is less change in length of day from season to season, have no anestrus period, at least in theory. Further, for reasons that are not clear, about twenty percent of domestic mares in the Northern Hemisphere will cycle the year round.
Family Equidae in addition to Equus, the family includes approximately 35 other genera, all extinct.
- Genus Equus
All species and subspecies
[extinct species are marked with †]
- Genus Equus
- Subgenus Equus
- Equus ferus Wild horse
- Equus ferus caballus Domestic horse
- †Equus ferus ferus Tarpan (historically extinct)
- Equus ferus przewalskii Przewalski's horse or Mongolian Wild Horse or takhi
- †Equus algericus (Pleistocene of Algeria)
- † North American caballid horses (Pleistocene; most likely synonymous with E. ferus):
- Subgenus †Amerhippus Hoffstetter, 1950 (jr synonym subgenus Tomolabis Cope, 1892) (this subgenus and the species therein are possibly synonymous with E. ferus)
- †New World stilt-legged horse (all following species within the group may be synonyms or regional races of a single species)
- Equus ferus Wild horse
- Subgenus Asinus
- Equus africanus African Wild Ass
- Equus hemionus Onager or Asiatic Ass
- Equus kiang Kiang
- †Equus hydruntinus European Ass (late Paleolithic of southern Europe)
- †Equus altidens (middle Pleistocene of Tadjikistan)
- †Equus tabeti (early middle Pleistocene of Algeria, known only from teeth and limb bones)
- †Equus melkiensis (late Paleolithic of Algeria, based on teeth and limb bones)
- †Equus graziosii (late Pleistocene of Italy)
- Subgenus Dolichohippus
- Subgenus Hippotigris
- Equus quagga Plains Zebra
- Equus quagga boehmi Grant's Zebra
- Equus quagga borensis Maneless Zebra
- Equus quagga chapmani Chapman's Zebra
- Equus quagga crawshayi Crawshay's Zebra
- Equus quagga burchellii Burchell's Zebra
- †Equus quagga quagga Gmelin, 1788 Quagga (South Africa, extinction in early 20th century)
- Equus quagga selousi Selous' Zebra
- Equus zebra L.,1758 Mountain Zebra
- †Equus mauritanicus Pomel, 1897 (Pleistocene transitional form between E. (Dolichohippus) and E. (Hippotigris), possibly via E. koobiforensis)
- Equus quagga Plains Zebra
- Subgenus †Parastylidequus
- †Equus parastylidens Mooser’s Horse
- incertae sedis
- †Equus simplicidens Hagerman Horse (North American Pleiocene - earliest known species of genus Equus) (perhaps closest to Dolichohippus)
- †Equus cumminsii Cope, 1893 (North America; based on a single, 3 million year old tooth)
- †Equus livenzovensis Baihusheva, 1978 (southern Russia and western Europe, Pleiocene - highly similar to E. simplicidens
- †Equus sanmeniensis Teilhard&Piveteau, 1930 (south China, late? Pliocene; similar but more derived than E.livenzovensis
- †Equus teilhardi Eisenmann, 1975 (south China, late(?) Pliocene; similar to E.sanmeniensis but smaller, likely synonym)
- †Equus numidicus Pomel, 1897
- †Equus plicidens Owen, 1844 (late Pliocene, only teeth known)
- †Equus stenonis group
- †Equus stenonis Cocchi, 1867 (Europe to China, late Pliocene)
- †Equus sivalensis (late Pleiocene (?), India; similar to E.stenonis in skull shape)
- †Equus stehlini (early Pleistocene, ~1 million years ago; similar but smaller than E. stenonis
- †Equus bressanus (early Pleistocene, ~1 million years ago; similar but much larger than E. stenonis
- †Equus sussenbornensis Wüst, 1901 (early middle Pleistocene of Europe)
- †Equus verae Sher, 1971 (middle Pleistocene of north eastern Asia)
- †Equus namadicus (middle paleolithic sites in India)
- †subgenus Allozebra & Hesperohippus (N. American lineage of middle to late Pleistocene)
- †Equus complicatus
- †Equus fraternus
- Equus major Boule (nomen dubium)
- †Equus giganteus group
- Subgenus Equus
Przewalski's horse, the only remaining type of "wild" horse that has never been domesticated
- Mule, a cross between a male donkey and a female horse. Mules are the most common type of hybrid equine and are renowned for their toughness, sure-footedness, and working ability.
- Hinny, a cross between a female donkey and a male horse. Considered a less desirable cross than a mule, generally smaller in size and not as hardy.
Any equine with partial zebra ancestry is called a zebroid.
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