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For other uses, see Equus.

Temporal range: 1.8–0Ma
Early Pleistocene to Recent
Plains Zebras
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Gray, 1821
Genus: Equus
Linnaeus, 1758

E. africanus - African Wild Ass
E. ferus - Wild Horse
   E. ferus caballus - Domestic Horse
E. grevyi - Grevy's Zebra
E. hemionus - Onager
E. kiang - Kiang
E. quagga - Plains Zebra
E. zebra - Mountain Zebra

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 any horse. The word comes from Latin equus, "horse",[1] cognate with Greek "ἵππος" (hippos),[2] which was derived from the earlier Ionic "ἴκκος" (ikkos), "horse",[3][4] which in turn derives from the Mycenaean Greek i-qo. I-qo is the earliest attested form of the word, and was found written in Linear B syllabic script.[5]


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, that 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.[6][7]

Equines which 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.[8] 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.[9] Further, for reasons that are not clear, about twenty percent of domestic mares in the Northern Hemisphere will cycle the year round.[9]


Family Equidae in addition to Equus, the family includes approximately 35 other genera, all extinct.

Extant species

All species and subspecies

[extinct species are marked with †]

  • Genus Equus
    • Subgenus Equus
      • Equus ferus Wild horse[10][11]
        • 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 therin are possibly synonymous with E. ferus)[13]
          • Equus andium
          • Equus neogeus
          • Equus fraternus Leidy, 1860 (North America)
          • Equus santaeelenae
          • Equus scotti Gidley
          • Equus niobrarensis Hay
          • Equus conversidens Owen, 1863 (Mexican horse) (North America)
      • New World stilt-legged horse (all following species within the group may be synonyms or regional races of a single species)[14]
        • Equus francisci Hay, 1915
        • Equus semiplicatus Cope, 1893 (North America)
    • Subgenus Asinus
      • Equus africanus African Wild Ass[11][12]
        • Equus africanus africanus Nubian Wild Ass
        • Equus africanus asinus Domestic Donkey, Ass or Burro
        • Equus africanus atlanticus Atlas Wild Ass
        • Equus africanus somalicus Somali Wild Ass
      • Equus hemionus Onager or Asiatic Ass
        • Equus hemionus hemionus Mongolian Wild Ass, Dziggetai or Gobi Kulan
        • Equus hemionus hemippus Syrian Wild Ass
        • Equus hemionus khur Indian Wild Ass or Khur
        • Equus hemionus kulan Turkmenian Kulan
        • Equus hemionus onager Persian Onager
      • Equus kiang Kiang
        • Equus kiang chu Northern Kiang
        • Equus kiang kiang Western Kiang
        • Equus kiang holdereri Eastern Kiang
        • Equus kiang polyodon Southern 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
      • Equus grevyi Grevy's Zebra
      • Equus koobiforensis Eisenmann, 1983 (East Africa, ~2 million years (late Pliocene), similar but more derived than E.simplicidens and E.sanmeniensis
        • Equus oldowayensis Hopwood, 1937 (East Africa, late Pleiocene, early Pleistocene, likely conspecific with E.koobiforensis
    • 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 zebra hartmannae Hartmann's Mountain Zebra
        • Equus zebra zebra Cape Mountain Zebra
      • Equus mauritanicus Pomel, 1897 (Pleistocene transitional form between E. (Dolichohippus) and E. (Hippotigris), possibly via E. koobiforensis)
    • 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[15])
      • 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 stenonis guthi (late Pliocene, France)
          • Equus stenonis senezensis (late Pliocene)
          • Equus stenonis pamirensis (Hippotigris pamirensis) (late Pliocene, central Asia)
          • Equus stenonis petraloniensis Tsoukala (Greece - Petralona cave)
          • Equus stenonis vireti (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 idahoensis
        • Equus (A.) occidentalis
        • Equus (A.) excelsus
        • Equus (H.) pacificus
      • Equus complicatus
      • Equus fraternus
      • Equus major Boule (nomen dubium)
      • Equus giganteus group
        • Equus giganteus
        • Equus pectinatus
        • Equus crinidens


Different species of Equus can crossbreed, though the ensuing offspring are usually infertile. Hybrids include:

  • 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, surefootedness, 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.

See also



  • Burke, et al. 2003. "The systematic position of Equus hydruntinus, an extinct species of Pleistocene equid". Quaternary Research 59(3):459–469.
  • Duncan, P. (ed.). 1992. "Zebras, Asses, and Horses: an Action Plan for the Conservation of Wild Equids". IUCN/SSC Equid Specialist Group. IUCN, Gland, Switzerland.
  • Equid Specialist Group 1996. "Equus ferus". In: IUCN 2004. 2004 IUCN Red List of Threatened Species. . Downloaded on 21 January 2006.
  • Equid Specialist Group 1996. "Equus ferus ssp. przewalskii". In: IUCN 2004. 2004 IUCN Red List of Threatened Species. . Downloaded on 21 January 2006.
  • "International Commission on Zoological Nomenclature. 2003. Opinion 2027 (Case 3010). Usage of 17 specific names based on wild species which are pre-dated by or contemporary with those based on domestic animals (Lepidoptera, Osteichthyes, Mammalia): conserved." Bull. Zool. Nomencl., 60:81–84.
  • Moehlman, P.D. 2002. "Equids. Zebras, Assess and Horses. Status Survey and Conservation Action Plan." IUCN/SSC Equid Specialist Group. IUCN, Gland, Switzerland. (

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