From New World Encyclopedia
Hyracotherium, also called "eohippus".
Hyracotherium, also called "eohippus".
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Gray, 1821
Genus: Equus
Linnaeus, 1758

E. asinus - Donkey
E. africanus - African wild ass
E. caballus - Domestic horse
E. ferus - Wild horse
E. grevyi - Grevy's zebra
E. hemionus - Onager
E. kiang - Kiang
E. quagga - Plains zebra
E. zebra - Mountain zebra
E. przewalskii - Przewalski's horse
  (or subspecies of wild horse or domestic horse)

Equidae is a family of odd-toed ungulate mammals of horses and horse-like animals. It is sometimes known as the horse family. All extant equids are in the genus Equus and include the three basic groups of horses, asses, and zebras. There are eight to ten extant species in Equidae, depending on the taxonomic scheme.

The extant horses (subgenus Equus) include the domestic horse (E. caballus) and Przewalski's horse. At one point, Przewalski's horse was considered extinct in the wild, until its reintroduction from zoos into the Mongolian steppe in the 1990s. It is sometimes considered a separate species, E. przewalskii, but also is listed as a subspecies of the domestic horse or as the last remaining wild horse (E. ferus), as the subspecies E. ferus przewalskii. The tarpan (Equus ferus ferus) became extinct in the early twentieth century.

The extant zebras include Grevy's zebra (E. grevyi), plains zebra (E. quagga), and mountain zebra (E. zebra). Some split the mountain zebra into two species, with Hartmann's mountain zebra (E. hartmannae) raised to species status.

The extant asses (subgenus Asinus) include the donkey or ass (E. asinus), African wild ass (E. africanus), onager (E. hemionus), and kiang (E. kiang). The domesticated donkey often is placed as a subspecies of its presumed wild ancestor, the African wild ass, which itself is variously designated as E. africanus or E. asinus.

With a fossil history tracing back to the early Eocene, 54 million years ago, numerous fossil equids are known. The intersection of equids with humans since the Paleolithic is evident in the drawings on cave walls. The subsequent domestication of the horse and donkey were watershed moments in humanity's culture and agriculture.


Equidae belongs to the order Perissodactyla, the odd-toes ungulates. In addition to equids, other odd-toed ungulates include the tapirs (Tapiridae) and the rhinoceroses (Rhinocerotidae). In members of Perissodactyla, the central axis of the foot passes through the third toe. In rhinoceroses, the first and fifth toes are lost and the animal walks on the remaining three toes. Tapirs have four toes on their front feet and three on their hind feet. In extant equids, only the third toe remains and supports the whole weight of the animal.

Extant equids are medium to large mammals, with long heads, and necks with a mane. Their legs are slender and end in the 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 range of equid monocular vision. Shaded areas represent blind spots.

The pinnae ("ears") of equids are mobile, enabling them to easily localize 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. Equids also have a vomeronasal organ, that allows males to use the flehmen, or "lip-curling" response to assess the sexual state of potential mates.

Equids 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, equids break down cellulose in the "hindgut" or cecum, a part of the colon. This is known as hindgut fermentation. Their dentition is almost complete, with cutting incisors to crop food, and grinding molars set well back behind a diastema. The high-crowned molars with complex ridges is effective for grinding grasses with high content of fibers (Moehlman 2004). The dental formula for equids is:


Wild equids are found in sub-Saharan Africa, Arabia, Middle East, Central Asia, and Mongolia; no wild equids are found in North America, only feral (formerly domesticated) animals (Moehlman 2004).

Equids 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, jennys, usually bear a single foal, after a gestation period of approximately 11 months. Young equids are able to walk within an hour of birth, and are weaned after 4 to 13 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 (Macdonald 1984).

Equids who 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 (Ensminger 1990). However, equids 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 (Eilts 2007). Further, for reasons that are not clear, about 20 percent of domestic mares in the Northern Hemisphere will cycle the year round (Eilts 2007).


The oldest known equid fossils date from the early Eocene, 54 to 55 million years ago. This species, Hyracotherium (formerly known as Eohippus), was a fox-sized animal with three toes on its hind feet, and four on the front feet. It was herbivorous browser on relatively soft plants, and already adapted for running. The complexity of its brain suggests that it was already an alert and intelligent animal (Palmer 1999). Later species had less toes and teeth more suited for grinding up grasses and other tough plant food.

The Equidae group became relatively large during the Miocene, with many new species appearing. By this time, equids were more truly horse-like, having developed the typical body shape of the modern animals. Many of these species bore the main weight of their bodies on their central, third, toe, with the others becoming reduced, and barely touching the ground, if at all. The surviving modern genus, Equus, had arisen by the early Pleistocene, and spread rapidly though the world (Savage and Long 1986).


Przewalski's horse, the only remaining type of "wild" horse that has never been domesticated
A domesticated horse
A donkey
An onager, one type of wild ass
A pair of Kiangs
Plains zebra
  • Order Perissodactyla (In addition to Equidae, Perissodactyla includes four species of tapir in a single genus, as well as five living species (belonging to four genera) of rhinoceros.) † indicates extinct species.
    • Family Equidae
      • Genus Hyracotherium
      • Genus Orohippus
      • Genus Epihippus
      • Genus Haplohippus
      • Genus Mesohippus
      • Genus Miohippus
      • Genus Archaeohippus
      • Genus Anchitherium
      • Genus Kalobatippus
      • Genus Hypohippus
      • Genus Sinohippus
      • Genus Megahippus
      • Genus Parahippus
      • Genus Merychippus
      • Genus Acritohippus
      • Genus Pseudhipparion
      • Genus Nannippus
      • Genus Neohipparion
      • Genus Hipparion
      • Genus Proboscidipparion
      • Genus Stylohipparion
      • Genus Cormohipparion
      • Genus Protohippus
      • Genus Parapliohippus
      • Genus Heteropliohippus
      • Genus Pliohippus
      • Genus Calippus
      • Genus Astrohippus
      • Genus Hippidion
      • Genus Onohippidium
      • Genus Dinohippus
      • Genus Equus
        • Subgenus Equus
          • Horse, Equus caballus
          • Wild Horse, Equus ferus
            • †Tarpan, Equus ferus ferus
            • Przewalski's Horse or Mongolian Wild Horse, Equus ferus przewalskii
          • †Yukon Horse, Equus lambei
        • Subgenus Asinus
          • African Wild Ass, Equus africanus
            • Nubian Wild Ass, Equus africanus africanus
            • †Atlas Wild Ass, Equus africanus atlanticus
            • Somali Wild Ass, Equus africanus somalicus
            • Donkey or Burro, Equus africanus asinus
          • †European Ass, Equus hydruntinus
          • Onager or Asiatic Ass, Equus hemionus, Equus hemionus onager
            • Mongolian Wild Ass, Equus hemionus hemionus
            • †Syrian Wild Ass, Equus hemionus hemippus
            • Gobi Kulan or Dziggetai , Equus hemionus luteus
            • Turkmenian Kulan, Equus hemionus kulan
            • Indian Wild Ass or Khur, Equus hemionus khur
          • Kiang, Equus kiang
            • Western Kiang, Equus kiang kiang
            • Eastern Kiang, Equus kiang holdereri
            • Southern Kiang, Equus kiang polyodon
            • Northern Kiang, Equus kiang chu
          • †Cummin’s Ass, Equus cumminsii
          • †Stilt-legged Onager, Equus calobatus
          • †Pygmy Onager, Equus tau
        • Subgenus Dolichohippus
        • Subgenus Hippotigris
          • Plains Zebra, Equus quagga
            • Quagga, Equus quagga quagga
            • Burchell's Zebra, Equus quagga burchellii
            • Grant's Zebra, Equus quagga boehmi
            • Selous' zebra, Equus quagga borensis
            • Chapman's Zebra, Equus quagga chapmani
            • Crawshay's Zebra, Equus quagga crawshayi
          • Mountain Zebra, Equus zebra
            • Cape Mountain Zebra, Equus zebra zebra
            • Hartmann's Mountain Zebra, Equus zebra hartmannae
        • Subgenus Amerhippus
          • †Scott’s Horse, Equus scotti
          • †Niobrara Horse, Equus niobrarensis
          • †Mexican Horse, Equus conversidens
        • Subgenus Parastylidequus
          • †Mooser’s Horse, Equus parastylidens
        • incertae sedis
          • †Hagerman Horse (or Hagerman Zebra), Equus simplicidens - perhaps closest to Dolichohippus
          • †Western Horse, Equus occidentalis
          • †Complex-toothed Horse, Equus complicatus
          • †Brother Horse, Equus fraternus
          • †Noble Horse, Equus excelsus - subgenus Equus?
          • †'Giant' Horses, Equus giganteus group
            • Equus giganteus
            • Equus pacificus
            • Equus pectinatus
            • Equus crinidens


a mule

Przewalski's horse and the domestic horse are the only equids that crossbreed and produce fertile offspring, although Przewalski's horse has 66 chromosomes while the domestic horse has 64. The offspring possess 65 chromosomes, but are not sterile and subsequent crossings can lead back to either species. Bell (2004) notes that "if the offspring is crossed back to a domestic horse, the resulting animal will have 64 chromosomes and very few Przewalski characteristics." Because these two animals can crossbreed, some consider the Przewalski's horse as a subspecies of the domestic horse (Equus caballus) as Equus caballus przewalskii. Other consider it a species, Equus przewalskii, while others consider it the last remnant of the wild horse (Equus ferus) as the subspecies Equus ferus przewalskii.

Other species of equidae can crossbreed but the ensuing offspring are usually infertile. Some hybrid equidae include:

  • Mule, a cross between a male donkey and a female horse. Mules are the most common type of hybrid equid 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.
  • Zeedonk or Zonkey, a cross between a donkey and a zebra.
  • Zony, a zebra/pony cross.
  • Zorse or zebrula, the offspring of a zebra stallion and a horse mare; the rarer reverse pairing is sometimes called a hebra.

Any equid with partial zebra ancestry is also called a zebroid.

ISBN links support NWE through referral fees

  • 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. ISBN 2831700523.
  • Ensminger, M. E. 1990. Horses and Horsemanship: Animal Agriculture Series, 6th edition. Interstate Publishers. ISBN 0813428831.
  • Groves, C. P., and H. B. Bell. 2004. New investigations on the taxonomy of the zebras genus Equus, subgenus Hippotigris. Mammalian Biology 69: 182-196.
  • Higuchi, R. G., L. A. Wrischnik, E. Oakes, M. George, B. Tong, and A. C. Wilson. 1987. Mitochondrial DNA of the extinct quagga: Relatedness and extent of postmortem change. Journal of Molecular Evolution 25: 283-287.
  • International Commission on Zoological Nomenclature (ICZN). 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.
  • Macdonald, D. W. (ed.). 1984. The Encyclopedia of Mammals. New York: Facts on File. ISBN 0871968711.
  • Moehlman, P. 2002. Equids—Zebras, Asses, and Horses: Status Survey and Conservation Action Plan. Gland, Switzerland: IUCN-the World Conservation Union. ISBN 2831706475.
  • Moehlman, P. D. 2004. Equidae. In B. Grzimek, D. G. Kleiman, V. Geist, and M. C. McDade, Grzimek's Animal Life Encyclopedia. Detroit: Thomson-Gale, 2004. ISBN 0307394913.
  • Palmer, D. 1999. The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. ISBN 1840281529.
  • Savage, R. J. G., and M. R. Long. 1986. Mammal Evolution: An Illustrated Guide. New York: Facts on File. ISBN 081601194X.


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