Equus (genus)

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For other uses, see Equus.
Temporal range: 1.8–0Ma
Early Pleistocene to Recent
Plains Zebra Equus quagga.jpg
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 horses.

The word comes from Latin equus, "horse",[1] cognate with Greek "ἵππος" (hippos), "horse",[2] and Mycenaean Greek i-qo /ikkʷos/ (cf. the alternative development of the Proto-Greek labiovelar in Ionic "ἴκκος" ikkos[3][4]), the earliest attested variant of the Greek word, 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 range of equine monocular vision. Shaded areas represent blind spots.

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:

A feral horse herd in the western United States
Mounted fossil of Equus occidentalis from the La Brea Tar Pits

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 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.[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[edit]

All species and subspecies[edit]

[extinct species are marked with †]


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

a mule
  • 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.

See also[edit]


  1. ^ equus, Charlton T. Lewis, Charles Short, A Latin Dictionary, on Perseus Digital Library
  2. ^ ἵππος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
  3. ^ ἴκκος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus Digital Library
  4. ^ Introduction to Ionic Dialect Brett Mulligan, "Introduction to Ionic Dialect", Haverford College Classics Department, accessed March 10, 2012
  5. ^ Palaeolexicon, Word study tool of ancient languages
  6. ^ Macdonald, D., ed. (1984). The Encyclopedia of Mammals. New York: Facts on File. pp. 482–485. ISBN 0-87196-871-1. 
  7. ^ "Environmental Assessment: Pryor Mountain Wild Horse Range FY2004: Fertility Control on Age-Specific Wild Horse Mares". BLM National Research Field Trials on Wild Horse Fertility Control, Summer 2004 accessed November 21, 2007.
  8. ^ Ensminger, M. E. Horses and Horsemanship: Animal Agriculture Series. Sixth Edition. Interstate Publishers, 1990. ISBN 0-8134-2883-1 p. 156
  9. ^ a b Eilts, Bruce E. "Aberrations of the Equine Estrous cycle", Louisiana State University school of Veterinary Medicine, last modified 15 August 2007. Accessed November 21, 2007
  10. ^ a b Don E. Wilson & DeeAnn M. Reeder, ed. (2005). "Equus caballus". Mammal Species of the World. A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. 
  11. ^ a b c d International Commission on Zoological Nomenclature (2003). "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. Opinion 2027 (Case 3010)." (Summary). Bull. Zool. Nomencl. 60 (1): 81–84. 
  12. ^ a b Don E. Wilson & DeeAnn M. Reeder, ed. (2005). "Equus asinus". Mammal Species of the World. A Taxonomic and Geographic Reference (3rd ed.). Johns Hopkins University Press. 
  13. ^ Orlando, L.; et al. (2008). "Ancient DNA Clarifies the Evolutionary History of American Late Pleistocene Equids". Journal of Molecular Evolution 66 (5): 533–538. doi:10.1007/s00239-008-9100-x. PMID 18398561. 
  14. ^ Weinstock, J.; et al. (2005). "Evolution, systematics, and phylogeography of Pleistocene horses in the New World: a molecular perspective". PLoS Biology 3 (8): e241. doi:10.1371/journal.pbio.0030241. PMC 1159165. PMID 15974804. Retrieved 2008-12-19. 
  15. ^ Hagerman Fossil Beds NM Hourse Quarry Page


  • 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. <www.iucnredlist.org>. Downloaded on 21 January 2006.
  • Equid Specialist Group 1996. "Equus ferus ssp. przewalskii". In: IUCN 2004. 2004 IUCN Red List of Threatened Species. <www.iucnredlist.org>. Downloaded on 21 January 2006.
  • Groves, C.P.; Bell, H.B. (2004). "New investigations on the taxonomy of the zebras genus Equus, subgenus Hippotigris". Mammalian Biology 69: 182–196. doi:10.1078/1616-5047-00133. 
  • Higuchi, RG, Wrischnik, LA, Oakes, E, George, M, Tong, B, Wilson, AC (1987). "Mitochondrial DNA of the Extinct Quagga: Relatedness and Extent of Postmortem Change". Journal of Molecular Evolution 25 (4): 283–287. doi:10.1007/BF02603111. PMID 2822938. 
  • "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. (http://www.iucn.org/themes/ssc/publications/actionplans.htm#Equids2002)
  • Orlando, L; Mashkour, M; Burke, A; Douady, CJ; Eisenmann, V; Hänni, C (2006). "Geographic distribution of an extinct equid (Equus hydruntinus : Mammalia, Equidae) revealed by morphological and genetical analyses of fossils". Molecular Ecology 15 (8): 2083–2093. doi:10.1111/j.1365-294X.2006.02922.x. PMID 16780426.