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This article is about the genus of mammals. For other uses, seeEquus.

Equus
Temporal range:5.33–0MaEarliest Plioceneto recent[1]
Clockwise (from top left):plains zebra(E. quagga),domestic horse(E. f. caballus),onager(E. hemionus)
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Mammalia
Order: Perissodactyla
Family: Equidae
Tribe: Equini
Genus: Equus
Linnaeus,1758
Type species
Equus caballus[2]
Linnaeus, 1758
Extant species

Equus(/ˈɛkwəs,ˈiːkwəs/)[3]is agenusofmammalsin thefamilyEquidae,which includeshorses,asses,andzebras.Within the Equidae,Equusis the only recognizedextantgenus, comprising seven living species. Like Equidae more broadly,Equushas numerousextinctspeciesknown only fromfossils.The genus originated in North America and dispersed into the Old World and South America during the Early and Middle Pleistocene.Equinesareodd-toed ungulateswith slender legs, long heads, relatively long necks, manes (erect in most subspecies), and long tails. All species areherbivorous,and mostly grazers, with simpler digestive systems thanruminantsbut able to subsist on lower-quality vegetation.

While the domestic horse and donkey (along with theirferaldescendants) exist worldwide, wild equine populations are limited to Africa and Asia. Wild equine social systems are in two forms; aharemsystem with tight-knit groups consisting of one adult male orstallion/jackass,several females ormares/jennets,and their young orfoals;and a territorial system where males establish territories with resources that attract females, which associate very fluidly. In both systems, females take care of their offspring, but males may play a role as well. Equines communicate with each other both visually and vocally. Human activities have threatened wild equine populations.

Etymology

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The wordequusisLatinfor "horse"[4]and is cognate with theGreekἵππος(hippos,"horse" )[5]andMycenaean Greeki-qo/ikkʷos/,the earliest attested variant of the Greek word, written inLinear Bsyllabic script.[6]Compare the alternative development of theProto-Greeklabiovelar inIonicἴκκος(ikkos).[5][7]

Taxonomic and evolutionary history

[edit]
Further information:Evolution of the horse
Equus
Cladogram ofEquusafter Vilstrupet al.(2013).[8]

The genusEquuswas firstdescribedbyCarl Linnaeusin1758.It is the only recognizedextantgenus in the familyEquidae.[9]The first equids were small, dog-sized mammals (e.g.Eohippus) adapted for browsing on shrubs during theEocene,around 54 million years ago (Mya). These animals had three toes on the hind feet and four on the front feet with small hooves in place of claws, but also hadsoft pads.[10]Equids developed into larger, three-toed animals (e.g.Mesohippus) during theOligoceneandMiocene.[9][10]From there, the side toes became progressively smaller through thePleistoceneuntil the emergence of the single-toedEquus.[11]

The genusEquus,which includes all extant equines, is believed to have evolved fromDinohippus,via the intermediate formPlesippus.One of the oldest species isEquus simplicidens,described as zebra-like with a donkey-like head shape. The oldest material to date was found inIdaho,USA. The genus appears to have spread quickly into the Old World, with the similarly agedE. livenzovensisdocumented from western Europe and Russia.[11]Molecular phylogenies indicate that the most recent common ancestor of all modern equines (members of the genusEquus) 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 mostrecent common ancestorwithin the range of 4.0 to 4.5 Mya.[12]

Mitochondrial evidence supports the division ofEquusspecies into noncaballoid (which includes zebras and asses) and caballoids or "true horses" (which includesE. caballusandE. ferus przewalskii,alternativelyE. przewalskii).[8][13]Of the extant equine species, the lineage of the asses may have diverged first,[9][contradictory]possibly as soon asEquusreached the Old World.[13]Zebras appear to bemonophyleticand differentiated in Africa, where they areendemic.[8]Members of the subgenusSussemionuswere abundant during the Early and Middle Pleistocene of North America and Afro-Eurasia,[14]but only a single species,Equus ovodovisurvived 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 theShang dynasty.[15][16]Genetic data fromE. ovodovihas placed theSussemionuslineage as closer to zebras and asses than to caballine horses.[16]

Molecular dating indicates the caballoid lineage diverged from the noncaballoids 4 Mya.[8]Genetic results suggest that all North American fossils of caballine equines, as well as South American fossils traditionally placed in the subgenusE.(Amerhippus), belong toE. ferus.[17]Remains attributed to a variety of species and lumped together asNew World stilt-legged horses(includingE. francisci,E. tau,andE. quinni) probably all belong to a second species that was endemic to North America.[18]This was confirmed in a genetic study done in 2017, which subsumed all the specimens into the speciesE. francisciwhich was placed outside all extant horse species in the new genusHaringtonhippus[19],although its placement as a separate genus was subsequently questioned.[20]A separate genus of horse,Hippidionexisted in South America.[21]The possible causes of the extinction of horses in the Americas (about 12,000 years ago) have been a matter of debate. Hypotheses includeclimatic changeandoverexploitationby newly arrived humans.[22][23]Horses only returned to the American mainland with the arrival of theconquistadoresin 1519.[24]

Extant species

[edit]
Subgenus Image Scientific name Common name Distribution
Equus Nokota Horses cropped Equus ferus przewalskii/E. przewalskiiandEquus caballus (Przewalski's horseand domesticatedhorse) Eurasia
Asinus Equus africanus African wild ass(includes domesticateddonkey) Horn of Africa, in Eritrea, Ethiopia and Somalia
Equus hemionus Onager,hemione, or Asiatic wild ass Iran, Pakistan, India, and Mongolia, including in Central Asian hot and cold deserts of Kazakhstan, Uzbekistan, Turkmenistan, and China
Equus kiang Kiang Tibetan Plateau
Hippotigris Equus grevyi Grévy's zebra Kenya and Ethiopia
Equus quagga Plains zebra south of Ethiopia through East Africa to as far south as Botswana and eastern South Africa
Equus zebra Mountain zebra south-western Angola, Namibia and South Africa.

Prehistoric species

[edit]

Many extinct prehistoric species ofEquushave been described. The validity of some of these species is questionable and a matter of debate. For example,Equus niobrarensisis likely synonymous withEquus scotti,whileEquus alaskaeis most likely the same species asEquus lambei,which itself may be a North American form of the livingEquus przewalskii.

DNA studies on American horse remains found frozen intopermafrosthave shown that several of the supposed American species, and the EuropeanEquus ferus,are actually one highly-variable widespread species. ,[25]as if the evolutionary process ofspeciationwas persistently being frustrated by large herds of the horses moving long distances and mixing, carrying theirgenesabout with them.

Domestic species

[edit]

Hybrids

[edit]
Main article:Equid hybrid
A mule (horse and donkey hybrid)

Equine species cancrossbreedwith each other. The most commonhybridis themule,a cross between a male donkey and a female horse. With rare exceptions, these hybrids aresterileand cannot reproduce.[28]A related hybrid, ahinny,is a cross between a male horse and a female donkey.[29]Other hybrids include thezorse,a cross between a zebra and a horse[30]and a zonkey or zedonk, a hybrid of a zebra and a donkey.[31]In areas where Grévy's zebras aresympatricwith plains zebras, fertile hybrids do occur.[32]Ancient DNA identifies the Bronze Agekungaas a cross between the Syrian wild ass and the donkey.

Biology

[edit]
Further information:Equine anatomyandHorse behavior

Physical characteristics

[edit]
From left to right: a cranium, a complete skeleton, a left forefoot frontal, and a left forefoot lateral from a Grévy's zebra

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 13.2hands(54 inches, 137 cm) and weighing up to 405 kg (890 lb).[33]Domesticated horses have a wider range of sizes. Heavy ordraft horsesare usually at least 16 hands (64 inches, 163 cm) high and can be as tall as 18 hands (72 inches, 183 cm) and weigh from about 700 to 1,000 kg (1,500 to 2,200 lb).[34]Someminiature horsesare no taller than 30 inches (76 cm) in adulthood.[35]Sexual dimorphismis 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. Theirdentitionis adapted forgrazing;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.[9][36]

Adun-colored coat withprimitive markingsthat include a dorsal stripe and often leg striping and transverse shoulder stripes reflect thewildtypecoat and are observed in most wild extant equine species.[37]Only the mountain zebra lacks a dorsal stripe.[38]In domestic horses, dun color and primitive markings exist in some animals across many breeds.[39]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.[40]With the exception of the domestic horses, which have longmanesthat lay over the neck and longtail hairgrowing from the top of the tailhead ordock,most equines have erect manes and long tails ending in a tuft of hair.[36]The coats of some equine species undergosheddingin certain parts of their range and are thick in the winter.[40]

Ecology and daily activities

[edit]
Group of onagers grazing

Extant wild equines have scattered ranges across Africa and Asia. The plains zebra lives in lush grasslands and savannas of Eastern and Southern Africa, while the mountain zebra inhabits mountainous areas of southwest Africa. The other equine species tend to occupy more arid environments with more scattered vegetation. Grévy's zebra is found in thorny scrubland of East Africa, while the African wild ass inhabits rocky deserts of North Africa. The two Asian wild ass species live in the dry deserts of the Near East and Central Asia and Przwelski's wild horse's habitat is the deserts of Mongolia. Only the range of the plains and Grévy's zebras overlap.[9]In addition to wild populations, domesticated horses and donkeys are widespread due to humans. In certain parts of the world, populations offeral horsesand feral donkeys exist, which are descended from domesticated animals that were released or escaped into the wild.[41][42]

Equines aremonogastrichindgut fermenters.[43]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 toruminants,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-likececum,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.[9][36]In the African savannas, the plains zebra is a pioneer grazer, mowing down the upper, less nutritious grass canopy and preparing the way for more specialized grazers such asblue wildebeestsandThomson's gazelles,which depend on shorter and more nutritious grasses below.[44]

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 in around in dust for protection against flies and irritation. Except the mountain zebra, wild equines can roll over completely.[36]

Social behavior

[edit]
Plains zebra group

Equines are social animals with two basic social structures.

Horses, plains zebras, and mountain zebras live in stable, closed family groups orharemsconsisting of one adult male, several females, and their offspring. These groups have their ownhome ranges,which overlap and they tend to be nomadic. The stability of the group remains even when the family stallion dies or is displaced. Plains zebra groups gather into large herds and may create temporarily stable subgroups within a herd, allowing individuals to interact with those outside their group. Among harem-holding species, this behavior has only otherwise been observed in primates such as thegeladaand thehamadryas baboon.Females of harem species benefit as males give them more time for feeding, protection for their young, and protection from predators and harassment by outside males. Among females in a harem, a lineardominance hierarchyexists based on the time at which they join the group. Harems travel in a consistent filing order with the high-ranking mares and their offspring leading the groups followed by the next-highest ranking mare and her offspring, and so on. The family stallion takes up the rear.Social grooming(which involves individuals rubbing their heads against each other and nipping with the incisors and lips) is important for easing aggression and maintaining social bonds and status. Young of both sexes leave their natal groups as they mature; females are usually abducted by outside males to be included as permanent members of their harems.[9][36][45]

In Grévy's zebras and the wild ass species, adults have more fluid associations and adult males establish largeterritoriesand monopolize the females that enter them. These species live in habitats with sparser resources and standing water, and grazing areas may be separated. Groups of lactating females are able to remain in groups with nonlactating ones and usually gather at foraging areas. The most dominant males establish territories near watering holes, where more sexually receptive females gather. Subdominants have territories farther away, near foraging areas. Mares may wander through several territories, but remain in one when they have young. Staying in a territory offers a female protection from harassment by outside males, as well as access to a renewable resource. Some feral populations of horses exhibit features of both the harem and territorial social systems.[9][36][45]

In both equine social systems, excess males gather in bachelor groups. These are typically young males that are not yet ready to establish a harem or territory. With the plains zebra, the males in a bachelor group have strong bonds and have a linear dominance hierarchy. Fights between males usually occur over estrous females and involve biting and kicking.[9][36][45]

Communication

[edit]
Przewalski's horsesinteracting

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.[36]

Equines produce a number of vocalizations and noises. Loud snorting is associated with alarm. Squealing is usually made when in pain, but bachelors also squeal while play fighting. The contact calls of equines vary from the whinnying and nickering of the horse and the barking of plains zebras to the braying of asses, Grévy's zebras, and donkeys. Equines also communicate with visual displays, and the flexibility of their lips allows them to make complex facial expressions. Visual displays also incorporate the positions of the head, ears, and tail. An equine may signal an intention to kick by laying back its ears and sometimes lashing the tail. Flattened ears, bared teeth, and abrupt movement of the heads may be used as threatening gestures, particularly among stallions.[36]

Reproduction and parenting

[edit]
Grévy's zebra foal

Among harem-holding species, the adult females mate only with their harem stallion, while in other species, mating is morepromiscuousand the males have larger testes forsperm competition.[46]Estrusin 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 theflehmen responseand 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.[36]Usually, only a single foal is born, which is capable of running within an hour. Within a few weeks, foals attempt to graze, but may continue to nurse for 8–13 months.[9]Species in arid habitats, like Grévy's zebra, have longer nursing intervals and do not drink water until they are three months old.[47]

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.[36]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.[47]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.[45]

Human relations

[edit]
Bronze Agepottery depicting horse and chariot

The earliest archaeological evidence for thedomestication of the horsecomes from sites inUkraineandKazakhstan,dating to around 4000–3500 BCE.[48][49]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.[50]The most recent, but most irrefutable, evidence of domestication comes from sites where horse remains were buried with chariots in graves of theSintashtaandPetrovkacultures c. 2100 BCE.[51]Studies of variation in genetic material shows that a very few wild stallions, possibly all from a singlehaplotype,contributed to the domestic horse,[52][53][54]mating with many mares in early domesticated herds.[55][56][57]

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 andE. caballusis estimated to have occurred 120,000–240,000 years ago, long before domestication. Of the caballine equines ofE. ferus,E. f. ferus,also known as the European wild horse or "tarpan", shares ancestry with the modern domestic horse.[58]In addition, tarpans that lived into modern times may have been hybridized with domestic horses.[52]

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 theHorn of Africa.Genetic evidence finds that the donkey was domesticated twice based on two distinctmitochondrial DNAhaplogroups.It also points to a single ancestor, the Nubian wild ass.[59]Attempts to domesticate zebras were largely unsuccessful, thoughWalter Rothschildtrained some to draw a carriage in England.[60]

Conservation issues

[edit]
Captive Przewalski's horse

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.[9]TheIUCNlists the African wild ass ascritically endangered,Grévy's zebra, the mountain zebra, and Przewalski's horse asendangered,the onager asvulnerable,the plains zebra asnear threatened,and the kiang asleast concern.[61][62][63][64][65]Przewalski's horse was considered to beextinct in the wildfrom the 1960s to 1996. However, following successful captive breeding, it has been reintroduced in Mongolia.[61]

Feral horsesvary in degree of protection and generate considerable controversy. For example, in Australia, they are considered a non-nativeinvasive species,often viewed as pests, though are also considered to have some cultural and economic value.[66]In the United States, feral horses andburrosare generally considered anintroduced speciesbecause they are descendants from domestic horses brought to the Americas from Europe.[67]While they are viewed as pests by many livestock producers, conversely, a view also exists thatE. caballusis areintroducedonce-native species returned to the Americas that should be grantedendangered speciesprotection.[68]At present, certain free-roaming horses and burros have federal protection as "living symbols of the historic and pioneer spirit of the West" under theWild and Free-Roaming Horses and Burros Act of 1971,[69]and inKleppe v. New Mexico,the United States Supreme Court ruled that the animals so designated were, as a matter of law, wildlife.[70]

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