Animal Diversity Web

Diversity

Equus is a relatively small group within the Order Perissodactyla which contains 8 different grassland herbivore species. These species include the zebras; Equus zebra, Equus grevyi, Equus burchellii (Groves and Bell 2004), and Equus quagga, the horse; Equus caballus, and the asses/donkeys; Equus asinus, Equus hemionus and Equus kiang (MacFadden and Guralnick 1999; Nacarino-Meneses et al. 2016). Due to the domestication of these animals, they are widespread throughout the world. In their native habitat they are secluded to areas where large expansions of grasses can be found or supplied.

There are over 350 breeds in species E. caballus, ranging from light and athletic horses, to miniatures, to large and thick body types meant for pulling heavy loads. Between these breeds there is a wide range of color and color patterns. There is such large diversity within E. caballus due to human exploitation of this species. Human need for transportation, war, agricultural, and pleasure has lead to the various body types across E. caballus (Macfadden and Guralnick 1999).

Along with asses and donkeys, there are over 150 breeds. While this group does not display vast changes in body size and type, there is still variation due mostly to human domestication. Asses and donkeys were used more universally for packing than the wider variety of duties seen in E. caballus (Nacarino-Meneses et al. 2016).

Zebras are less diverse in breed, size and color variation. Largely because they are much less domesticated than horses and donkeys/asses. They all display similar black and white color pattern (Fischhoff et al. 2007).

Geographic Range

Equus species are limited to areas with access to large landscapes of grasses. This includes grasslands, savannas, tundras, forests, and mountain ranges. However, due to their domestication, Equus species are extremely widespread and can be found where ever there are humans. As long as grasses can be supplied through human interaction, q.Equus can be found anywhere.

The native geographic region of Equus asinus is Northern Africa, in the Arabian Peninsula. Feral and domesticated E. asinus can be found in all parts of the world due to human intervention. Equus hemionus has been extinct since 1927. Their native range was mainly in southern Mongolia but supposed subspecies are found in China, Russia, Iran, and India. Equus kiang prefer desert or semidesert areas and their native range is China, Tibet, India, and Nepal (Nacarino-Meneses et al. 2016). Equus burchellii is naturally found in southeastern Africa, namely Kenya and Tanzania. Equus grevyi is found in Africa and mostly found in East Africa. Equus quagga has been extinct since 1883 but had a native range in southern Africa. The mountain zebra, Equus zebra is naturally found in Cape Mountain in southern Africa and the Kamanassie Mountains in south west Africa (Fischhoff et al. 2007). Equus caballus is the most widespread species within Equus. Due to their domestication, they are found generally where ever there are humans. E. caballus native ranges included the northern tip of Africa, China, North America, Mongolia, and Europe (Macfadden and Guralnick 1999).

• Biogeographic Regions
• nearctic
  • introduced
• palearctic
  • native
• oriental
  • introduced
• ethiopian
  • native
• neotropical
  • introduced
• australian
  • introduced

• Other Geographic Terms
• cosmopolitan

Habitat

Equus species occupy grasslands, savannas, mountain ranges, tundras, deserts, swamps, wetlands, woodlands and temperate grasslands. They rely on a habitat that includes their dietary needs of large ranges of grasses. Although some species can survive in harsher climates, such as tundras and deserts, they still rely on grass vegetation as their staple diet. Domesticated species can be found in every habitat as long as their diet can be supplemented through human intervention (Van der Made 2012).

• Habitat Regions
• temperate
• terrestrial

• Terrestrial Biomes
• tundra
• desert or dune
• savanna or grassland
• chaparral
• forest
• scrub forest
• mountains

• Wetlands
• marsh

• Other Habitat Features
• urban
• agricultural

Systematic and Taxonomic History

There is an on going debate on the wild horse, Equus ferus przewalskii. Previously believed to be a subspecies of E. caballus but now new molecular data disagrees with this hypothesis. Differences in chromosomes leave the relationship of this wild horse species unsure (George and Ryder 1986; Steiner et al. 2012).

• Synonyms

  • Equus caballus caballus Linnaeus, 1758
  • Equus ferus caballus Linnaeus, 1758
  • Asinus vulgaris Gray, 1824
  • Equus asinus asinus Linnaeus, 1758
  • Equus asinus palaestinae Ducos, 1968
  • Equus africanus asinus Linnaeus, 1758
  • Equus sylvestris von den Brincken, 1826
  • Equus gmelini Antonius, 1912
  • Equus caballus ferus Boddaert, 1785
  • Equus ferus ferus Boddaert, 1785
  • Equus faurei Matschie, 1898
  • Equus grevyi berberensis Pocock, 1902
  • Equus zebra hartmannae Matschie, 1898
  • Equus penricei Thomas, 1900
  • Hippotigris hartmannae matschiei Zukowsky, 1924
  • Equus hemionus hemippus I. Geoffroy Saint-Hilaire, 1855
  • Equus hemionus blanfordi (Pocock, 1947)
  • Equus hemionus khur Lesson, 1827
  • Equus caballus przewalskii Poliakov, 1881
  • Equus ferus przewalskii Poliakov, 1881
  • Equus zebra zebra Linnaeus, 1758
  • Equus wardi Ridgeway, 1910
  • Hippotigris campestris Gray, 1852
  • Equus indica Trouessart, 1898
  • Equus montanus Burchell, 1822
  • Equus zebra frederici Trouessart, 1905
  • Asinohippus Trumler, 1961
  • Asinus Brisson, 1762
  • Asinus Gray, 1824
  • Caballus Rafinesque, 1815
  • Dolichohippus Heller, 1912
  • Grevya Hilzheimer, 1912
  • Hemionus F. Cuvier, 1823
  • Hemionus Stehlin and Graziosi, 1935
  • Hemippus Dietrich, 1959
  • Hippotigris C. H. Smith, 1841
  • Ludolphozecora Griffin, 1913
  • Megacephalon Hilzheimer, 1912
  • Megacephalonella Strand, 1943
  • Microhippus Matschie, 1924
  • Onager Brisson, 1762
  • Pseudoquagga Hoffstetter, 1951
  • Quagga Shortridge, 1934
  • Quaggoides Willoughby, 1974
  • Zebra J. A. Allen, 1909
  • Berberensis Pocock, 1902
  • Faurei Matschie, 1898
  • Couagga Desmarest, 1822
  • Danielli Pocock, 1904
  • greyi Lydekker, 1902
  • Lorenzi Lydekker, 1902
  • qouagga Lesson, 1827
  • Quaccha Gray, 1827
  • trouessarti Camerano, 1908

• Synapomorphies

  • Number of functional digits
  • Degree of isolation of protocone and hypocone
  • Presence and size of preorbital facial fossa
  • Presence and degree of development of secondary infundibular fold on I2
  • Size and position of inferior canines
  • Presence and degree of development of isthmus on inferior molar
  • Proportions of metacarpal and metatarsal III
  • Shape of lambdoidal crest
  • Relative length of snout
  • Relative depth of rostrum
  • Presence and degree of development of secondary infundibular fold in 13
  • Presence of "cranial broadening complex"
  • Degree of "cranial flexion
  • Relationships of mastoid, paramastoid, and mastoid part of temporal bone
  • Orientation of postorbital bars relative to horizontal plane
  • Presence of supraorbital boss
  • Size and presence of postglenoid notch in crista temporalis, or ear size
  • Presence of "frontal doming"
  • Relative skull breadth
  • Presence of flattened incisors, lacking the secondary infundibular fold, which also lack the posterior commissure
  • Presence of opisthotic dolichocephaly

Physical Description

Equus are odd-toed ungulates with oval shaped hoofs. They feature large and long heads with eyes facing sideways, as is characteristic with prey animals. Equus have teeth consistent with herbivores. They all have manes and tails. Asses, donkeys, and zebras only have tufts of hair at the end of a bony tail while horses tails have hair from the top to the bottom. The manes and tails can be curly, straight, or wavy. All variations in coloring is represented from stripes, spots, and mixed coloring. Horses can be solid colored, spotted, and blended (roan), in many colors (Macfadden and Guralnick 1999). Equus caballus also commonly have white markings on the legs and face. Zebras are striped black and white. Asses and donkeys are mostly solid brown, black, or grey but there are exceptions. Equus are stocky animals built for speed. These animals can be all types of sizes from 36-200 cm tall and are usually measured from the ground up to where their back meets their neck (the withers). They also vary in weight from 400-1600 pounds. Males are typically bigger and thicker than females (Steiner et al. 2012).

• Other Physical Features
• endothermic
• bilateral symmetry

• Sexual Dimorphism
• male larger

Reproduction

Equus species characteristically live in herds where one main male watches over a band of females. In this group, the one male mates with the different females in his band.

• Mating System
• polygynous

Breeding can occur in the spring months. They can only reproduce once a year. The litter size across the genus is typically one (Van der Made 2012). Zebras have a gestation period of 10-12 months. They reach sexual maturity at 3-4 years of age. Donkeys have gestation period of 11-14 months. Sexual maturity is reached at about 2 years of age. Horses typically have a gestation period of 11 months and are sexually mature at about 1 year of age. Offspring of this genus are fairly mature at birth and can stand up, nurse, and run soon after birth (Macfadden and Guralnick 1999). (MacFadden and Guralnick, 1999; Van der Made, 2012)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sexual

Lifespan/Longevity

Lifespan is highly variable in equids, especially between captive and wild individuals. In the wild, the struggle for resources, shelter, and surviving predation are factors that account for lower lifespans in Equus species. The zebras, (Equus burchellii, Equus grevyi, and Equus zebra) have a life expectancy of 20 years in the wild and 40 years in captivity. The asses and donkeys (Equus kiang and Equus asinus) (St-Louios 2009) have a wild life (feral populations) expectancy of 20 years and a captive life expectancy of 30 years. Equus caballus is mostly found in captivity and have a life expectancy of 25-30 years, but feral populations have a life expectancy of 15 years (Van der Made 2012).

Behavior

All species in this genus are herd animals and social beings. Since these animals are all prey, they live in groups called herds ranging from 5 to 400 individuals.

Feral populations of Equus caballus have very complex behavior and social structure. A wild herd consists of an alpha male and females with young males. The Alpha male is usually the one who reproduces with his herd of females. He will protect his herd against rival males looking to take over. A younger or strong male may try to overthrow the alpha which leads to aggressive fights which involve kicking, striking, and biting. When females are ready to birth, they will go to a private area away from the herd, give birth, and return. Females give birth at night to help avoid predation on their vulnerable young. Their young can be on their feet and able to run within a few hours of birth. An established dominance order is important to the specie’s social structure. Pecking order is determined through small fights, such as biting or kicking. They communicate their feeling through facial expressions, especially position of their mobile ears. Positioning of their body, swishing tail, and vocal cues are other behaviors that help them communicate. Since predation is a constant threat to E. caballus, they must stay vigilant even in their sleep. When sleeping horses can lock their knees in order to sleep standing up. Because of this they can be ready to get away from danger quickly. They will also break into small groups where all are sleeping laying down but one or two which sleep standing up, waiting guard.

Equus asinus lives in herds averaging about 5 individuals but they can be as large as 100. They are active during cooler hours such as morning and evening. E. asinus are always alert and are cautious animals. They tend to be more stubborn than other species in Equus, and do not flee as quickly as other species. E. asinus also interact well with other animals such as horses, sheep, goats, and cows. Due to their more aggressive and stubborn nature, E. asinus are often used as protection animals for horses or other farm animals. (Nacarino-Meneses et al. 2016).

Equus kiang live in cohesive herds ranging from 5 to hundreds of individuals. Due to their cohesive nature,<< E. Kiang>> tend to do everything together. They eat, drink, and sleep in unison and move in single file. These herds are led by an old female. Unlike other herd structures, E. kaing herds are made up of mostly one gender but the males begin to follow the females in July. During this time the males fight among each other for breeding rights. Swimming is something this species is quite good at. They have been noted to go for swims for pleasure (Nacarino-Meneses et al. 2016).

Equus bruchellii live in groups consisting of one male and one to six females with their young. Bond is important in this group. Grooming and greeting rituals are common in this species. Hierarchy is important in these herds and usually ranges from male to females to young. Age also plays a role in this hierarchy. Young will leave the herd around 2 years but the males typically leave earlier than the females. The young males find bachelor groups until they can acquire their own harem of females. Infanticide has been noted in this species. If a new male takes over a harem of females he may kill previous offspring belonging to the previous alpha male (Groves and Bell 2004).

Equus zebra follows the common social structure. They live in breeding herds with a dominant male and breeding females with their young. One difference is that during reproductive times, females rise in hierarchy ranks. They are bonded and grooming, playing, greeting, and scratching are common behaviors within herds (Fischhoff 2007).

Equus grevyi are one of the less social and structured species belonging to Equus. Their social groups can vary day to day. Males are more territorial versus concerned about a specific herd. They have the right to breed with females in their territory more than their own harem (Groves and Bell 2004).

• Key Behaviors
• cursorial
• terricolous
• diurnal
• motile
• nomadic
• social
• colonial
• dominance hierarchies

Communication and Perception

Horses, donkeys, asses, and zebras share similar forms of perception and communication. They have long hairs on their face to detect touch and vibrations. They have large ears that allow them to have very acute hearing to listen for predators or other stimuli. They also have good eyesight and can see fairly well in the dark. This helps avoid predation even in the dark. (van der Made 2012) Their eyes face sideways to allow for more peripheral vision to see around them while grazing, but have poor vision right in front and behind them. Their main form of communication is their facial expressions, namely their ears. Their ears can swivel and move back and forward. This allows for better audio perception but also for more communication (Pegar 2016). Laying their ears back is a sign of hostility and forward is a form of alertness. Positioning of their body is also a form communication. Presenting their rear end is a threat and may lead to aggressive behavior such as kicking. These animals also communicate orally with, neighing, nickering, braying, and other audio communication (Macfadden and Guralnick 1999).

• Communication Channels
• visual
• tactile
• acoustic

• Perception Channels
• visual
• tactile
• acoustic
• chemical

Food Habits

The members of the Equus are all herbivorous grazers. They rely on grasses but occasionally munch on herbs, leaves, bark and twigs. Captive species are often supplemented grains, mineral/salt blocks, and prepared grasses in the form of grass, alfalfa, or a combination. In the wild Equids are limited to large areas of grass vegetation and must continue moving as the grasses are depleted. Water is also a large factor and these animals may travel far distances to water sources (Dalquesst 1979).

• Primary Diet
• herbivore

Predation

These large animals live in big herds in an attempt to protect against predation and remain more vigilant to the presence of predators. Their main way to avoid predation is to flee as they can reach speeds up to 40 miles per hour. When attacking they bite, kick or strike with extreme power and accuracy. Predators will attack with large concerted efforts, surprise attacks, and will attack mostly the young, old, or weak (Van der Made 2012). When under attack alpha males attack if necessary, while females protect their young. Members of this genus have the ability to lock their knees and sleep standing up in order to be ready to flee at any time. Members of the zebra species (Equus burchellii, Equus grevyi, and Equus zebra) use their stripes to blend members together and confuse predators (Fischhoff 2012).

• Anti-predator Adaptations
• cryptic

• Known Predators

  • lions Panthera leo
  • spotted hyenas Crocuta crocuta
  • African wild dogs Lycaon pictus
  • cheetahs Acinonyx jubatus
  • leopards Panthera pardus
  • Nile crocodiles Crocodylus niloticus
  • gray wolves Canis lupus
  • humans Homo sapiens
  • coyotes (Canis latrans)
  • mountain lions Puma concolor

Ecosystem Roles

Since Equid species are grazers and are a main food sources for keystone species, they have large implications on ecosystem roles. They clear out old vegetation to make room for new growth. Fertilization is another role they play in ecosystem stability. Their manure provides nutrients to the land to help with primary production. Equid species are also help in dispersal of seeds. Seeds can get stuck in the fur of grazing while eating and transported as the animals moves. Seeds can also be dispersed from manure. These animals are all prey for large keystone species that help keep ecosystems in balance.

• Ecosystem Impact
• disperses seeds
• soil aeration

Economic Importance for Humans: Positive

Equid species have brought numerous positive economic importance for humans. Horses have played large roles in all areas of the world. In the past they were largely used for transportation, pulling heavy loads, aided in mining, war, sport, and pets. They cut cost and labor of farm work and transport. Since their need for transportation and work has been reduced throughout the years, they have become a positive economic importance through other ways. They have switched their positive economic importance for humans through sport and as pets. More exotic members of this group bring positive economic benefit through tourist interest, such as zebras in zoos. On a different angle, these animals are commonly used as food or even making products such as glue from their hooves. Their manes and tails can be used to make products such as bows for string instruments. Their manure can be used as cheap fertilizer to improve agricultural benefit.

• Positive Impacts
• pet trade
• food
• body parts are source of valuable material
• ecotourism
• produces fertilizer

Economic Importance for Humans: Negative

While Equid species have many positive economic importance, they also bring some negative impacts as well. Donkeys, asses, zebras, and horses consume huge amounts of grass limiting resources for other grazers of interest, such as buffalo. Remaining populations of North American wild horses and other feral equids species result in resource depletion from agricultural needs, erosion, and habitat degradation. The cost of maintaining feral populations of equid species is an ongoing economic problem. These species can also become invasive and are known to outcompete other grazing species. Introducing new pathogens to agricultural livestock can cause huge economic downfalls.

• Negative Impacts
• injures humans

Conservation Status

Equus burchellii are the only zebra that is not severely threatened or endangered. They are currently facing factors that could change this such as hunting and habitat loss. Equus grevyi is endangered with population estimated to be only about 2,500 in 2009. There is a conservation plan in action. The species Equus zebra is not quite endangered but is vulnerable due to hunting and habitat loss. The species Equus kiang's conservation status in undetermined but hunting has decreased population. Domestic populations of Equus caballus and Equus asinus are not endangered. While they are not endangered rare breeds (less than 500 worldwide) such as the American Cream, Caspian, the Cleveland Bay, the Hackney Horse, the Newfoundland Pony, and the Sufflock. Wild populations of Equus caballus, such as the subspecies, Equus caballus przewalskii (only 200 remaining), are endangered or vulnerable due mostly to habitat loss. Extinct members of Equus include Equus quagga which went extinct in 1883, and Equus hemionus went extinct 1927 but about 6 subspecies are thought to be alive but in very small numbers (Steiner 2012; IUCN).

• IUCN Red List [Link]

  Not Evaluated

Other Comments

Culturally, animals in the genus Equus are important. They have been a symbol of wealth and royalty in many different cultures such as Middle Eastern, European, Irish, and others. In Chinese culture, the horse is the seventh symbol in the Chinese calender.Several horses appear in Greek mythology such as Pegasus. Horses, especially are common in western and native folklore. Equids are also mentioned numerous times in religious stories including the bible. Mary arrives in Bethlehem aboard a donkey which displayed the humbleness and humility of the birth of Jesus. There are over 116,200 fossil specimens recorded and found everywhere but the Antarctic. (MacFadden and Guralnick, 1999).

Contributors

Liz Akers (author), Colorado State University, Tanya Dewey (editor), University of Michigan-Ann Arbor.

References

Dalquest, W. 1979. The little horses (genus Equus) of the Pleistocene of North America. American Midlan Naturalist, 101: 241-244.

Fischhoff, I., S. Sundaresan, D. Rubenstein. 2007. Habitat use and movements of plains zebra (Equus burchelli) in response to predation danger from lions. Behavioral Ecology, 18/4: 725-729.

Ishida, N., T. Hasegawa, H. Mukoyama. 1996. DNA polymorphism of the ryanodine receptor gene, exon 17 among six Equus species. Aminal Genetics, 27: 1.

MacFadden, B., A. Azzaroli. 1987. Cranium of Equus insulatus (Mammalia, Equidae) from the middle Pleistocene of Tarija, Bolivia. Journal of Vertebrate Paleontology,, 7: 325-334.

MacFadden, B., R. Guralnick. 1999.

Horses in the Cloud: big data exploration and mining of fossil and extant Equus (Mammalia: Equidae)

Nacarino-Meneses, C., X. Jordana, M. Kohler. 2016. First approach to bone histology and skeletochronology of Equus hemionus. Comptes Rendus - Palevol, 15: 267-277.

Peglar, M., T. Nerad, R. Anderson. 2016. Stenamoeba polymorpha, a New Species Isolated from Domesticated Horse Equus ferus caballus. Journal of Eukaryotic Microbiology, 63: 698-708.

St-Louios, A., S. Cote. 2009. Equus kiang (Perissodactyla: Equidae). American Society of Mammalogists, 839: 1-11.

Steiner, C., A. Mitelberg, R. Tursi, O. Ryeder. 2012.

Molecular phylogeny of extant equids and effects of ancestral polymorphism in resolving species-level phylogenies

Van der Made, J. 2012. First description of the large mammals from the locality of Penal, and updated faunal lists for the Atapuerca ungulates Equus altidens, Bison; and human dispersal into Western Europe. Quaternary Internationa, 1: 1.

Wallner, B., F. Piumi, G. Brem, M. Muller, R. Achmann. 2004.

Isolation of Y Chromosome-specific Microsatellites in the Horse and Cross-species Amplification in the Genus Equus