Equidae

(vertebrate zoology) A family of perissodactyl mammals in the superfamily Equoidea, including the horses, zebras, and donkeys.

(Equidae)

Class: Mammalia

Order: Perissodactyla

Family: Equidae

Thumbnail description
Medium-sized herbivore with long legs, hard, single-toed hooves, erect mane, long tail, and short coat; some species have a striped coat color

Size
Head and body length 77–118 in (195–300 cm); shoulder height 45–63 in (115–160 cm); weight 440–990 lb (200–450 kg)

Number of genera, species
1 genus; 7 species

Habitat
Savanna grassland, shrub-land, semiarid grassland, and desert grassland

Conservation status
Extinct in the Wild: 1 species; Critically Endangered: 1 species; Endangered: 2 species; Vulnerable: 1 species; Lower Risk/Least Concern: 2 species

Distribution
Sub-Saharan Africa, Middle East, Arabia, Central Asia, and Mongolia

Evolution and systematics

The fossil record of the family Equidae begins 55 million years ago in the Eocene with the small "dawn horse," Eohippus. The main trends in the evolution of Equidae were an increase in body size, lengthening of the legs, reduction in the number of toes culminating with the single-toed hoof, increasing height and complexity of crown patterns in the cheek teeth, lengthening and deepening of the skull to accommodate the high-crowned cheek teeth, and an increase in the size and complexity of the brain. The mainstream of equid evolution occurred in North America and during the Pleistocene, when the modern genus Equus appeared and spread to Eurasia and Africa. In North America, there were approximately eight equid species. However, during the late Pleistocene, there was a mass extinction of mammals in North America and all the equid species disappeared. In Eurasia and Africa, seven species of equids survived.

Equidae is in the order Perissodactyla, the odd-toed ungulates. This order also includes Rhinocerotidae and Tapiridae. Equidae has one genus, Equus. Current taxonomy identifies seven species in this genus. However, taxonomy is an evolving science and questions remain concerning evolutionary and genetic relationships and whether some species should be split or combined.

Physical characteristics

Equids are medium-sized odd-toed ungulates. Anatomically, they are adapted for moving quickly and efficiently (long legs), feeding on higher-fiber grasses (high-crowned premolars and molars), quicker digestion of high-fiber forage (single stomach and rear-gut fermentation), and moving over hard and rocky substrate (single-toed hoof).

The three species of zebra each have different stripe patterns. Within a species, it is possible to identify individuals by their unique rump and side stripe patterns. The African wild ass (Equus africanus) has stripe patterns on its legs that make it possible to distinguish individuals. The Asiatic wild ass (E. hemionus) and the kiang (E. kiang) do not have stripes and the identification of individuals is more difficult. Przewalski's horse (E. caballus przewalskii) may have leg stripes. All equid species have a short coat, although in temperate latitudes (e.g., the Przewalski's horse) they may grow a thicker, longer coat. Male and female may differ by less than 10% in terms of height and weight. There is little sexual dimorphism in equid species.

Distribution

Equids are found in sub-Saharan Africa, the Middle East, Arabia, Central Asia, and Mongolia. Although most of equid evolution occurred in North America, no wild equids are found there today. The radiation of the genus Equus occurred in Eurasia and into Africa. Four species are found in Africa and the remaining three species occur in the Middle East, Arabia, central Asia, and Mongolia.

Habitat

Equids are mainly grazers and the different species can thrive in habitats from below sea level in extreme deserts (e.g., the Danakil Desert) to mesic grasslands (e.g., the Serengeti Plains) to dry mountain grassland and shrubland.

Behavior

Social organization and reproductive strategies in equids are a complex interaction of individual feeding requirements, access to females, and defense against predation. Individuals will form groups when benefits exceed costs in terms of feeding, predation, disease, and reproduction. The density, continuity of distribution, and biomass of forage are key factors in the stability of association and the spacing of equids. However, water distribution and predation pressure are also important factors. When forage and water availability allows females to be gregarious and form stable groups, then a male can attempt to control access to these females. Among equids, these female groups form strong bonds, and if a male is removed, the females often maintain a stable group. Age and fighting ability and the adult sex ratio can also affect a harem male's success in defending his female group against bachelor males. The harem-defense type of social organization is characterized by long-term, stable non-territorial groups composed of one adult male and one or more females and their offspring (e.g., plains zebra, Equus burchellii; mountain zebra, E. zebra; Przewalski's horse. Other males live in what are often termed bachelor groups. The adult females remain in the social group through time, but another male might displace the current harem male. The group male has exclusive mating rights. Occasionally, there is a subordinate adult male within the group that is reproductively active, but has lower reproductive success. Foals born into a group usually remain with it for two to three years or longer. Dispersal time of young males and females and the causation varies. In plains zebra, a young female might be abducted during her first estrus, but young males tended to leave on their own. In mountain zebra, young females might be driven away by their mothers and young males might be chased away by the dominant male. Among equid populations that have a harem mating system (female-defense polygyny), the following have been observed:

• Multi-male harem groups in which all males defend the females and the dominant male achieves the most copulations.
• Harem males that form alliances and cooperatively defend their harems.
• Populations in which adult male sex ratio is significantly low and single-male harems encounter less intrusion/harassment from bachelor males.

Harassment involving chases and copulations can negatively affect a female's feeding rate and may even result in abortion or involve infanticide.

A prerequisite for the cost-effective viability of female-defense polygyny (stable family/harem groups) is a spatial and temporal patterning of resource availability such that it is possible for females to feed in close proximity. In more mesic habitats, forage for ungulates tends to be more abundant with a more continuous distribution. Large stable groups are more likely to form when one individual's foraging does not adversely affect a conspecifics' foraging. Consequently, closer spacing and larger aggregations are possible when food is abundant. Conversely, food shortages will tend to limit group size and stability.

Predation pressure on large mammals, like equids, should increase the tendency to form groups in order to improve detection of and/or defense against predators. The potential for polygyny among equids is further enhanced because females are able to provide nutritional care for their young, and females do not come into estrus synchronously, which enables a male to mate with several females. Thus, in a mesic habitat, a male can control access to multiple females by virtue of their gregariousness and their non-synchronous estrus. From the female point of view, abundant food allows closer spacing with other females and gregariousness enhances predator detection. In addition, the presence of a dominant male precludes harassment by other males in the population.

In more arid environments, limited food availability (both spatially and temporally) usually does not permit females to forage in close proximity and/or to be associated consistently. In dry habitats, equids exhibit the same nutritional and reproductive characteristics (e.g., females provide nutrition and females tend to come into estrus asynchronously), which allow males to attempt multiple matings, but indirectly control access to the females. In most cases, they actually control access to a critical resource, i.e., water. In the resource-defense type of social organization, the only stable groups are a female and her offspring. No permanent bonds persist between adult individuals (African wild asses, Grevy's zebra, E. grevyi; and Asiatic wild asses). Some males are territorial, dominate their areas for years, and have exclusive mating rights within their territories. Conspecifics of both sexes are tolerated in these territories.

Feeding ecology and diet

Equids are primarily grazers and have dental adaptations for feeding on grasses. Their high-crowned molars with complex ridges allow them to effectively grind grasses with higher-fiber content. Though individuals will select the most nutritious and lower-fiber forage, they can process senescent and higher fiber grasses. Equids also have a single stomach and hindgut fermentation. This allows them to digest and assimilate larger amounts of forage during a 24-hour period. By contrast, ruminants with a four-chambered stomach are limited in the volume of forage that can be digested in a 24-hour period. Equids are more effective in assimilating forage and can tolerate and survive on a greater breadth of diet in terms of relative forage quality/nutrition.

Reproductive biology

Equids are polyestrous and their estrous cycles are 19–35 days long. They breed seasonally and will cycle until conception or the end of the season. In the temperate zone, the breeding season is in the spring with the appearance of better forage and weather. In the tropics, breeding usually occurs during the rainy season. The mating system of equids tends to vary, depending on environmental conditions. Equid gestation is 11–12 months in duration. Soon after the birth of the foal, the female will come into postpartum estrus (7–18 days). This means that a female has the capability of producing a foal every year at approximately a 12-month interval. However, wild equids rarely produce a surviving foal every year. Normally, they will have a foal every other year if nutritional conditions permit. Natality in the African wild ass correlates significantly with rainfall during the previous 12 months, e.g., the period of gestation.

Wild equids have been observed to reach puberty at one to two years of age. However, wild equids normally produce their first foal at three to five years of age. In the more arid habitats, age of first reproduction may be five years. There is limited information on natality and survivorship in wild equids. Natality can be 0.0–1.0 in African wild ass, but with many wild equids, the average hovers around 0.5. Data on foal and yearling survival are equally sparse. More is known about adult survival in the plains zebra. Based on aging of skulls, adult female annual survival was 0.9–1.0 and most females died by age 16. Plains zebras live in mesic grasslands, and during most years, nutrition may be less of a limiting factor. Wild equids living in more extreme arid environments may have lower survival rates. There is useful information from domestic and feral horses and donkeys. However, these data need to be used with caution as these populations have a long history of domestic breeding and have been introduced to their current habitats.

Data on recently introduced wild equids (e.g., Asiatic wild ass) indicate that, with good environmental conditions, they can exhibit a rapid growth rate. But most wild equids either have stable populations (plains zebra in the Serengeti) or exhibit severe declines due to severe winters and severe drought. Predation can also impact population growth rates and stability. Once again, there are insufficient data as to what age and sex classes are most affected. Predation normally has the most impact on the survival of foals. Disease can also be a source of major mortality in equid populations. Predation by humans, transmission of disease from domestic livestock, and competition for forage and water are major threats to the continued existence of wild equids.

Conservation status

Family Equidae contains one genus, Equus, and seven species. Of these, one is Critically Endangered, two are Endangered, one is Vulnerable, and one is Extinct in the Wild. Only two species have large enough populations to be considered Lower Risk/Least Concern. But one of these, the Asiatic wild ass, illustrates the vulnerability of all wild equids. Kulan, the subspecies of Asiatic wild ass in Turkmenistan, were reduced from a population of approximately 6,000 to roughly 700 individuals in a few years time. This was attributed to human hunting.

Wild equids in their native habitats are threatened by hunting for food and medicine, competition with livestock and people for access to water and forage, fragmentation and reduction of habitat, small population size, and inter-breeding with domestic horses and donkeys.

Significance to humans

Since Paleolithic times, wild equids have been a source of inspiration to artists on cave walls and canvas. Their beauty and speed have been the personification of independence and freedom. Their domestic relatives have had major significance in the social and agricultural history of man. Less well understood is the significant role of wild equids in the ecology of multiple-grazer/browser ecosystems.

Species accounts

Resources

Duncan, P. Horses and Grasses. New York: Springer-Verlag Inc., 1991.

Moehlman, P. D., ed. Equids: Zebras, Asses and Horses. Status Survey and Conservation Action Plan. Gland, Switzerland and Cambridge, UK: IUCN/SSC Equid Specialist Group, IUCN, 2002.

Nowak, R. M. and J. L. Paradiso. Walker's Mammals of the World, 4th ed. Baltimore: John Hopkins University Press, 1983.

Wilson, D. E., and D. M. Reeder. Mammal Species of the World: A Taxonomic and Geographic Reference, 2nd ed. Washington DC: Smithsonian Institution Press, 1993.

IUCN Species Survival Commission, Equid Specialist Group. Box 2031, Arusha, Tanzania. E-mail: tan.guides@habari.co.tz

[Article by: Patricia D. Moehlman, PhD]

A family of mammals, members of which have a single functional digit although the second and third digits persist as splint bones. Includes horses, wild horses, asses (donkeys) and zebras. Quaggas were a member of the family but are now extinct.

The noun has one meaning:

Meaning #1: horses; asses; zebras; extinct animals
  Synonym: family Equidae

Equidae Fossil range: 54–0 Ma PreЄ Є O S D C P T J K Pg N Early Eocene to Recent
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	Perissodactyla
Family:	Equidae Gray, 1821

Look up Equidae or equid in Wiktionary, the free dictionary.

Equidae (sometimes known as the horse family) is the taxonomic family of horses and related animals, including the extant horses, donkeys, and zebras, and many other species known only from fossils. All extant species are in the genus Equus. Equidae belongs to the order Perissodactyla, which includes the extant tapirs and rhinoceros, and still more fossils.

The term equid refers to any member of this family, including any equine.

Evolution

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Main article: Evolution of the horse

The oldest known fossils assigned to Equidae date from the early Eocene, 54 million years ago. They formerly were assigned to the genus Hyracotherium (sometimes known as Eohippus), but the type species of that genus now is regarded to be not a member of this family (see Hyracotherium). These early Equidae were fox-sized animals with three toes on the hind feet, and four on the front feet. They were herbivorous browsers on relatively soft plants, and already adapted for running. The complexity of their brains suggest that they already were alert and intelligent animals.^[1] Later species reduced the number of toes, and developed teeth more suited for grinding up grasses and other tough plant food.

The family became relatively diverse 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 sole surviving genus, Equus, had evolved by the early Pleistocene, and spread rapidly though the world.^[2]

Classification

Hyracotherium

Orohippus

Hypohippus

Mesohippus

Hipparion

Equus (Przewalski's horse)

• 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 taxa.
  • Family Equidae
    • Subfamily †Hyracotheriinae
      • Genus †Epihippus
      • Genus †Haplohippus
      • Genus †Heptaconodon
      • Genus †Hyracotherium (possibly paraphyletic with some species belonging to the family †Palaeotheriidae)
      • Genus †Orohippus
      • Genus †Xenicohippus
    • Subfamily †Anchitheriinae
      • Genus †Anchitherium
      • Genus †Archaeohippus
      • Genus †Desmatippus
      • Genus †Hypohippus
      • Genus †Kalobatippus
      • Genus †Megahippus
      • Genus †Mesohippus
      • Genus †Miohippus
      • Genus †Parahippus
      • Genus †Sinohippus
    • Subfamily Equinae
      • Genus †Merychippus
      • Tribe †Hipparionini
        • Genus †Eurygnathohippus
        • Genus †Hipparion
        • Genus †Hippotherium
        • Genus †Nannippus
        • Genus †Neohipparion
        • Genus †Pseudhipparion
      • Tribe Equini
        • Genus †Astrohippus
        • Genus †Calippus
        • Genus †Dinohippus
        • Genus Equus (22 species, 7 extant)
        • Genus †Hippidion
        • Genus †Onohippidium
        • Genus †Pliohippus
        • Genus †Protohippus
    • Genus †Acritohippus
    • Genus †Eurohippus
    • Genus †Heteropliohippus
    • Genus †Parapliohippus
    • Genus †Proboscidipparion

References

1. ^ Palmer, D., ed (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. pp. 255. ISBN 1-84028-152-9. 
2. ^ Savage, RJG, & Long, MR (1986). Mammal Evolution: an illustrated guide. New York: Facts on File. pp. 200–204. ISBN 0-8160-1194-X. 

v • d • e Equine Equine science and management Equine anatomy · Equine nutrition · Horse behavior · Horse care · Horse breeding · Equine conformation · Equine coat color · Horse gait Equestrianism and sport Glossary of equestrian terms · Horse tack · Bit · Bridle · Saddle · Harness · English riding · Western riding · Driving · Horse training · Horse racing · Equestrian at the Summer Olympics · Horse show · Equitation Evolution and history Domestication  · In warfare  · In the Middle Ages  · Horses in East Asian warfare  · History of the horse in South Asia  · Horses in the Napoleonic Wars  · Horses in World War I  · Horses in World War II Horse breeds, types and other Equidae Horses List of horse breeds · Wild horse · Feral horse · Stock horse · Gaited horse · Draft horse · Warmblood · Sport horse Other Equus Donkey · Zebra · Onager Hybrids Hinny · Mule · Zebroid Category: Equidae

v • d • e Extant Perissodactyla (Odd-toed ungulates) species by suborder Kingdom Animalia · Phylum Chordata · Class Mammalia · Infraclass Eutheria · Superorder Laurasiatheria Hippomorpha Equidae (Horse family) Equus (including Zebras) Subgenus Equus: Wild horse (E. ferus) · Domestic Horse (E. ferus caballus) Subgenus Asinus: African Wild Ass (E. africanus) · Donkey (E. asinus) · Onager (E. hemionus) · Kiang (E. kiang) Subgenus Dolichohippus: Grévy's Zebra (E. grevyi) Subgenus Hippotigris: Plains Zebra (E. quagga) · Mountain Zebra (E. zebra) Ceratomorpha Rhinocerotidae (Rhinoceros) Rhinoceros Indian Rhinoceros (R. unicornis) · Javan Rhinoceros (R. sondaicus) Dicerorhinus Sumatran Rhinoceros (D. sumatrensis) Ceratotherium White Rhinoceros (C. simum) Diceros Black Rhinoceros (D. bicornis) Tapiridae (Tapirs) Tapirus Baird's Tapir (T. bairdii) · Malayan Tapir (T. indicus) · Mountain Tapir (T. pinchaque) · Black Lowland Tapir (T. pygmaeus) · South American Tapir (T. terrestris) Category

v • d • e Genera of extinct Equidae (Horse family), arranged by subfamily Kingdom Animalia · Phylum Chordata · Class Mammalia · Infraclass Eutheria · Superorder Laurasiatheria · Order Perissodactyla Hyracotheriinae Epihippus · Haplohippus · Heptaconodon · Hyracotherium · Orohippus · Xenicohippus Anchitheriinae Anchitherium · Archaeohippus · Desmatippus · Hypohippus · Kalobatippus · Megahippus · Mesohippus · Miohippus · Parahippus · Sinohippus Equinae Merychippus Tribe Hipparionini: Eurygnathohippus · Hipparion · Hippotherium · Nannippus · Neohipparion · Pseudhipparion Tribe Equini: Astrohippus · Calippus · Dinohippus · Equus · Hippidion · Onohippidium · Pliohippus · Protohippus None Acritohippus · Heteropliohippus · Parapliohippus · Proboscidipparion

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