Horses, zebras, and asses

(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

List of Species

African wild ass

Equus africanus

taxonomy

Equus africanus Heuglin and Fitzinger, 1866, Atbara River, Sudan.

other common names

English: Somali wild ass, Nubian wild ass, Abyssinian wild ass; French: Ane sauvage; German: Wildesel.

physical characteristics

Body length 78.7 in (200 cm); shoulder height 49.2 in (125 cm); weight 615 lb (280 kg). Is a medium-sized, long-eared, long-legged, hoofed ungulate, with a short shiny coat that is tan to gray in color with white belly and chest. The mane is erect, pale in color with a dark edge. The muzzle is white with gray between and around the nostrils and on the lips. There is a stripe down the back. The Somali subspecies, E. africanus somaliensis, has leg stripes and occasionally a shoulder stripe. The Nubian subspecies, E. africanus africanus, has a shoulder stripe, but no leg stripes.

distribution

Within the last 20 years its historic range has been reduced by more than 90%. They are currently found in low density in Eritrea and Ethiopia. Small populations may persist in Somalia and Sudan.

habitat

They live in extreme desert conditions (less than 7.8 in [less than 200 mm] of rainfall), mostly in the rift valley of the Horn

of Africa. They range from below sea level to approximately 2,000 ft (700 m). The substrate can vary from sandy soil to lava rock. Like all wild equids, they need to have access to water and it is estimated that during the dry season they stay within 18.6 mi (30 km) of permanent water sources.

behavior

Lives in small temporary groups that are typically composed of fewer than five individuals. The only stable groups are composed of a female and her offspring. In temporary groups, the sex- and age-group structure varies from single-sex adult groups to mixed groups of males and females of all ages. Adult males typically are solitary, but on occasion associate with other males. Adult females were usually associated with their foal and/or yearling. Some adult males are territorial and only territorial males have been observed copulating with estrous females.

feeding ecology and diet

They feed primarily on available grasses, but will also utilize browse.

reproductive biology

Probably polygamous. Polyestrous, and most foals are born during the rainy season from October–February. A few females have produced foals every other year, but other females have surviving foals at longer inter-birth intervals. Females have not been observed to produce their first foal before the age of five years. Natality is strongly correlated with rainfall during the previous 12-month period. Gestation is approximately 12 months.

conservation status

Critically Endangered, with a 90% reduction in range in the last 20 years. Major threats are hunting for food and medicine, potential competition for water and forage with domestic livestock, and possible interbreeding with domestic donkeys.

significance to humans

In some areas where they occur, they are used for meat and medicine. In Eritrea, they are conserved by the local Afar pastoralists as an important part of the natural environment.

Grevy's zebra

Equus grevyi

taxonomy

Equus grevyi Oustalet, 1882, Galla country, Ethiopia.

other common names

French: Zebra de Grevy; German: Grevyzebra.

physical characteristics

Body length 118.1 in (300 cm); shoulder height 63 in (160 cm); weight 990 lb (450 kg). Is the largest wild equid; a medium-sized, long-legged, hoofed ungulate with large rounded ears and a short coat that is striped black and white. The belly is white and the mane is erect and striped. The muzzle is white with gray between and around the nostrils and on the lips. There is a dark stripe with white margins down the back.

distribution

Currently found in low density in Kenya from the Laikipia Plateau to the Ethiopian border. They are also found in southern Ethiopia in the Chalbi and Borana reserves. The northernmost population is found in the Alledeghi Wildlife Reserve of Ethiopia. A small population may persist in southeastern Sudan.

habitat

Live in arid and semiarid grasslands. They need to have access to water. Lactating females need access to water every one to two days.

behavior

Has a territorial mating system; territorial males dominate on large resource territories, usually in the vicinity of permanent water sources. Lactating females with young foals (less than three months of age) tend to stay on these territories for daily access to water. Thus, when they come into postpartum estrus, the territorial male has better access to reproductive females. Live in small temporary groups and the only stable social group is composed of a female and her offspring. In temporary groups, the sex and age-group structure varies from single-sex adult groups to mixed groups of males and females of all ages. Adult males typically are solitary, but on occasion associate with other males.

feeding ecology and diet

Feed primarily on available grasses, but will also utilize browse during drought periods.

reproductive biology

Polygamous. Polyestrous, and most foals are born after periods of good forage availability. During droughts, females will be anoestrous. Gestation is approximately 13 months, and age of puberty at three to four years.

conservation status

Endangered, with a 70% reduction in population size in the last 30 years. Major threats are hunting for food, medicine, and hides, competition for water and forage with people and domestic livestock, and loss of habitat.

significance to humans

In some areas where they occur, they are used for meat and medicine. In recent times, they were one of the most important herbivores in the arid and semiarid grasslands of Kenya and Ethiopia. Due to their severe decline in numbers, they no longer play an important role in the biodiversity of these grassland ecosystems.

Mountain zebra

Equus zebra

taxonomy

Equus zebra Linnaeus, 1758, Paardeburg near Malmesbury, southwest Cape Province, South Africa.

other common names

English: Cape mountain zebra, Hartmann's mountain zebra; French: Zebre de montagne; German: Bergezebra.

physical characteristics

Body length 102 in (260 cm); shoulder height 59.1 in (150 cm); weight 750 lb (340 kg). Is a medium-sized, long-legged, hoofed

ungulate, with a short coat that is striped black and white with wider stripes on the rump and a white belly. A dewlap gives a distinctive appearance. The mane is erect, and also striped. The muzzle is tan to dark gray between the nostrils and on the lips.

distribution

Occurs in small relict populations in Cape Province of South Africa. The largest populations occur in the Mountain Zebra National Park and the Karoo National Park. The Hartmann's mountain zebra occurs in small numbers in northwestern South Africa. The main population is in Namibia and occupies most of its historic range.

habitat

Live in semiarid mountainous grassland and shrubland. During the hotter months, Cape mountain zebra use the more open grasslands, and move to the ravine and wooded hills in the cold months.

behavior

Has stable family (harem) groups composed of one male and one to five females and their offspring. Bachelor males are usually less than five years of age and travel in less stable groups. In the Mountain Zebra National Park family groups have overlapping home ranges.

feeding ecology and diet

Very selective in their grazing, e.g., more leaf than stalk. However, when forage quality decreases, they can feed on higher-fiber, more senescent grasses. They will also feed on browse when grass availability declines.

reproductive biology

Polygamous. Polyestrous, and age at puberty may range 13–30 months. Males normally do not attain harem male status until they are five years old. Gestation is about 12 months.

conservation status

Endangered, due to small population size. Major threats are fragmented and small populations, droughts and reduced access to water and forage, and interbreeding between the two subspecies.

significance to humans

They are a strikingly beautiful animal and are an important ecological component of their grassland and shrubland ecosystems. In Namibia, they are used for meat and the sale of skins.

Plains zebra

Equus burchellii

taxonomy

Equus burchellii Gray, 1824, north Cape Province, South Africa.

other common names

English: Common zebra, Burchell's zebra, painted quagga; French: Zebre de steppe; German: Steppenzebra.

physical characteristics

Body length 98 in (250 cm); shoulder height 55 in (140 cm); weight 772 lb (350 kg). Is a medium-sized, long-legged, hoofed ungulate, with a short coat that is striped black and white. The stripe patterns vary with subspecies and geographic location. The mane is erect and striped.

distribution

Occurs in eastern sub-Saharan Africa from Sudan and Ethiopia to Namibia, Botswana, and South Africa. The largest populations occur in Kenya and Tanzania.

habitat

Live in mesic grasslands and are capable of migrating long distances. Their ability to utilize coarser vegetation of poorer quality means that they can thrive and survive in a range of habitats.

behavior

Lives in stable family (harem) groups composed of a male and one to six females and their offspring. Both male and female offspring disperse from the natal group. Young males and deposed harem males live in bachelor groups that are loose aggregations. Family groups and bachelor groups will often form into larger aggregations/herds.

feeding ecology and diet

Feed primarily on grasses. They are selective in their feeding, but can feed on higher-fiber grasses when there is no choice. They must have access to water.

reproductive biology

Polygamous. Polyestrous, and most foals are born during the rainy season. Age of puberty has been estimated at 15–22 months in the female. Males can reach puberty at 24 months, but rarely take over harems until they are older. Gestation is 12 months. Normally, less than 50% of the females had foals in any year. Throughout the species range, mortality rate can vary in the first year, from 19–47%. Adult mortality is lower and varies from 3–17%.

conservation status

Lower Risk/Least Concern; however, one subspecies, the quagga, is already extinct and three subspecies are Data Deficient. Threats to this species include loss of habitat and overhunting.

significance to humans

In most parts of their range, they are used for meat and their hides are sold commercially. This utilization is potentially sustainable, but needs to be studied and closely monitored. The plains zebra is a symbol of the African savannahs and wildlife and is important to photographic tourism. This medium-sized herbivore plays an important role in grassland ecosystems.

Asiatic wild ass

Equus hemionus

taxonomy

Equus hemionus Pallas, 1775, Transbaikalia, Russia. There are six subspecies currently recognized.

other common names

English: Kulan, onager, khulan, khur, dzigettai.

physical characteristics

Shoulder height 43–50 in (108–126 cm); weight 441–573 lb (200–260 kg). Is a medium-sized, long-eared, long-legged, hoofed ungulate, with a short coat that is tan to reddish in color with a white belly and chest. They also have distinctive white markings on the posterior portion of the shoulder and the anterior portion of the rump. The mane is erect, and dark in color. The muzzle is white with gray between and around the nostrils and on the lips. There is a stripe down the back that is edged in white.

distribution

The five remaining subspecies are found in restricted ranges in Mongolia, Kazakhstan, Uzbekistan, Turkmenistan, Iran, and India. They have been reintroduced in Israel and Saudi Arabia.

habitat

Live in habitats ranging from semiarid grasslands to extreme salt desert conditions.

behavior

Females live in small temporary groups that are typically composed of fewer than five individuals. The only stable groups are composed of a female and her offspring. Males may form small seasonal harem groups and hold seasonal territories near permanent water sources. Some females may stay on one territory during the entire breeding season.

feeding ecology and diet

Little is known about the feeding ecology of the Asiatic wild ass. When grass is abundant, they are primarily grazers. During drier seasons or in drier habitats, they will also feed on browse.

reproductive biology

Monogamous. Polyestrous, breeding is seasonal, and gestation is 11 months. Most foals are born during a two- to three-month period. Foaling and mating occurs during the rainy season when vegetation is most abundant.

conservation status

Listed as Vulnerable. Only one subspecies, the khulan, has a substantial population size. Major threats are over-hunting, loss of habitat, and competition for forage and water.

significance to humans

In some areas where they occur, they are used for meat. In all areas where they occur, they are an important component of the grassland and desert ecosystems.

Kiang

Equus kiang

taxonomy

Equus kiang Moorcroft, 1841, Ladak, Kashmir, India.

other common names

English: Tibetan wild ass.

physical characteristics

Shoulder height 39–56 in (100–142 cm). Is a medium-sized, long-legged, hoofed ungulate; the color of the coat changes seasonally and is red-chestnut in summer and dark brown in the winter. During the winter, the length of the hair doubles to approximately 1.2 in (3 cm). Has a white belly, whitish legs, and distinctive patches of white on the neck, chest, and shoulder. The mane is erect, short, and dark in color. The muzzle is white with gray between and around the nostrils and on the lips.

distribution

Found in China, India, Nepal, and Pakistan.

habitat

Live in alpine grasslands and arid steppes. They are found at high altitudes ranging 8,800–17,400 ft (2,700–5,300 m).

behavior

Has a social organization similar to other arid habitat equids. Information is limited, but during the breeding season males are solitary and their spacing suggests that they are territorial.

feeding ecology and diet

Feed primarily on available grasses, in particular Stipa spp.

reproductive biology

Gestation is approximately 12 months. The peak in foaling and breeding occurs from June–September when forage is most abundant. Mating system not known.

conservation status

Listed as Lower Risk/Least Concern. The total kiang population is estimated to be 60,000–70,000. Major threats are commercial hunting, loss of habitat, and competition for forage and water.

significance to humans

In some areas, they are hunted as a source of meat. In the Chang Tang Reserve in the Tibet Autonomous Region, they are one of the largest herbivores and their large herds have the potential to draw admiring tourists from throughout the world.

Przewalski's horse

Equus caballus przewalskii

taxonomy

Equus ferus przewalskii (Groves, 1986).

other common names

English: Przewalski's wild horse, Asiatic wild horse, Mongolian wild horse, tahki.

physical characteristics

Shoulder height 49–58 in (124–146 cm); weight 772 lb (350 kg). Is a medium-sized, long-legged, hoofed ungulate. During the summer months, its coat is short and reddish-brown. During the colder winter months, hairs grow thicker and longer and provide good insulation. They have erect manes and the top of the tail has short hairs. The muzzle is white with dark gray around the nostrils and on the lips.

distribution

Extinct in the Wild; it has been reintroduced to Hustain Nuruu and Takhin Tal in Mongolia.

habitat

Last observed in the arid cold steppes of the Gobi desert. It may have also lived in the more mesic Eurasian steppes.

behavior

Social organization and mating system is a stable family (harem) group that is composed of one male and several females and their offspring. Young males form bachelor groups. Dispersal of young males and females from their natal groups occurs 12.5–30.5 months.

feeding ecology and diet

Feed primarily on available grasses.

reproductive biology

Polygamous. Reaches puberty at two to three years of age. Foaling is seasonal from April–August, with a significant peak in May. The age of first foaling is 4.5 years. In females over the age of five years, natality ranges 56–100%.

conservation status

Extinct in the Wild; they are being reintroduced at several sites in Mongolia. Threats that contributed to their extinction were hunting for food, reduced access to water sources, loss of habitat and competition for water and forage with domestic livestock, and zoological capture expeditions. Currently, a major threat is loss of genetic diversity.

significance to humans

The Przewalski's horse is of great significance to the government and people of Mongolia. Concerned individuals and governments from Holland, Switzerland, and France are donating significant time, energy, and funds to make the re-introduction of the Przewalski's horse into the wild a reality.

Resources

Books

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.

Organizations

IUCN Species Survival Commission, Equid Specialist Group. Box 2031, Arusha, Tanzania. E-mail: [email protected]

Patricia D. Moehlman, PhD