The English-speaking world measures the height of horses in hands, abbreviated "h" or "hh"; one hand is equivalent to 4 inches. Horses are measured at the highest point of an animal's withers (at the base of the neck). A horse described as 15.2 hh tall is 15 hands, 2 inches (62 inches).
A horse can rest and even doze while standing by locking one of its hind legs at the stifle joint (basically, the knee). A group of ligaments and tendons called the stay apparatus holds the leg in place with minimal muscle involvement. Horses will switch from leg to leg to prevent fatigue in the locked leg.
A female horse is called a mare and a male horse is called a stallion. In the wild, the mare decides when the herd moves on and usually only one stallion will stay with a herd.
Horses live in well-structured groups with clear followers and leaders. Without any human training, horses will line up behind a lead mare according to their rank in the herd, usually with a stallion guarding the rear.
A person who makes horse shoes and fits them onto the horses is called a farrier. Farriers also clip hooves, which grow like fingernails, to keep them from getting overgrown, as well as caring for hooves more generally.
The famous mustangs of the American West, like many other "wild" populations, are actually considered feral, descended from escaped domesticated horses. The only truly wild horses live in Mongolia; the Przewalski.
The Przewalski horse of Mongolia was declared extinct in the wild in the late 1960s. Thanks to an existing captive breeding program, a group was reintroduced to the wild in 1992, and has now successfully reproduced.
THE ORIGIN OF HORSES
The horse evolved 55 million years ago. A close, early relative of the horse is Hyracotherium, also known as an eohippus. The size of a large fox, Hyracotherium stood 10 inches high at its shoulders and had four toes on its front feet and three on its back.
The only surviving branch of the horse family is the genus Equus, which includes zebras, asses, and donkeys along with the horse.
Rhinoceroses and tapirs are the horse's closest living relatives outside the horse family.
As horses adapted to eating tough grasses, their teeth became tougher too. Longer teeth evolved that could wear down without wearing out.
The first domesticated horses were probably kept primarily as a source of food, rather than for work or for riding.
During the Han dynasty, the Chinese mounted an expedition to Fergana, in present-day Uzbekistan, to acquire superior horses. Fergana horses were famous for sweating blood—a mystery now thought to be caused by parasites under their skin.
Kazakh horse herders milk mares and ferment the milk to make koumiss, a mildly alcoholic drink thought to have healthful properties.
HORSES AT WORK
In 1532, 168 Spanish soldiers, 62 on horseback, faced off against 80,000 Inca on foot in western South America and captured the emperor, Atahuallpa.
To protect against the poison gases used in World War I, both soldiers and horses wore gas masks. Horses' noses were covered but their eyes were not, since they could tolerate the poisons better than humans.
Though cavalry charges are now a thing of the past, there are still places where a horse is more useful than a truck. In 2002, for example, during the war in Afghanistan, some U.S. Special Forces rode horses in areas where the rugged terrain and lack of fuel made auto transport impractica
In 1900, around 130,000 horses worked in Manhattan—more than ten times the number of yellow cabs on the streets of New York City today. A typical city horse produced up to 20 kilograms (45 pounds) of manure and 7.5 liters (2 gallons) of urine a day.
In April 1860, a new postal service called the Pony Express was launched. With railroads handling the eastern leg and horsemen racing day and night across the west, from Missouri to California, the Pony Express could get a letter from coast to coast in just 10 days for just five dollars, later reduced to one dollar. Prior to this, the fastest way to get a letter cross-country was still by horse-drawn stagecoach, which took 25 days or more.
To learn more about the powerful and continuing relationship between horses and humans and to explore the origins of the horse family, extending back more than 50 million years, visit the special exhibition The Horse at the American Museum of Natural History, May 17, 2008, through January 4, 2009.
Imagine a world in which horses of all colors, shapes and sizes roamed the world, some barely larger than a small dog. That world no longer exists—but once it was real. Today's horses represent just one tiny twig on an immense family tree that spans millions of years. All the other branches of the horse family, known as Equidae, are now extinct. The earliest known horses evolved 55 million years ago and for mupuch of this time, multiple horse species lived at the same time, often side by side, as seen in this diorama.Ancient Horses
Some 10 million years ago, up to a dozen species of horses roamed the Great Plains of North America. These relatives of the modern horse came in many shapes and sizes. Some lived in the forest, while others preferred open grassland.
Here, two large Dinohippus horses can be seen grazing on grass, much like horses today. But unlike modern horses, a three-toed Hypohippus tiptoes through the forest, nibbling on leaves. A small, three-toed Nannippus, shown here eating shrubs, ate both grass and leaves.
In the background are several other large mammals alive at that time, including Procamelus, a camel relative; a herd of Dinohippus horses; Gomphotherium, a distant relative of true elephants; and Teleoceras, a hornless rhinoceros.
A Brief History of Horses
By 55 million years ago, the first members of the horse family, the dog-sized Hyracotherium, were scampering through the forests that covered North America. For more than half their history, most horses remained small, forest browsers. But changing climate conditions allowed grasslands to expand, and about 20 million years ago, many new species rapidly evolved. Some—but not all—became larger and had the familiar hooves and grazing diets that we associate with horses today. Only these species survived to the present, but in the past, small and large species lived side by side.
Changing Sizes
Horses were once much smaller than they are today. But there was not a steady increase in size over time. Little Nannippus, shown in the diorama at full adult size, was actually smaller than its predecessors.
Mesohippus ~45 kg
Merychippus ~100 kg
Nannippus ~75 kg
Equus ~500 kg
Dinohippus
Single hoof
Ate grass
11 to 4 million years ago
The Dinohippus shown grazing on the left is a close relative of horses today. Like modern-day Equus, Dinohippus had single-toed hooves and ate mostly grass. The other extinct species shown in the diorama had three toes and never developed single hooves.
On Your Toes
Horses, humans and all other mammals share a common ancestor—one with five toes. So how did horses end up with single-toed hooves? Over millions of years, many horse species lost most of their side toes. The middle toe evolved into a single large hoof, while the other toes became smaller and ultimately functionless.
Only one species in this scene, the grazing Dinohippus, has a single hoof. What's the connection between hooves and grazing? Hooves and long legs help horses run farther and faster on the open prairie, helping them flee from predators and find fresh grass for grazing. In the forest, where the ground is softer, many horses retained three toes.
Stretch Your Legs
Comparing a human leg to a horse leg shows which bones give horses their great speed. Horses that moved onto grasslands have longer legs than their forest-dwelling ancestors. But their leg bones did not all lengthen equally. Mostly it was the bones of the foot that grew longer, with the ankle moving relatively higher up on the leg. Long, sturdy, light-weight legs help a horse run faster—a useful trait on the open prairie, where there's no place to hide.
Adapting To A Changing Climate
Two major changes in climate affected the evolution of early horses. First, about 55 million years ago, global temperatures abruptly rose by 5 to 10 °C (9 to 18 °F), turning much of North America into a warm, wet, subtropical forest—much like what you'd see today in the Brazilian Amazonian rainforest. Small, leaf-eating horses thrived.
Then, about 35 million years ago, global temperatures dropped, creating a climate more similar to today's. Thereafter, dry grasslands replaced much of the North American forest, leading to rapid evolution among horses. By about nine million years ago, most forest browsers had disappeared, leaving primarily grass-eating grazers like those alive today.
Hypohippus
Three toes
Ate soft leaves
18 to 9 million years ago
Long after hoofed, grass-eating grazers evolved and adapted to the American plains, three-toed forest browsers like the Hypohippus in this scene still continued to thrive for millions of years. This three-toed lineage is now extinct, but in the past many diverse horses lived side by side.
On Tiptoe Through Time
The earliest horses had three or four functional toes. But over millions of years of evolution, many horses lost their side toes and developed a single hoof. Only horses with single-toed hooves survive today, but the remains of tiny vestigial toes can still be found on the bones above their hoofs.
Where Did Horses Come From?
The majority of horse species evolved in North America. From there, they occasionally walked to other continents. This map shows how horses spread around the world at three different times.
About 20 million years ago, three-toed horses called anchitheres crossed to Asia and continued to Europe and Africa.
About 11 million years ago, three-toed horses called hipparions spread from North America around the globe.
About three million years ago, hoofed Equus, the ancestor of living horses, spread to several continents including South America.
The End Of An Era
About 10,000 years ago, horses became extinct in North America and South America. Equus, the ancestor of all horses today, survived only in Eurasia and Africa. What ended their 55-million-year run in North America? The prime suspects are changes in the environment, disease and overhunting by humans who likely killed them for food.
Talking Teeth
You can tell a lot about what a horse eats from its teeth—even if the teeth are fossils. The first horses all had short, broad chewing teeth, like ours. Later horses had teeth three times longer. Why?
Short teeth are fine if you're browsing on soft leaves, like the Hypohippus shown eating in the forest. But grazing on tough grasses would quickly wear short teeth down to nothing. For prairie horses like Dinohippus, evolution favored longer teeth that could handle the grind of grazing—as a tooth wears down, more emerges.
Glass-In Grass?
Grazing is hard on teeth for two reasons. Grasses contain bits of the mineral silica that resemble glass and wear teeth away like sandpaper. Chomping grass close to the ground also picks up gritty soil that wears teeth away. Grass-eating horses evolved longer teeth that can withstand this wear.
Until recently, scientists thought that all horses with long teeth grazed on grass. But new evidence shows that some long-toothed species also browsed on leaves. How do scientists know? Tiny scratches on fossil teeth, and chemicals preserved in the teeth, provide clues about what the horses ate.
Meet The Relatives
The horse family (Equidae) today is quite small. All horse breeds, from slim thoroughbred racehorses to stocky plow horses to tiny ponies, belong to a single species, Equus caballus. What's more, all surviving branches of the horse family tree are also members of this same genus Equus, which now consists of only seven living species. Other equids include donkeys, asses and zebras.
The horse (Equus caballus) includes all domesticated horse breeds. Some scientists also consider the Asiatic wild horse, or Przewalski horse (above), to be a variety of Equus caballus, though it is often called a separate species, Equus przewalskii. Domestic horses are thought to have been bred from the European wild horse, or tarpan, extinct since 1919.
Przewalksi horse group
Przewalksi horse group
Nigel Bean/Naturepl.com
The donkey (Equus asinus) is a domesticated African ass native to eastern Africa.
The onager and kulan are varieties of the Asiatic ass (Equus hemionus), which has five subspecies in the Middle East and Asia.
The kiang (Equus kiang) is an Asian ass with three subspecies ranging from China to India.
Grevys Zebra
Grevy's zebra
Leonard Lee Rue III/Photo Researchers
The largest zebra, Grevy's zebra (Equus grevyi) of eastern Africa, has the narrowest stripes.
Known for the "gridiron" stripes on its rump, the mountain zebra (Equus zebra) of southern Africa is endangered due to poaching and habitat loss.
Burchell's zebra (Equus burchelli) has wide stripes. It has several subspecies with distinctive patterns.
The quagga, a form of Burchell's zebra, sometimes considered its own species, disappeared in the mid-1800s. It formerly lived in southeastern Africa.
About Scientific Names
Scientists use scientific names to catalog life-ideally, each true species should have a name different from every other. With very closely related organisms, however, it may be difficult to draw a sharp line between species. While many scientists think that all living horses can be grouped in one species (Equus caballus), as in this exhibition, agreement is not universal.
Next of Kin
The only surviving branch of the horse family is the genus Equus, which includes zebras, asses and donkeys along with the horse. But which living animals outside the horse family are the horse's closest relatives? Hint: you won't find them on a farm.
Brontothere by Charles Knight
The Field Museum
Here's another hint: follow the feet. Horses belong to a group of mammals with an odd number of toes. That rules out mammals with two toes, or "cloven hooves," like goats, pigs, cows, deer and camels.
So who are the other odd-toed, plant-eating animals? Most members of this group, known as perissodactyls, are extinct. But several species survive at present. They include rhinoceroses and tapirs, the horse's closest living relatives.
Extinct Relatives
and my favorite :The Legendary SECRETARIAT
Horses are more closely related to extinct perissodactyls like this brontothere than they are to cows, pigs and goats.
