The HORSE is a large land mammal notable for its speed, strength,
and endurance. Horses are members of the Equidae family, which also includes
zebras and asses. Like all equids, the horse is extremely well adapted to
traveling long distances with great efficiency and to surviving on a diet
of nutrient-poor, high-fiber grasses. The horse is an intensely social animal,
forming strong associations with members of its herd and possessing a keen
ability to recognize subtle social cues. These instinctive behaviors form
the basis of the horse’s ability to bond with and obey a human trainer.
The horse’s influence on human history and civilization make it one
of the most important domestic animals. Horses were domesticated in Eurasia
around 6,000 years ago. Throughout much of human history, they have provided
humans with mobility and have served in agriculture, warfare, and sport.
Today domestic horses are found throughout the world, with a total population
estimated at 60 million. So-called wild horses, such as those found in the
American West, are actually feral animals, free-living descendants of domestic
horses that escaped or were turned loose.
The wild ancestors of the
modern horse evolved for millions of years in North America. They spread
to other parts of the world by traveling southward to South America and
by crossing land bridges that connected North America to Europe and Asia
during the ice age. Horses vanished from both North and South America in
a wave of extinctions that occurred near the end of the Pleistocene Epoch,
about 15,000 years ago. They were not seen in the Americas again until 1494,
when Italian explorer Christopher Columbus transported them on ships from
Spain on his second voyage to the New World.
which is believed to be the only truly wild horse to survive to modern times,
probably became extinct in the wild in Mongolia in the 1960s. About 1,100
Przewalski's horses survive today in captivity in zoos and wildlife parks.
As a result of deliberate breeding by humans, horses display
a remarkable variation in size, body shape, and coat color. Traditionally,
a horse’s size is measured at the withers—an elevated part of the spine
between the neck and the back. The measurement is made in hands; one hand
equals about 10 cm (4 in). Typical riding horses stand 14 to 16 hands high
and weigh 400 to 500 kg (900 to 1,100 lb). The smallest horse on record,
a Falabella miniature pony, stood 48 cm (19 in), or just under 5 hands,
and weighed 14 kg (30 lb). The largest horse on record was a Belgian that
stood 1.8 m (6 ft) tall, or 18 hands, and weighed 1,450 kg (3,200 lb).
The horse has a hairy coat and a long mane and tail. A heavy winter
coat grows in the fall and sheds in the spring. Typical coat colors include
black, brown, gray, cream, gold, and white. The mane and tail can be the
same or different from the body color, and many variations in color can
result from inherited traits that cause spotting, dilution of the basic
coat colors, or a sprinkling of white hairs in the coat. Many color patterns
have specific names, such as bay (brown with black mane and tail), chestnut
(reddish brown with mane and tail of the same or lighter color), and palomino
(gold with a creamy white mane and tail).
A horse’s head is composed
of the cranium, which encloses the animal’s large, complex brain, and the
face, distinguished by a long muzzle consisting of the nose and lips. The
muzzle provides enough distance between the horse’s mouth and its eyes so
that it can graze and watch for danger at the same time.
the largest eyes of any land mammal. The large eyes protrude from the sides
of the head, enabling horses to see almost directly behind themselves, even
while facing forward. Their night vision is excellent. Horses have limited
color vision, which appears to be similar to one of the less common forms
of color blindness in humans—they perceive red and blue, but they cannot
distinguish between green and shades of gray.
Horses have powerful
teeth and jaws to grind and break down plant fibers. Their teeth grow continuously
as their surfaces wear down. Male horses usually have 40 teeth and females
have 36. Between the front incisors and the rear molars is a gap called
the diastema, where the bit is placed. Horses can close their wide nostrils
against dusty winds, and they can move their large ears to detect sounds
from various directions.
A horse’s head is held up by its long, flexible
neck, which lets the horse reach down to the ground to feed, rise to a high
vantage point to sight danger, and bite itches on the front part of its
body. The horse’s body has a wide chest, which holds its enormous lungs
and heart; and a muscular back, beneath which lie the horse’s internal organs
for digesting food and reproducing. A horse’s long, flowing tail helps keep
its hindquarters warm and is used to swish away insects.
structures of the horse’s legs make the animal a very efficient runner.
What we think of as the horse’s knee is actually the equivalent of a human’s
ankle, so from the knee down the leg is really a highly elongated foot.
The lowest part of the foot is the tip of a single toe, which corresponds
to the tip of a person’s middle toe. This large, strong toe tip is well
protected by a tough, curved hoof. By “standing on its toes,” the horse
has a very long leg for an animal of its size, but also a very light leg,
since toes are lightweight structures, carrying a minimum of bone and tendon
and no muscle at all. Like a person’s foot, a horse’s foot has a sole. In
the horse, the sole includes a rubbery, V-shaped structure called the frog,
which helps absorb the impact of the foot against the ground.
of the joints in horses’ legs are comparable to hinges that permit forward
and backward motion only. This type of joint requires fewer muscles than
are needed for the kind of ball-and-socket joint that occurs in the human
hip, which can rotate in any direction. This yields a further savings in
weight. Long, light legs allow a horse to move very efficiently. A long
leg produces a long stride, and a light leg allows the horse to swing its
limbs back and forth quickly with a minimal expenditure of energy. The top
speed of the horse is about 70 km/h (45 mph).
The horse has very efficient respiratory and circulatory
systems that enable it to race at high speeds without running short of air.
While walking, a horse consumes only 1 liter (about 0.25 gallon) of oxygen
a minute, but at a racing gallop, its oxygen
consumption can approach 60 liters (nearly 15 gallons) per minute. At the
gallop, the horse’s head and neck move up and down in rhythm with each stride.
This motion tends to squeeze and expand the lungs, so that a galloping horse
automatically takes exactly one breath per stride. This mechanism ensures
that the faster the horse gallops, the more air it takes in.
horse has a single stomach and a large digestive organ called the cecum,
which forms a dead-end alley at the junction of the large and small intestines.
Microorganisms that live in the cecum break down cellulose, a tough substance
within the walls of plant cells, making it possible for the horse to digest
grasses. The cecum has a comparable role to the rumen, a specialized stomach
chamber present in ruminants, or cud-chewing animals, such as cows and sheep.
Horses cannot extract as much energy out of food as ruminants do, but they
are able to digest food more quickly. As a result, a horse can eat more
food each day than a cow of the same size. Due to this difference, horses
can survive on stemmy, high-fiber roughage that would not sustain a cow.
Horses reach sexual maturity at about one and a half years.
The estrous cycle in the mare—a mature female horse—typically lasts 21 days.
During the first five days of the cycle, the mare is usually receptive to
mating. The estrous cycle stops during winter and resumes in the spring,
which is the start of the breeding season. A stallion—a mature male horse—approaching
a mare in estrus engages in various courtship rituals. These include uttering
nickering sounds and sniffing and licking the mare’s genital area.
The gestational period in the horse averages 11 months. Mares generally
give birth to a single offspring, or on rare occasions, twins. Young horses
that have not yet been weaned are called foals. Young female horses are
called fillies, and young males are called colts.
Among feral horses,
stallions guard a harem of mares and compete with other stallions for “ownership”
of mares. A harem commonly consists of a single stallion, one to three mares,
and their immature offspring. Stallions challenge one another by competing
in lengthy squealing contests; often a horse that squeals the longest is
able to claim the superior position without physical combat. Stallions that
take over a harem from another male will often cause abortions in pregnant
mares by chasing and aggressively attacking them. This allows the new stallion
to immediately rebreed the mares and produce his own offspring.
control aggressive behavior in stallions, which is closely linked to the
hormone testosterone produced in the testes, horse owners usually castrate
males that will not be used for breeding. A castrated male horse is called
As herd animals, horses have highly developed social behaviors that help
hold the group together and maintain the ranking of each individual within
the group. Horses have a basic instinct to form fixed friendship bonds with
other members of their group. Mares in feral herds or farm groups invariably
pair off with particular other mares. These pairs often engage in mutual
grooming, which involves standing side by side and head to tail while each
one scratches the other’s neck and back with her teeth.
As with all
group animals, horses establish and defend a strict pecking order, which
helps them avoid constant fighting over access to food, water, and mates.
They respond to subtle social signals, such as pinned-back ears, which signal
aggressiveness. Once its place in the social hierarchy is established, a
lower-ranking horse almost always gives way to a higher-ranking horse without
a fight. Most communication between horses takes the form of physical gestures
rather than sounds. This behavior reflects horses’ evolution in open, unforested
habitats where they relied heavily on vision for survival. The horse’s repertoire
of vocal signals is quite limited compared to many other mammals.
Humans, in establishing their relationship with domestic horses, exploit
both the horse’s bonding instinct and its instinctive recognition of the
pecking order. Trainers often initiate a horse’s training in the spring,
when horses are shedding heavily and appreciate being groomed. Grooming
helps cement the friendship bond and makes the horse willing to allow the
human to invade its personal space. By establishing a position as a higher-ranking
member in the group hierarchy, the human trainer can generally get a horse
to cooperate with a minimal use of physical force or punishment.
Breeds of Horses
Selective breeding by humans has produced more than 100 breeds
of horses, many of which are characterized by distinctive traits such as
size, appearance, or temperament. Some breeds are the product of deliberate
efforts over many centuries to develop horses suited for specialized tasks,
such as racing, herding livestock, or pulling plows, wagons, or carriages.
Other breeds simply reflect regional differences that have accumulated over
years as relatively isolated populations of animals were bred together.
Horse breeds are often divided into three broad classes: light horses,
heavy horses, and ponies. These are not strict categories, however, and
do not, as is sometimes claimed, mean that these types of horses descended
from different populations of wild horses.
Light horses include saddle
horses, such as thoroughbreds, quarter horses, and Arabians; and light harness
horses, such as standardbreds and Morgans. The thoroughbred is the preeminent
racehorse breed; thoroughbreds are also used as hunters and jumpers. All
thoroughbreds are descendants of three Arabian stallions that were brought
to England in the late 1600s and early 1700s and bred with native European
mares. Quarter horses were developed in America from crosses between thoroughbreds
and descendents of Spanish horses. Their name reflects their use in quarter-mile
races. Quarter horses are widely used for work on cattle ranches, most notably
as cutting horses, which are trained to separate out a single head of stock
by moving deftly to cut it off as it tries to return to the herd. Another
distinctive American breed is the Morgan, developed in Vermont from matings
between various female horses and a single, famous male—a dark bay called
Justin Morgan who was born in the late 18th century. Used originally to
pull light carriages, the Morgan is now considered a multipurpose breed
and is popular as a saddle horse. The standardbred, developed from crossing
thoroughbreds with Morgans and other light horses, is used in harness racing.
Heavy horses include draft horses and coach horses. Draft horses were
developed in the Middle Ages as the heavy chargers ridden into battle by
armor-clad knights. They were later used to pull plows and heavy wagons
and perform other farm work; they have largely been displaced in the 20th
century by tractors. Draft horse breeds include the Clydesdale, Belgian,
Percheron, and Shire. Coach horses were bred for pulling large carriages
and for light farm work. The Cleveland Bay is an example of a typical coach
Ponies are usually defined as any horse that stands
less than 14.5 hands high. The most familiar pony breeds are the Welsh mountain
pony and the smaller Shetland pony, which is usually less than 11 hands
high. Ponies have a reputation for being smart and wily.
equid family consists of horses, zebras, and asses. All of these animals
diverged from a common ancestor about 4 million years ago. Relatives in
the equid family, such as an ass and a horse, can interbreed, but the resulting
offspring are nearly always infertile.
The evolution of equids did not proceed in a straight line
to culminate in today’s horses and their relatives. Instead, the modern
equids are a small remnant of a once vast and diverse family. This family
came into being about 55 million years ago with the emergence of Hyraco
therium , which is commonly known as the dawn horse, or eohippus. Hyracotherium
weighed about 35 kg (80 lb) and lived in forests in North America. It had
four toes on its front feet, three toes on its rear feet, and small teeth
suitable for a diet of fruit and leaves.
A turning point in the evolution
of equids occurred about 20 million years ago, when the dense forests of
North America gave way to more open grasslands. At this time, the equids
underwent an evolutionary explosion that produced a wealth of species displaying
a wide variety of physical types, all of which were well adapted to their
particular environments. Some of these horses, such as Merychippus, which
weighed some 200 kg (450 lb), showed a trend toward the large, modern onetoed
horse with broad-surfaced teeth well adapted to chewing grass. Others, such
as Nannippus, a tiny browser that ate leaves and fruits, filled a very different
ecological niche. Most horses from this period had three toes on each foot,
but in one branch, the Hipparion species, the two side toes did not touch
the ground. In the line that would lead to the modern horse, the side toes
became increasingly reduced until they finally disappeared. As equids increased
in diversity, they also increased their range by spreading across North
America and, via land bridges, to Europe and Asia.
History of the Modern Horse The horse is a
well-documented case study in evolution. The fossil record shows clear steps
in the progression from a four-toed, small browsing animal—one of a line
that gave rise to tapirs, rhinoceroses, and other mammals in addition to
horses—to the modern horse, a large grazing animal with a single leg bone
and enlarged middle toe. The doglike eohippus of 60 million years ago had
molars with small grinding surfaces to chew the succulent leaves of its
forest habitat. With the spread of Miocene grasslands 25 million years ago,
only the descendants whose teeth had adapted to grinding survived. A drying
climate produced harder ground, and the middle digit of Merychippus, expanded
to bear the strain of its increased weight, became a single digit in Pliohippus.
The horse’s sturdy legs evolved to pound the ground at speeds fast enough
to outrun predators.
Horses were widespread across North America,
Europe, Asia, and Africa during the ice ages. But as the climate warmed
and open tundra gave way to forest around 15,000 years ago, the habitat
for the horse began to vanish. In North America, where horses also suffered
from being hunted by Paleo-Indians, they became extinct. Horses nearly became
extinct in the rest of the world as well; by about 7,000 years ago the world’s
only horses were confined to a small area in the still-open grassland steppes
of Ukraine and Central Asia.
Humans Horses were widely hunted as a source
of food by early humans in North America and Europe. About 6,000 years ago,
the peoples living north of the Black Sea in a region between the forest
and the steppe began to face dwindling supplies of forest game, such as
boar and deer. They began to exploit the steppe-dwelling horses for meat.
Archaeological evidence cannot clearly establish whether the horse was domesticated
at this time as a source of food, or whether the horses remained wild and
were hunted. But not long after the peoples of this region began consuming
large amounts of horsemeat, they also began riding horses. Horses from this
period were buried in ritual graves along with perforated antler tines that
appear to be the cheek pieces for a rope bridle. Microscopic analysis of
the teeth of these ritually buried horses show wear patterns that are unique
to horses that have carried a bit in their mouths.
Over the next
thousand years, the horse staged a dramatic comeback, repopulating Europe
and Asia, but now as a domestic animal under the control of humans. Archaeological
remains show that tribes that possessed horses suddenly became larger, possessing
greater material wealth and prospering with larger households. Horses enabled
them to exploit the resources of the steppes, trade with distant lands,
and bring sudden, ferocious warfare upon their less mobile neighbors. The
association of the horse with warfare dates from earliest times and persisted
into the 20th century. By 2000 BC, the chariot, pulled by a pair of matched
and well-trained horses, was well established as the supreme weapon of war
in Egypt and western Asia. Charioteers in ancient Egypt were exclusively
noblemen of high status. This reflected the huge cost of maintaining horses
in the ancient world; the feed for a pair of chariot horses is estimated
to have taken the entire crop from 4 hectares (10 acres) of barley each
Modern equestrian recreations such as horse racing, hunting,
and polo also date back to ancient times. The Iliad contains an account
of chariot racing at the time of the Trojan War, which was fought in the
late 13th or early 12th century BC. Throughout the Middle Ages (around the
5th century to the 15th century AD) and even until modern times, the horse
played a pivotal role in expanding trade, in exploring new lands, and in
providing the motive power for farm work.
Today most horses are pleasure
and sport animals. There are more than 7 million horses in the United States
today, more than there were in the 1940s when the U.S. Cavalry was disbanded.
Popular activities on horseback include trail riding and competition in
horse shows and rodeo events.
Training a horse is a complex art. Trainers typically begin a young horse’s
training by introducing the horse to human contact and teaching it to follow
on a lead rope. One method of training involves working the horse on a lunge
line, a long rope attached to a halter placed over the horse’s head. In
this method, the trainer keeps the horse moving in a large circle. Horses
can be taught to respond to voice commands, such as “walk,” “trot,” and
“whoa,” while being worked on a lunge line. Horses need to be gradually
accustomed to a saddle and bridle and to bearing weight on their back.
More advanced training involves teaching the horse to respond to signals
from a rider’s legs and hands. A well-trained horse will learn to change
gaits or move from side to side with a very subtle pressure from the rider’s
legs or a small pull on the reins. The reins are used in several ways to
communicate with the horse. In neck reining, a rein is laid against one
side of the neck; this signals the horse to turn in the opposite direction.
Neck reining is used mostly in Western riding and by polo players who keep
only one hand on the reins. Horses are similarly taught to move in a direction
away from the pressure of the rider’s leg. Reins can also be used to apply
direct pressure via the bit to one side of the mouth or both to signal the
horse to turn or slow down. Training for harness horses begins with a person
holding long reins and walking behind the horse. Once the horse learns to
respond to basic commands, it can graduate to pulling a cart or carriage.
The basic method used in all training is to reward a correct response,
thus helping the horse to make an association between the trainer’s signal
and its own response. Horses have excellent memories. Their ability to form
associations is often strongly influenced by individual temperament; nervous
or high-strung horses and excessively shy horses are poor learners. Most
training of horses uses what animal behaviorists call negative reinforcement,
which means that the reward is the removal of an unpleasant stimulus. For
example, to get the horse to move forward, the rider squeezes with his or
her legs; once the horse moves forward the rider stops squeezing, thereby
rewarding the horse by removing the stimulus. This is different from punishment,
which is applied after an incorrect response. Punishment is generally much
less effective than negative reinforcement, although it is occasionally
necessary to maintain the trainer’s position as the dominant member of the
Caring for a Horse
The amount of food and care a horse needs varies according
to how much the animal is worked. Many horses that are lightly worked or
not worked at all thrive without any difficulty on grasses found in pastures,
and without any special food. All horses, however, need continual access
to fresh water and mineral salt blocks that provide needed trace minerals
in their diet. In areas with mild winters it is often possible to stockpile
grass in one portion of a pasture by leaving it to grow in late summer and
fall and then allowing horses to graze during the winter. In such cases,
horses do not even need to be fed hay. When winter pasture is not available,
a 500 kg (1,100 lb) horse typically requires about 7 kg (15 lb) of average
quality hay a day to maintain its weight and health.
are worked several hours a day or more generally need some supplementary
grain in their diet. Individual horses vary considerably in their needs,
which are also affected by weather. A horse kept in a warm stall or turned
outside with a blanket on will need less feed, and a horse that is let outside
in cold weather or that has had its coat clipped will need more feed. Working
horses typically need several quarts of grain a day in addition to hay.
Horses’ needs for shelter also vary widely. Except in severe climates,
horses can generally be left outside without harm. Show horses and racehorses
are usually kept in stalls almost all the time they are not working. Keeping
them in stalls protects them from injury, keeps them clean, and ensures
that they receive constant care and attention. Stalls need to be supplied
with a heavy layer of bedding, such as sawdust or straw, and must be cleaned
daily. Horses that are stabled need regular exercise. Most pleasure horses
need only be brought into a stall on cold winter nights. Open shelters that
horses can enter and leave as they please are a good means of protecting
horses from wind and rain, and from strong sun and flies in the summer.
In addition to food and shelter, horses need other care to keep them
healthy. All horses need annual vaccinations to protect against a number
of highly contagious, and often fatal, diseases. These diseases include
tetanus, rabies, influenza, and Potomac fever. Horses also need oral medication
at least every two months to kill intestinal parasites (see Diseases of
Animals). Horses that are ridden regularly on surfaces other than grass
or soft ground need to be fitted with shoes, and this can represent a considerable
part of the expense of keeping a horse. Working horses generally need new
shoes every six to eight weeks, or even more often. A horse’s teeth need
to be checked periodically, and they may require filing to remove sharp
edges and align the biting surfaces.
Daily grooming is important
in maintaining the bond between a horse and its owner. It also helps to
keep a horse looking neat and provides a regular chance to check for injury
or other health problems. Thorough brushing of the region under the saddle
and girth—the strap below a horse’s belly that holds the saddle in place—is
especially important in preventing the skin from becoming irritated by dirt
and grime. A horse’s feet need to be picked out frequently to remove stones
that can cause bruises. Picking out the hooves also helps to prevent an
infectious condition known as thrush, which is caused by microorganisms
that grow in the absence of air.
Scientific classification: Horses belong to the family Equidae
of the order Perissodactyla. The domestic horse is classified as Equus caballus,
and Przewalski’s horse is classified as Equus caballus przewalskii.