Introduction
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.
Przewalski's horse, 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.
Physical Characteristics
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.
Horses have 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.
The specialized 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.
Many 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).
Internal Organs
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.
The 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.
Reproduction
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.
To 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 a gelding.
Behavior
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 horse breed.
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.
The modern 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.
Evolution
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.
Evolutionary
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.
Horses and 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 year.
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
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 social hierarchy.
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.
Horses that 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
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.
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.
Evolutionary
History of the Modern Horse