mates, mark the location of food, or
warn other animals of danger.
The human body makes more than 20
major hormones. Growth hormone
helps to build muscles. Adrenaline, also
called epinephrine, makes the heart beat
fast during stressful times. Insulin controls
the level of sugar in the blood.
Estrogen causes girls to grow into
women. Testosterone causes boys to
grow into men.
How HormonesWork
In humans, hormones travel through the
blood to cells throughout the body.
Many different hormones may be in the
blood at one time. Each hormone can
affect only a certain type of cell. These
cells are known as target cells. A hormone
travels to its own target cells and
locks onto them. When this happens,
the hormone causes the cells to start or
to stop certain activities.
#More to explore
Blood • Cell • Endocrine System
One type of hormone controls how much a
person grows. A man who had too much
growth hormone grew to be more than 8
feet (2.4 meters) tall.
84 Hormone BRITANNICA STUDENT ENCYCLOPEDIA
Horn
Many animals have hard structures,
called horns, growing from their heads.
Most horned animals also have hooves.
Sheep, cattle, goats, and antelope are
some of the most common horned
animals.
Structure
Horns are hollow and pointed. They are
attached to the bone of an animal’s
head. Horns are made of keratin, a type
of protein. Keratin is the same material
that makes up hooves, hair, fingernails,
and feathers. Animals may be born with
those things, but animals are not born
with horns. The horns grow as the animals
get older.
Horns grow in pairs, one on each side of
the head. Horns vary greatly in size and
shape. Bighorn sheep have large, curled
horns. Antelope have long, thin horns
that point upward. Oxen have long,
heavy horns that stick out from the sides
of their heads.
Uses
Animals use horns as weapons to protect
themselves from enemies. They also use
their horns in battles with other animals
of their kind. In some types of animals,
only the male has horns. In others, both
males and females have horns. A large
set of horns may impress mates. They
show that the animal has lived a long
time and can defend itself.
False Horns
Hornlike antlers grow from the heads of
deer, elk, and moose. Antlers are not
true horns, however. They are bone with
a velvety covering. Antlers fall off every
year. Horns never fall off.
Many lizards have bony stalks that look
like horns. Giraffes have knobby
growths made of bone covered with skin
and hair. The single horn of a rhinoceros
is actually made of hardened hairs that
are tightly bunched together.
#More to explore
Hair • Hoof • Nail and Claw
Horse
The horse is a mammal that people have
valued for thousands of years. In the
past people commonly used horses to
get from place to place and to pull heavy
loads. People still use horses in sports
and recreation. The scientific name of
the horse is Equus caballus.
Where Horses Live
Horses once roamed free over grasslands
in North America, South America,
Europe, Asia, and Africa. Today, however,
almost all horses are domesticated,
or tamed. Domesticated horses are kept
Animal horns grow in many different shapes. by people all over the world.
People make
horns into
tools, containers,
musical
instruments,
and even
traditional
medicines.
BRITANNICA STUDENT ENCYCLOPEDIA Horse 85
The only surviving type of horse that
has never been tamed is called Przewalski’s
horse. It now lives mostly in zoos.
Other horses that roam free came from
horses that were raised by people. These
horses include the mustangs of western
North America.
Physical Features and Breeds
Horses are large and very strong. A typical
domesticated horse stands 5 feet (1.5
meters) tall at the shoulder. It weighs
about 1,150 pounds (520 kilograms).
A horse’s legs are strong even though
they look very slender. A horse’s foot is
actually a single toe protected by a hard
covering called a hoof. The hoof is made
of the same material as a human toenail
or fingernail.
Over the centuries people have bred
horses to have features that they considered
useful or desirable. The breeds can
be grouped into three general types:
light horses, heavy horses, and ponies.
Light horses are used mostly for riding.
They include the Thoroughbred, a
breed that is used for racing. Heavy
horses, also called draft horses, can pull
heavy wagons. Some heavy horses weigh
more than 2,000 pounds (900 kilograms).
The smallest breeds are called
ponies. Some ponies weigh only 385
pounds (175 kilograms) or less.
Behavior
Horses eat grass. Most free-living horses
graze and wander in groups called
bands. One type of band is called a
breeding band. A breeding band usually
has one adult male, several adult
females, and their young. A bachelor
band consists of males only.
Horses have several gaits, or ways of
stepping. The fastest gait is called a gallop.
A slower gallop is called a canter.
The trot is slower than the canter. The
walk is slowest of all.
Life Cycle
About 11 months after mating, a female
horse usually gives birth to a single
Przewalski’s horse has a short mane that
stands upright.
A pony ride is a treat for a child.
86 Horse BRITANNICA STUDENT ENCYCLOPEDIA
