plant or animal that is in danger of disappearing
forever. If a species, or type, of
plant or animal dies out completely, it
becomes extinct.
How Plants and Animals
Become Endangered
Animals and plants depend on each
other and their environment to survive.
When their environment, or habitat,
changes or disappears, plants and animals
may become endangered. Natural
The California condor was
hunted and poisoned until it
became an endangered species.
Now it is protected by law.
An X-ray photograph shows encephalitis on
the right side of a brain.
BRITANNICA STUDENT ENCYCLOPEDIA Endangered Species 133
disasters or changes in the climate can
harm or destroy habitats.
Humans can cause species to become
endangered as well. Humans pollute
habitats with garbage, car fumes, and
factory wastes. They destroy habitats by
clearing land for roads, buildings, and
farms. They also may hunt and kill too
many of certain types of animals.
The arrival of a new species in a habitat
is another threat. Often the new species
spreads freely because it has no natural
enemies. The species already in the habitat
may not be able to compete against
the newcomer. For example, mongooses
brought to Jamaica in the 1800s killed
many native animals.
Endangered Species Today
The list of endangered animals includes
many large, well-known mammals. The
tiger and both African and Asian
elephants are endangered. Several types
of whale are also endangered. The giant
panda survives only in remote parts of
China and in zoos.
Endangered birds include several types
of crane, eagle, and parrot. Among the
endangered reptiles are several types of
sea turtle—for example, the leatherback
turtle. There are also endangered
amphibians, fishes, mollusks, insects,
and other animals.
Endangered plant species range from
tiny mosses to towering trees. The mandrinette
is an endangered shrub with
bright red flowers. Foreign species of
plant have been taking over its habitat
on the island of Mauritius. In the early
21st century fewer than 50 of these
plants were growing in the wild.
Another endangered plant, the Bermuda
cedar tree, grows in the wild only in
Bermuda. People have cut down many
of these trees to build houses, ships, and
furniture.
Protection
In the 1960s people became aware that
many species were endangered. Today
many countries have made it illegal to
harm, capture, kill, or sell endangered
species. In addition, many organizations
work to keep endangered species from
becoming extinct. The International
Union for Conservation of Nature and
Natural Resources keeps lists of endangered
species. The Nature Conservancy
and theWorldWildlife Fund help to set
aside land for threatened wildlife.
Nature preserves and zoos help endangered
animals to reproduce. Such programs
have helped to increase the
populations of the black-footed ferret,
the California condor, and other endangered
animals.
#More to explore
Animals, Extinct • Conservation
• Environment • Habitat
Endocrine
System
In the human body, a group of organs
called glands make up the endocrine
system. Glands make hormones. These
are chemicals that help control activities
of the body. Insects, fish, amphibians,
One endangered
species
that was saved
from extinction
is a bird called
the whooping
crane. In
1937 fewer
than 20
remained.
Now there
are several
hundred.
134 Endocrine System BRITANNICA STUDENT ENCYCLOPEDIA
reptiles, birds, and mammals all have
endocrine systems.
Glands
The glands of the human endocrine
system make more than 20 major hormones.
The hormones travel in the
blood to cells throughout the body. A
part of the brain called the hypothalamus
helps to control the release of many
hormones.
The major glands of the endocrine system
include the pituitary, thyroid, and
adrenal glands, as well as the pancreas.
The ovaries are glands found only in
women. Men have glands called testes.
Pituitary Gland
The pituitary gland is a bean-sized gland
at the base of the brain. It sits beneath
the hypothalamus. The pituitary gland
makes hormones that control many
body functions. One hormone controls
the growth of the body. Another helps
women deliver babies and helps produce
milk so that women can nurse their
babies. Other hormones made by the
pituitary gland control how other endocrine
glands work.
Thyroid and Parathyroid Glands
The thyroid gland is located at the base
of the neck. It produces a hormone that
controls how fast cells burn food to
release energy. Four parathyroid glands
are attached to the thyroid gland. They
make a hormone that helps control the
amount of calcium in the blood. Calcium
is an important mineral that helps
bones to grow.
Most glands of the endocrine system are
found in both women and men. Only
women have ovaries. Only men have testes.
BRITANNICA STUDENT ENCYCLOPEDIA Endocrine System 135
Adrenal Glands
One adrenal gland sits on top of each
kidney. The two adrenal glands are
divided into two main parts. One part
produces hormones that do many jobs.
For example, they control the mixture of
salt and water in the body. The other
part makes hormones that raise blood
pressure and heart rate when the body is
under stress.
Pancreas
