Britannica Student Encyclopedia - 2010

to avoid contact with the substance that

causes the allergy. Unfortunately, substances

such as dust and pollen are

everywhere. Taking drugs known as antihistamines

can decrease the allergic reactions.

Antihistamines prevent histamine

from reaching the cell tissues.

#More to explore

Asthma • Immune System

Alligator

Alligators are large lizardlike animals

with long, rounded snouts and powerful

tails. They spend most of their time in

the water, but they also travel on land.

Alligators belong to the group of animals

called reptiles. They are related to

crocodiles.

Where Alligators Live

Alligators live along the edges of

swamps, lakes, and slow-moving rivers.

There are two species, or types, of alligator.

The American alligator lives in the

southeastern United States. The Chinese

alligator lives in the Yangtze River region

in China. It is very rare.

Physical Features

Male American alligators measure about

11 feet (3.4 meters) in length on average.

The females are slightly smaller.

Young American alligators are black

with yellow stripes on the tail. The

adults are brownish.

The Chinese alligator normally grows to

a length of about 5 feet (1.5 meters). It is

blackish with faint yellowish markings.

Alligators have many features in common

with crocodiles. They have thick

skin composed of many scales and

plates. They each have a long body and

four short legs. The eyes, ears, and nostrils

are located on top of the head.

These show above the water when the

animal floats.

However, there are easy ways to tell alligators

and crocodiles apart. One of the

most obvious is that alligators have

broad, rounded snouts, while most

crocodiles have narrow, pointed snouts.

Behavior

Alligators eat mainly fish, small mammals,

and birds. But they sometimes kill

American alligator prey as large as deer or cattle.

A view from above shows the different

snouts of an alligator and a crocodile.

BRITANNICA STUDENT ENCYCLOPEDIA Alligator 85

 

Alligators usually mate during the

spring. The female builds a nest of mud

and grass. She lays 20 to 60 white eggs

in the nest. The eggs hatch after about

65 days. The young alligators live with

their mother for as long as a year.

#More to explore

Crocodile • Reptile

Allosaurus

Allosaurus was a fierce dinosaur that

roamed Earth before the similar but

more well-known Tyrannosaurus rex

appeared. The name Allosaurus means

“other lizard.” Allosaurus is a member of

the group of dinosaurs known as theropods.

The theropods were meat-eating

dinosaurs that walked on their two back

legs.

When and Where Allosaurus

Lived

Allosaurus lived about 163 to 144 million

years ago. Most Allosaurus fossils, or

remains, have been found in North

America, in Colorado, Utah, New

Mexico, Montana, South Dakota, and

Wyoming.

Physical Features

Allosaurus reached up to 39 feet (12

meters) in length, stood about 15 feet

(4.5 meters) tall, and weighed up to 2

tons. The massive head of Allosaurus

measured about 3 feet (1 meter) long

and was supported by a short, thick

neck. The sharp teeth of Allosaurus were

curved and had sawlike edges. Allosaurus

had sharp claws on the three toes of each

hind foot. It also had long, grasping

claws on the three toes of each front foot.

The long, heavy tail of Allosaurus may

have helped the animal balance itself.

Behavior

Allosaurus was a meat-eating dinosaur

that preyed on plant-eating dinosaurs.

When it hunted alone, Allosaurus would

hide among thick plants and then jump

out on its prey. Some scientists believe

that Allosaurus also hunted in packs.

Together, a pack of Allosaurus could

bring down very large dinosaurs, such as

Apatosaurus and Diplodocus. Scientists

believe that Allosaurus reproduced by

laying eggs.

#More to explore

Dinosaur • Tyrannosaurus rex

Allosaurus

86 Allosaurus BRITANNICA STUDENT ENCYCLOPEDIA

 

Alloy

An alloy is a substance made of two or

more metals or of a metal and nonmetallic

materials. To make an alloy, the

ingredients are melted and mixed

together. When the mixture cools, it

hardens.

Alloys are generally more useful than

pure metals. Pure metals may rust or

wear away easily. They may also be too

soft to hold a shape. But mixing other

substances with a pure metal often

results in a hard and strong substance.

People first learned to make alloys more

than 5,000 years ago. Ancient peoples

used a combination of copper and tin,

called bronze, to make tools and weapons.

Today such things as church bells

and statues are made of bronze. Brass, a

combination of copper and zinc, is

another ancient alloy. People today use

brass to make such things as doorknobs

and musical instruments.

Steel is one of the most important modern

alloys. Steel is a mixture of iron,

carbon, and other substances. Steel is

useful in building construction and

many other industries. Another important

modern alloy contains aluminum

and small amounts of copper or other

substances. Aircraft and other vehicles

are often made of aluminum alloys.

#More to explore

Brass • Bronze • Metals • Steel

Almond

Almonds are commonly called nuts, but

they are actually seeds. They come from

a tree that is closely related to the peach

tree. People eat almonds as a snack and