What are Animals?
Author: Andrew Tomkinson

With almost two million species having been identified to
date animals are the most varied living thing on the
planet. For over one billion years they have adapted to
the changing world around them, developing a vast array of
different lifestyles in the struggle that they face to
survive. At one extreme animals include fast moving
predators such as big cats, sharks and birds of prey; at
the other extreme there are the inconspicuous animals
living in the soil or on the sea bed.

Animals are usually easy to distinguish from other forms of
life because most of them have the ability to move. This
rule works very well for most of the animals that live on
the land but it is not always the case for those that live
in water. Here they live in water and in some cases have
trailing arms or tentacles that make them look like plants.
A reliable way of identifying animals is by their basic
biological features. Their bodies are composed of many
cells and they have nerves and muscles that enable them to
respond to the world around them. They get the energy that
they need by taking in food.

Animals are highly complex and responsive compared to other
forms of life. Even the most simplest of animals react
quickly to changes around them, shrinking away from
potential danger or reaching out for food. Animals that
have well developed nervous systems are able to learn from
experience which is unique to the animal world. The worlds
largest living animals, baleen whales can live up to 25
metres long and weigh 120 tons. At the other end of the
scale are microscopic organisms and sub microscopic flies
and beetles. These animals are so tiny that their weight
is negligible but they still possess the body systems that
are needed for survival.

The different body sizes allow animals to live in different
ways. Whales have few natural predators and the same is
true of elephants, which is the largest land animal. Their
huge bodies are very energy efficient because they process
food on such a large scale. However they take a long time
to reach maturity, which means that they are slow to
reproduce. Insects on the other hand are easy prey for
many animals and their small size means that their bodies
are not as energy efficient as large animals. But because
they can breed very quickly when the conditions are in
their favour, their numbers can rise at a very fast rate.

Almost all of the world's largest and most familiar animals
are vertebrates; these are animals that have backbones.
They include the fastest animals on land, sea and air and
also the world's most intelligent species (homo sapiens).
Vertebrates are all related to each other, sharing a common
ancestry that goes back millions of years. However despite
the fact that vertebrates lead the animal kingdom in many
fields, they only make up a small minority of the animal
species known today. The vast majority of species are
invertebrates - animals without backbones.

Invertebrates often have very little in common with each
other (unlike vertebrates), apart from their lack of a
backbone. The giant squid which is the largest
invertebrate, can measure over 16 metres long but it is
very much an exception. Most of the invertebrates are very
small and live in inaccessible habitats.

Most animals are cold blooded (ectothermic), which means
that their body temperature is determined by that of their
surroundings. Birds and mammals are warm blooded
(endothermic), which means that they are able to generate
their own heat and maintain a constant internal temperature
regardless of the conditions outside. This difference in
body temperature has some far reaching effects on the ways
that animals lead their lives; this is because animal
bodies work best when they are warm. Cold blooded animals,
such as reptiles, amphibians and insects are able to
operate very effectively in warm conditions, but they can
slow down if the temperature drops. They are able to
absorb some heat by being out in the sunshine but if the
temperature falls below about 50 degrees fahrenheit, their
muscles work so slowly that they find it difficult to move.
Mammals and birds are hardly affected by this kind of
temperature change. Their internal heat and good
insulation enable them to remain active even when the
temperature falls below freezing.

In a physical way vertebrates function as separate units
even though they may live together in families or in larger
groups. In the invertebrate world it is not unusual for
animals to be permanently linked to together, forming
clusters that are known as colonies. These colonies often
look and behave like single animals. Most are static but
some, particularly those that live in the sea, are able to
move around. Colonial species include some of the world's
most amazing invertebrates. Pyrosomes, for example, form
colonies that are shaped like test tubes which are large
enough for a diver to swim enter. However in ecological
terms the most important colonial animals are reef building
corals, which create complex structures that provide havens
for a range of other animals. In reef building corals, the
members of each colony are usually identical. But in some
colonial species, the members have different shapes that
are designed for different tasks. An example of this is
the Portuguese man o' war which looks like separate
animals, called polyps that capture food, digest it, or
reproduce. They dangle beneath a giant gas filled polyp
that that acts as the colony's float.

Animals obtain their energy from organic matter or food.
They are able to break food up by digesting it and then
absorb the substances that are released. These substances
go into the animals cells where they are combined with
oxygen to release energy. This process is called cellular
respiration and is a controlled form of burning, with food
acting as the fuel.

The majority of animals are either herbivores who eat only
plants, omnivores which eat both plants and other animals
and carnivores which eat other animals. There are also
scavengers that feed on dead matter. All animals,
regardless of their lifestyle ultimately provide food for
other animals. All are connected by food chains which pass
food and energy to one another. However food chains are
not often more than six links because 90 per cent of an
animal's energy is used in making its own body work.


About the Author:

Andrew Tomkinson is a writer of Articles on animals.
http://www.animalsrmagic.com