External covering is the outside appearance of an animal. Animals can have fur, feathers, hair, short hair, long hair, smooth hair, bristles, skin, thick skin, moist skin, dry skin, scales, waterproof scales, small scales, overlapping scales, spikes, hard shells, soft shells, smooth shells, rough shells, wool, or no covering at all.
Different external body coverings are designed for different purposes, such as to fly fast, to swim between reeds, to walk in swamplands, or to endure freezing temperatures. Animals have body covering for protection against diseases, attack from other animals (predators), weather, their habitat, or to keep their species alive.
Animals also have external covering for insulation to keep them cool in summer and warm in winter. Other functions include: protect their insides (such as heart and lungs), camouflage (to blend into the environment so that predators don’t attack them), to signal a mate (by showing their feathers or cleaning their fur), for flying, as a sun shield, waterproofing, or for receiving information from the environment. For example, skin can tell animals whether there is moisture in the air, or if there is a scent from another animal, or if it is windy, or too hot or too cold so that they can cool down or warm up.
Below is more information about the types of animals that have each type of external (outer) covering. There is information on: (1) Dry hard exoskeletons: Anthropods, (2) Overlapping scales: Fish, (3) Smooth and moist skin: Amphibians, (4) Dry scales: Reptiles, (5) Feathers: Birds, and (6) Fur, wool, and hair: Mammals.
Dry Hard Exoskeletons: Arthropods
Arthropods, such as insects, snails, and crabs, have dry hard coverings. Their covering is called an exoskeleton — which means ‘outside skeleton’ — on their bodies to protect their internal organs, because they do not have bones. A shell is an exoskeletion.
When arthropods grow, they must shed (get rid of) their exoskeleton and replace it with a new one. Arthropods have new exoskeletons that grow back very quickly.
Overlapping Scales: Fish
Bony fish, such as whiting, carp, and perch, have overlapping wet scales. Each scale is attached to an inner layer of skin. On top of the scales is a slimy mucous liquid, which is why fish feel slippery. Glands (special holes in the skin) secrete (pump out) this mucous from underneath the scales. The mucous protects fish from disease and helps to lubricate fish so that they can swim easily.
Sharks and rays are different. Sharks and rays are not bony fish—they are cartilaginous fish—so they have small tooth-like interlocking dermal scales, which is why they look smooth but feel rough (like sandpaper).
Smooth and Moist Skin: Amphibians
Amphibians, such as frogs, toads, and salamanders, have smooth and moist coverings. They have no scales, no feathers, and no fur. They must be kept moist so that they do not dehydrate (lose moisture) and die. The coverings of amphibians also have mucous glands, like fish, that secrete slime-like liquid. It keeps them protected from dehydration. Moist skin also helps them breathe.
Dry Scales: Reptiles
Reptiles, such as turtles, snakes, alligators, and crocodiles, have dry scales. Their skin does not have sweat glands, so they need dry horny scales to protect them from dehydration. Most reptiles also renew their scales periodically by removing the whole covering in patches or large pieces, and not individual scales.
Birds, such as penguins and eagles, have feathers on their bodies and scales on their feet. Feathers are made from keratin. Keratin is a hard substance in fingernails, toenails, feathers, and hair. Most birds have glands that secrete oil to make their feathers waterproof. Birds have different types of feathers, such as down feathers (small and soft to insulate them – to keep them warm and dry), filoplume feathers (hair-like feathers), and contour feathers (larger, outside feathers for flying). Flight feathers are called retrices and are stiff and hard. Not all birds have filoplumes.
Fur, Wool and Hair: Mammals
Mammals, such as cats, dogs, sheep, elephants, and donkeys, have hair or fur or wool. Mammals have glands that secrete oil to keep the fur waterproof. The oil is sometimes called lanolin. Fur also protects mammals from the weather. They often lick their fur to keep cool in hot weather, and when it is cold their fur becomes thicker or fluffier to keep them warm. Elephants do not have fur like rabbits, but they have hair on very thick leathery skin. Whales are mammals too and it looks like they don’t have hair or fur, but they do. Whales and dolphins have bristles, which are short, rough hairs.
Photographer: Martina Nicolls
Martina Nicolls: SIMILAR BUT DIFFERENT IN THE ANIMAL KINGDOM