Category Zoology

          Some people take a holiday each year in warmer parts of the world. Some animals do so, too, although for them it is not for pleasure but for survival. These animals go on migrations—long-distance, usually seasonal, journeys. They stay in an area where there is enough food, water, shelter or other needs. When conditions change, the animals travel on to find better surroundings, especially a place to breed.

          In some cases, the journeys can be fairly random. Herds of wildebeest (gnu) and zebra wander the African plains. They stay in an area while there are plants to eat. Once these are gone, they set off to find an area where recent rains have produced new plant growth. There is usually a yearly pattern to their movements. But sometimes an extra-long dry spell drives them far away.

          For most migrating creatures the journeys are at a regular time each year and usually follow the same routes as well. In the polar lands of the far north, the summer is short but the long hours of daylight and warmth allow plenty of plant growth. There are few resident animals to eat the plants. So birds such as geese fly up from the south in the spring to feed and raise their young in the Arctic. Then in the autumn, before the long, dark, icy winter grips the Polar Regions. They return south to temperate Europe, Asia and North America.

          Another group of migrating birds, such as swallows and swifts spend spring and summer in Northern temperate lands, feeding and breeding. Then, in autumn, they fly south to the warmth of the tropics.

          Birds are the main group of migrating animals because their power of flight allows them to cover long distances rapidly. Some land mammals migrate too, such as caribou. In Australia, herds of kangaroos and flocks of emus travel hundreds of kilometres across deserts in search of areas where rain has brought fresh plant growth.

OCEAN WANDERERS

          Animals migrate through oceans as well as across land. Some are regular to-and-fro migrants with the seasons, like grey whales. Their total yearly journey is more than 20,000 kilometres, making this whale the longest-travelled mammal. Green turtles are probably record-holders for reptiles. Some groups feed off the tropical coasts of South America and then swim over 2000 kilometres to breed on lonely Ascension Island in the middle of the Atlantic Ocean.

          Some ocean migrations are less regular. Salmon grow up in European and North American rivers, then swim out to sea. They wander the oceans for 10,000 kilometres or more before returning to their home rivers.

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          Many animals, from earthworms to whale sharks, lead simple lives. Their behaviour is limited and they only encounter others of their kind briefly to mate. Other creatures have much more complex behaviour. They form groups, have contests for group supremacy, mates and territories, help each other when feeding and even look after each other’s offspring.

          Living things strive to survive so that they can breed and pass on their genes to their offspring. An animal must choose its breeding partner carefully. Courtship behaviour and mating displays pick out a partner of the same species, the opposite sex, sexually mature, strong, fit and healthy. This increases the chances of the offspring being fit and healthy, too.

          In some animals, survival depends on having a territory. This is a patch of land or water where the owner can live and feed without being in direct competition with rivals of its own kind. Owners often mark their territories by rubbing scents, spraying urine and leaving piles of droppings around the borders. They defend the territory against other of their kind by calls and songs, and also by visual displays.

HELPING OTHERS

          Life in the wild is a battle to stay alive. But sometimes helping others can increase an individual’s own chances of survival. Some living things form partnerships with other, quite different species where both partners gain. This is known as symbiosis. Cleaner fish are small fish, such as wrasses, that tend to larger fish. The big fish could easily eat the cleaner. But the cleaner nips fish lice and other pests from its body, mouth and gills. The big fish is relieved of these parasites and the cleaner fish gets its meal.

          The hermit crab forms a symbiotic partnership with the calliactis sea anemone. The anemone protects the crab with its stinging tentacles. In return the crab carries the anemone to new places to catch victims. Also each partner may share in the leftovers of the other’s meal.

          Many group-living animals produce alarm calls and actions if they spot danger. This warns others in their group. Likewise if one group member finds a plentiful supply of food, the others gather round to share it. A few kinds of animals even work together to hunt prey. These co-operative killers include wolves, lions and African wild dogs.

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          Meat-eating predators, or carnivores, obtain their food by hunting and attacking other creatures—their prey. Most prey have some form of selfdefence and so the predator must overcome this. Many predators have strong and agile bodies, quick reactions, keen senses and hunting weapons such as sharp teeth and long claws. Some use speed to race after their prey. Others lurk hidden among leaves or long grass—perhaps also disguised by camouflage—then ambush their victims.

          Owls use silence and stealth to swoop on prey. The owl’s feathers have very soft edges so they make hardly any sound as the bird flaps and glides. The owl can see well at night with its huge eyes. It can hear even better and pinpoint a mouse’s footsteps or chewing noises in the darkness.

          Many animals use chemical weapons rather than physical ones. Some snakes and spiders have poisonous bites, while wasps and scorpions have venomous stings. The poison can also be used for selfdefence as well as subduing prey. These predators usually give warning that they are about to bite or sting an attacker. For example, a rattlesnake shakes its tail, while a poisonous spider rears up to show its fangs. This is because the supply of venom is limited and predators need it to hunt, so they try to avoid using it in defence unnecessarily.

DEFENCE

          Plant-eating animals need to defend themselves from carnivores. One defensive strategy is to fight back. Elephants, wild boar and warthogs can slash with their tusks. Gazelles, antelopes and wild cattle like musk oxen jab enemies with their sharp horns. Some, like zebras, can kick out powerfully with their hard hooves. Safety in numbers also helps—many eyes and ears are more likely to detect approaching predators. Musk oxen form a circle around their young to keep away wolves.

SPINES AND POISONS

          One type of physical protection is a hard body case, as in turtles, tortoises, armadillos, snails and beetles. Another strategy is prickles, spikes or spines, as in hedgehogs, porcupines and porcupine fish. Many animals, from mice to deer, rely on their sharp senses, speed and agility to escape as they dodge or outrun their predators.

          Certain fish, beetles, caterpillars, moths and butterflies have horrible-tasting or poisonous flesh. Predators soon learn to avoid them because they advertise this form of defence with bright body patterns called warning colours. Some animals puff up to look bigger, such as puffer fish, toads and lizards. Another strategy is suddenly to flash bright colours and patterns at the enemy, especially eye spots which resemble or mimic the eyes of an even bigger predator!

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          The process of evolution ensures that only living things that are suited, or adapted, to their environment will survive. An animal or plant that is poorly adapted to its surroundings soon loses out in the struggle for survival. Animals are adapted to three major features of their environment: the climate (including temperature and rainfall), the food sources available, and avoidance of predators.

          Life thrives in its greatest diversity in warm and moist or wet conditions, such as tropical rainforests and coral reefs. Places that are very cold or very dry provide the greatest challenge to animal survival. Yet some creatures can live in even the driest deserts. They include large animals such as camels and oryx, and smaller creatures like jerboas, lizards, scorpions and insects.

          In hot deserts, smaller animals often hide from the scorching sun by day and come out in the cooler night. They produce very little sweat, urine or other body liquids, thus saving valuable water.

          Desert creatures have adapted in various ways to moving over soft sand. The camel has very wide feet so it does not sink in. Jerboas and gerbils hop and leap on their large back feet. The sidewinder snake moves its body diagonally in a series of Z-shaped stages, pushing sideways against the loose grains. Some desert-dwellers, like the water-holding frog, burrow underground and sleep through the worst of the drought.

        Only warm-blooded birds and mammals are able to live on land or in the air in the Earth’s coldest regions, the Arctic and Antarctic icecaps. Reptiles and amphibians would simply be too cold to move. Musk ox, yak, seals and polar bears have thick furry coats to keep out the chill.

          Marine polar mammals like walruses and whales have almost no fur at all. But they do have a thick layer of fat under the skin, called blubber, to keep in body warmth. Birds such as penguins also have blubber. Other birds, like the ptarmigan and snowy owl, have extra-thick plumage. They can fluff out their feathers to trap a layer of air which keeps out the cold.

          One of the most varied habitats is the coast. The main change here is the twice-daily rise and fall of the tide. Animals such as crabs, worms, shrimp and shellfish are adapted to being active when they are covered by the seawater—whether it is day or night, winter or summer. Shellfish like limpets have strong outer cases to prevent them drying out at low tide and also being smashed by the waves. Soft-bodied worms and fish burrow in sand or hide under rocks for protection against winds and waves.

          One of the most constant habitats is the bottom of the sea. It is always dark and cold, with few water currents. The main problem at great depths is the enormous pressure of the water. Deep-sea fish, starfish and sea cucumbers would go soft and floppy (and die) if brought to the surface.

          One of the key features of an animal is that it moves. It moves parts of its body when it opens its mouth, bends its neck or curls its tentacles. Most creatures also move about in their surroundings. They run, walk, jump, hop, slither, swim or fly. A few animals do not move about, at least as adults. Barnacles and mussels are stuck to seashore rocks. But they were mobile during their young, or larval, stages.

MOVING IN WATER

          Many animals live in either the sea or in rivers and lakes. Water is much denser than air and so resists movement more. To travel through water quickly, creatures must be smooth and streamlined so the water slips past them easily (submarines are a similar shape for this reason). Fish such as sharks swish their tails from side to side to provide the forward propulsive force for swimming. A fish’s fins and a dolphin’s flippers provide control for steering, slowing and going up or down. Penguins flap their wings and “fly” through the water.

MOVING IN AIR

          Only three groups of living animals truly fly in a sustained, controlled way—birds, bats and insects. Air is so thin it produces little resistance to movement, but it provides very little buoyancy either. Fliers flap their wings down to create a lifting force as well as back to push themselves forwards.

          Birds and bats have very thin, light bones to reduce body weight and so save on the energy needed to stay airborne. Even so, they must take in almost twice the amount of energy as food, compared to their ground-dwelling counterparts. The down flapping wing muscles in the chest are by far the largest muscles in the body of a bird or bat. Insects and bats control their movement by tilting or twisting their wings. Tiny insects like gnats and midges flap their wings nearly 1000 times each second to stay aloft. Birds control their flight by fanning and twisting their feathers. Hummingbirds are the fastest-flapping birds, with up to 80 wing beats each minute.

          All animal movement is made possible by muscles. A gorilla has about 640 muscles, making up about three-fifths of its body weight. A fast fish has 30-40 large muscles along each side of its body. Forming nine tenths of its body weight, they pull on the backbone to swish the tail from side to side.

MOVING ON LAND

          Large land animals move in a way suited to their habitat. On open plains, cheetahs, gazelles and antelopes have long legs and run very fast. Forest animals like deer are slower in straight-line speed, but more agile as they zigzag between trees. Other animals, such as squirrels and sloths, are adapted to moving in the trees themselves. They have long, sharp claws to grip the bark. Central and South American monkeys have long tails, called prehensile tails, that curl around branches like a fifth limb. Limbs are not vital for movement. Many snakes are limbless, yet they can slither on the ground, swim in water, and climb trees— and even glide!

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          Most animals move around as they search for food, shelter or mates and avoid danger. So they have senses to detect what is going on around them. We have the same five main senses as many animals—sight, hearing, smell, taste and touch. Our main sense is sight. Compared to many animals, our eyes see clearly, in detail and in an especially wide range of colours. However, some animals have much better sight and other senses than we do. Some can even detect what we cannot, like tiny pulses of electricity.

          Some animals are nocturnal or active at night. They include cats, mice, bats, owls and moths. Their large eyes pick up as much of the faint light as possible. Animals that live in total darkness, like moles and cave salamanders or fish at the bottom of the sea have tiny eyes or none at all.

          The eye contains specialized nerve endings that detect patterns of light and send information about them to the brain. Other senses work in a similar way. In the ear, the eardrum is a thin piece of skin that vibrates when sounds hit it. Again, nerve endings detect these vibrations. Mammals, birds, lizards and frogs have eyes and ears on the head. However, some animals have them in other places on the body. A snail has eyes on flexible stalks. A clam has a row of small, simple eyes in the fleshy frill or mantle along the gaping edge of its shell. A grasshopper has eardrums on its knees.

SMELL AND TASTE

          Smell and taste are chemosenses. They are based on the presence of chemical substances, called odorants for smells and flavorants for tastes. We smell airborne odorants with the nose and taste flavorants in food and drink when they touch the tongue. Some animals have chemosensors on other parts of the body, too. A fly can taste with its mouthparts, its antennae (feelers) and its feet. A male moth’s feathery antennae can detect special floating chemicals given off by the female moth even if she is two or three kilometres away.

          A dog can smell scents up to 10,000 times weaker than we could detect.

          For animals in water, smell and taste are much the same. A shark has groups of chemosensors (taste buds) all around the inside of its mouth and also on the front of its snout. They are especially sensitive to blood and body fluids. A catfish has so many chemosensors in the skin all over its body that it is like a “living tongue”. Some fish including sharks, rays and elephant-snout fish (mormyrids) can detect the tiny electrical pulses given off by the active muscles of other animals. They use their electrosense to find prey in cloudy water or hiding in sand and mud.

         Electricity travels well in water, so many water animals have evolved to sense it. Electricity does not travel through air so land animals do not sense it. There are other senses that we lack and that we find difficult to imagine. Some animals migrate vast distances across featureless oceans with amazing accuracy. They may be able to sense Earth’s natural magnetic field or the way our planet’s downward pull of gravity varies slightly from place to place.

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