Category Science

          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.