My Science School

 

                         Cleaners are tiny fish living in coral reefs. They regularly clean parasites from much larger fish. Even large predatory fish queue up to be cleaned of skin parasites. The cleaner fish (or sometimes a shrimp) even swims into the predator’s mouth and gills without being eaten.

                        Birds can do a similar job. A basking crocodile opens its jaws so small birds can pick pieces of food and parasites from between its teeth. Small birds ride on the backs of grazing animals on the African grasslands, picking out skin parasites and feeding on insects as the animal feeds. At the same time the birds act as an early warning system for the host animal, by flying away when danger is near.

                  Predators usually live singly or in small groups, but their prey animals often group together in large numbers.

                  The most important reason for this behaviour is that there is safety in numbers. A predator will often become confused by a large, rapidly shifting group of prey. For example, a tuna will find it very difficult to select one target fish from a closely packed shoal containing several thousand fish. Similarly, a lion will usually not try to catch one of a panicking herd of wildebeest, but will attack  stragglers or injured animals.

                        Sometimes a herd has a defensive function. Adult buffalo and musk ox group together to form a protective circle around their vulnerable youngsters when a predator is near. The predator will not usually attack a fully-grown adult.

 

                           Scientists used to study just the appearance and structure of animals, but in recent years the study of their behaviour has become just as important.

                           The new science of ethology measures and records animal behaviour. We can now describe behaviour in terms of what the animal actually does, and not as a result of guessing what is going on in an animal’s mind. These studies have shown, for example, that animals such as birds have many inbuilt types of behaviour, which are inherited and not learned from other birds.

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               Parasites are animals that live at the expense of other animals. They rob the host animal of food, and can make it become sick. However, in other forms of relationship different animals can help one another. Some hermit crabs place sea anemones on their shells, hiding under their protective stinging tentacles. At the same time, the sea anemone benefits because it shares the crab’s food. Similarly, a species of shrimp digs a burrow that it shares with the small goby fish. The fish benefits from being able to hide in the burrow, while acting as a lookout to warn the shrimp of approaching predators. This kind of relationship can only be revealed by the careful study of animal behaviour. Most true parasites are very simple animals, because they do not need complicated organs to digest their food. Some parasites are simply a mass of reproductive organs.

 

 

                There is evidence from prehistoric cave paintings, and from bones showing signs of cooking or carving, that humans lived alongside some very large extinct animals.

                 Just after the last Ice Age, woolly mammoths, woolly rhinoceroses and giant cave bears lived in Europe. They were hunted by humans and, together with the warming climate; this may have led to their extinction. The giant ox was also prey for hunters. It became extinct in 1627, having survived in the forests of Eastern Europe.

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There are no large marsupial predators living in Australia today, but the thylacine ‘wolf’ only became extinct recently. There may still be a few survivors in Tasmania.

The thylacine was a large, dog-like animal with striped sides and an enormous mouth lined with fierce teeth. Sheep farmers hunted it to the edge of extinction in Tasmania, in order to protect their flocks. The last known thylacine died in a zoo in 1936, but there are still occasional reports of their being seen in the thick bush.

The thylacine was once common throughout Australia, but it was pushed out by the success of the dingo, a semi-domesticated dog introduced into Australia by the Aborigines when they colonized this country.

 

           The dodo was a large, flightless relative of the pigeon that once lived on the island of Mauritius in the Indian Ocean.

           It was discovered in 1507 but by 1681 it was extinct, although some relatives living on nearby islands survived for a few more years. The dodo was very large and fat, weighing around 23 kg, so sailors hunted it for food. Being a very trusting and slow-moving creature, it was easy to catch. The dodo also suffered from attacks by introduced animals such as pigs and rats, which destroyed its eggs and young.

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               All the larger kinds of whale are threatened with extinction. Although whales are protected from hunting by international law, there are many loopholes that are exploited. A new threat comes from the commercial exploitation of krill, the tiny crustaceans that form the bulk of the diet of the huge blue whales and some of smaller relatives. Krill are being harvested in huge quantities by fishing boats, for use as fertilizer and as food. If their stocks are reduced, the whales will have no alternative food sources.

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                    The desire of some people to keep exotic pets threatens many parrots and wild cats. Spectacular parrots like the macaw are captured in large numbers and exported for the pet trade. This practice has greatly reduced their numbers in the wild, and this trade is now strictly controlled in most countries. Captive breeding programmes have been set up to avoid the need to capture wild parrots. Hawks are subject to the same pressures, and are protected in many countries.

                    Some of the smaller wild cats, such as the ocelot, are popular among the wealthy as prestigious pets. However, these creatures are not really suited to captivity.

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                    Turtles are threatened because of hunting and loss of habitat. They have been a favourite food for thousands of years, and their population is unable to sustain the present level of hunting. The worst threat comes from the disturbance and destruction of the turtles’ breeding sites. The ideal beaches for turtles to lay their eggs on are also the ideal beaches for holidaymakers. The result is that hotels have been built near these beaches, roads carry cars whose headlights confuse turtles heading for the beach at night, and tourists disturb the turtles as they lay their eggs.

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                    According to the theory of evolution, some animal species become extinct because they are less successful than other species that gradually replace them.

                   These so-called ‘failed’ animals are also unable to adapt to changing circumstances. Humans have accelerated their extinction by changing the environment so rapidly that animals do not have time to adapt. For example, the destruction of Indonesian rainforests has left nowhere for the orangutan to live. It would take millions of years for the animal to evolve into a ground-living creature. Hunting is the reason for the reduced numbers and probable extinction of animals such as the tiger, the blue whale and the giant panda.

 

            The giant squid is by far the largest invertebrate known. This monstrous creature lives in the ocean depths, and is only occasionally washed ashore. The specimens found so far have been 20 m in length, and their eyes are 40 cm across (these are the largest eyes known).

            Squids have tentacles like an octopus that surround a powerful beak. Two of these tentacles are enormously long, with gripping suckers at the end. The giant squid’s suckers have a ring of sharp teeth around them to help grip their prey. These suckers leave scars on the sperm whales that feed on them, and some of these scars are from suckers nearly three times bigger than those on the largest squid found so far. So there are much larger giant squids still to be discovered!

 

 

 

 

            Both starfish and sea urchins move about on tiny tube feet. These creatures have the same internal structure. Imagine the sea urchin as a starfish rolled up and joined along the edges of its arms.

            Coral reefs are built up from the skeletons deposited by millions of tiny invertebrate animals. These coral animals live only in warm water, feeding on plankton and also living on food substances produced by microscopic plants living inside their tissues. The reef builds up over thousands of years. Eventually the coral skeletons form complete islands that are colonized by plants and land animals.

 

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            The long arms that trail beneath the umbrella section of a jellyfish carry thousands of microscopic stinging cells. These cells have triggers, which cause the stinging cell to fire out a sharp thread when they are touched. The thread injects venom into any creature that contacts the stinging cells. Once the jellyfish’s tentacles have gripped the prey and the stings have immobilized it, the prey is drawn up to the jellyfish’s mouth and eaten.

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