Transcripts
Atlantic salmon are born in freshwater rivers, where they grow into adults. Then, they swim out to the saltwater ocean, where they live their lives. The extraordinary part is that, when it’s time to breed, each salmon returns to exactly the same river in which it was born. Incredible! How do they navigate across the ocean and back to the same place? But before we try to answer ‘how’, lets look at ‘why’. What makes it worth these exhausting journeys and taking such enormous risks? The three main reasons for migration are climate, food and breeding. First, let’s consider climate. Snow, ice and temperatures below zero Celsius all make life more difficult and dangerous for most animals. So in the northern hemisphere, animals travel from north to south as winter draws near. To escape the freezing mountain temperatures of the Himalayas, bar-headed geese head for the warmth of South- East Asia. The migration of these birds is extreme in another way: they often fly up to eight kilometers above sea level. That’s about the same altitude as a jumbo jet! Climate is connected to a second reason animals migrate – food. Generally speaking, food is most readily available in the spring and summer months. The humpback whale travels over 16,000 kilometres from Mexico to feeding grounds in the Arctic. That’s the longest ocean migration of any animal. This brings us to the third reason for migration – breeding. Whales cover the thousands of kilometres from the Arctic to a warm sea in which to have their calves. Whale calves are born in the warmer waters of Mexico, away from the cold of the Arctic. Then, when they are old enough and strong enough, they swim back north to feed. So how do all these animals know when it is time to go? And come back? How do they know the destination and how to get there? And how do they navigate? Weather is a key factor. When the temperature starts to drop and the first snow falls, animals sense that it is time to change location. In the USA, the pronghorn antelope spends the summer in Wyoming. However, because the snows are so heavy in this region in winter, the pronghorn starts to move south in the autumn. The timing has to be very accurate – otherwise the path will be blocked by snow. So, antelopes must have some device, some way of judging very precisely when it is time to leave. How do animals know where they need to go? Again, no-one knows, exactly. Some information may be contained in their genes – the genetic information they inherit from their parents. Young animals also learn from observing and being with their parents and other animals in their group. After a few years of travelling together, they have memorized the route. Scientists have discovered that when babies grow up with no other animals to learn from, they don’t know the way. For example, a group of whooping cranes had to be taught where to migrate by humans using a light aircraft. Animals also use visual aids to navigate. Some birds orientate
198 English for the 21st Century • Transcripts
themselves using the path of the sun. However, when it is cloudy or they are travelling at night, they need other ways of navigating. Just like sailors of ancient times, they use the stars. Some ducks work out which way is north by using Polaris – also known as the North Star – which is one of the brightest stars in the sky. But how they use this information to calculate their route over thousands of kilometres remains a mystery. Despite migrating many thousands of miles, whales move in astonishingly straight lines. Most pods of whales stray off course by no more than one degree, despite big changes in sea currents. Again, no-one is entirely sure how they do it. They may use magnetic fields to help them navigate. They also appear to use the position of the sun. Being able to process these different kinds of data in order to keep on course suggests that whales have an extraordinary additional sense or a vast amount of data processing power. Scientists think some animals also use smell. For example, salmon use their sense of smell to find the river where they were hatched. Here they lay their own eggs. Wildebeest follow the smell of rain on the dry Serengeti soils to reach greener pastures. And it is thought that migrating birds can smell particular lakes from many hundreds of kilometres away. Animals have extraordinary and unexplained abilities that help them to process the environmental, visual and other information needed to navigate great distances. They time these journeys with pinpoint accuracy in dangerous and difficult circumstances. Despite scientists’ best efforts, their exact methods remain largely a mystery. Migration is truly an amazing phenomenon.
Unit 5
5.1 The human brain is the most complex organ in the body and is made up of many different sections, which each control a number of functions. It is perhaps useful to start with the six main sections. The cerebrum is the largest part of the brain but that in itself can be divided into four main sections. First, there is the frontal lobe which deals with high-level thinking – intellectual actions like problem-solving, for example. Then there is the parietal lobe. When the brain receives sensory information like pain or touch, the brain needs to process that information and this is done in the parietal lobe. In other words, it helps us to make sense of the world. Next, we have the occipital lobe and this is the visual part of the brain. It deals with information received through the eyes, so shapes, colours, and so on. The final part of the cerebrum is the temporal lobe and this is where the brain processes language, for example, sounds and understanding speech. The temporal lobe also helps us to form memories. Moving away from the cerebrum, we have the cerebellum, or ‘little brain’. The cerebellum is in charge of movement, which covers things like physical actions, balance and posture, and so on. Finally, one should not forget the brain stem. This is where basic but absolutely vital body functions are controlled, for example, breathing, swallowing and sleep.
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