(1) Rats, mice, hamsters, and squirrels belong to a large group of animals called rodents. It is estimated that there are about 2,000 species of rodent, and they are believed to be one of the most successful groups of animals we know. They are considered successful because the rodent group accounts for over 40% of all mammal species on earth. Of all the rodents, the mouse-like rodents, such as those mentioned above, are probably the most successful, and it has been suggested that what makes them so is their teeth.

(2) Rodents’ teeth enable them to eat wide range of food, such as leaves, roots, nuts, and seeds. All rodents have four very sharp teeth at the front of their mouths ー two at the top and two at the bottom. These teeth are called incisors, but unlike the incisors of most other mammals, they never stop growing. So whatever their age, rodents will always have fresh, sharp teeth to eat with. In addition, they also have extremely strong muscles around their mouths, which enables them to bite through the shells of nuts and seeds. Many other mammals and birds cannot do this, so the seeds they eat pass through their bodies undigested, and the nutrition within them is unused.

(3) Nuts and seeds are very nutritious. Some seeds, such as sunflower seeds, sesame seeds, and peanuts, are very high in fat and protein. There are benefits of being able to eat these energy-rich foods: a lot of energy is gained in a short period of time; spending less time outside the home searching for food reduces the risk of being killed by other animals; and the time saved can be spent reproducing and looking after their young.

(4) Successful reproduction is essential to the survival of a species. However, it can be a double-edged sword. That is, it has good points and bad points. Many rats and mice are eaten by other animals before they die of natural causes, so being able to reproduce quickly is a good thing as it increases the species’ chances of survival. For example, a Norwegian rat could have 56 children and over 400 grandchildren within 20 weeks of being born. On the other hand, if reproduction is too successful and there is a huge increase in population in a short time, then there will not be enough food available for all of them to survive. When this happens, thousands and thousands die, resulting in population crash.

(5) The survivors often survive, not always by chance but because they are slightly different from the general population in some way. Perhaps they can run faster or further and so get to food before the others; or they have a better sense of smell and find food that others cannot; or they have stronger jaws and sharper teeth and can open nuts that others cannot open. Which of these slight advantages they have over those that die is not clearly understood; however, those that do survive have the opportunity to reproduce and pass on their advantageous traits to a new generation.

(6) So together with their high rates of reproduction and the survival of those that have adapted in some beneficial way, mouse-like rodents are in a strong position to survive well into the future. Ultimately, however, what probably enables these mouse-like rodents to reproduce so successfully is the fact that their teeth allow them to eat highly nutritious nuts and seeds so they do not have to spend most of their time engaged in the dangerous activity of gathering food.

① most birds and mammals can also digest the seeds they eat.

② mouse-like rodents are not as successful as other animals.

③ mouse-like rodents cannot bite into the center of nuts and seeds.

④ rodents have four incisors and powerful muscles around their mouths.

① Eating food high in fat and protein helps rodents develop muscular bodies.

② It takes little time to get enough food to satisfy the body’s energy requirements.

③ Less time is spent being exposed to the dangers of being eaten by another animal.

④ The time that is not spent searching for food can be used for raising a family.

① the good point of having many young is that they exhibit different traits.

② the increase in the population of a species may lead to food shortages.

③ the Norwegian rat is a good example of surviving food shortages.

④ the population of rodents is kept down by natural death.

① the breeding habits of rodents and other methods of repopulation.

② the different kinds of successful rodent subspecies.

③ the disadvantages that the surviving rodents face.

④ the possible reasons why some rodents survive population crashes.

① the advantages of using incisors for eating high energy food.

② the mouse-like rodents and the reasons for their success.

③ the relationship between the rodent population and the food supply.

④ the rodents which account for the 40% of mammal species.

(a) sharp teeth and a powerful bite

(b) The likely explanation for survival

(c) Reproduction and population crashes

(d) A background to mouse-like rodents

(e) The advantages of high energy food

(f) Conclusion

① (b)→(a)→(c)→(d)→(e)→(f)

② (b)→(a)→(e)→(c)→(d)→(f)

③ (d)→(a)→(c)→(b)→(e)→(f)

④ (d)→(a)→(e)→(c)→(b)→(f)