IELTS Online Practice Materials

Mammals

A) A mammal is an animal that gives birth to live young and feeds them on milk. It is warm blooded, with specialised features distinguishing it from other animals, such as middle ear bones allowing good hearing and jaws with teeth to cut, chew or grind food, allowing most efficient digestion of nutrients and calories. Mammal embryos in the uterus are initially protected from rejection by the mother's immune system and subsequently nourished by a placenta, which means they can stay in the womb long enough to develop specialised features such as flippers or wings and are generally born fully formed. The mother-child bond is strong and young animals learn by copying adult behaviour.

B) The range of mammals is wide, including marsupials, rodents, predators, large plant eaters, aquatic mammals and primates. Each species' physiology is highly adapted to its environment and lifestyle. Mammals also have large brains and highly developed nervous systems, enabling more intelligence and resourcefulness than other animals, a key survival feature. Learned behaviour is passed on from one generation to another, so younger generations benefit from their ancestors' experience. This was vital to human evolution and flexible adaptation and problem solving skills are still an exceptional feature of our species.

C) The earliest true mammals, appearing over 200 million years ago, were small shrew-like tree dwellers. Mammal lineages developed to some extent until the demise of the dinosaurs, around 65 million years ago, after the cataclysmic collision of a meteor 6 miles across into what is now the Yucatan desert caused dramatic changes in climate and conditions worldwide. Fossil evidence shows that, once they could exploit Earth's resources freely, mammals diversified enormously. There are now around 4600 species of mammals, in all continents. Most live on land and move about on four legs but some are aquatic (e.g. the whale) and some fly (e.g. a Thai forest bat the size of a bee).

D) The separation and (in some places) the reformation of land masses resulted in groups of species evolving on their own for many millions of years. Some eventually succumbed to environmental changes, geographic events or other animals; other more successful types flourished and adapted to become even more successful, perhaps also migrating to other areas.

E) Variations in size and specialisations are enormous. Lifespan ranges from less than a year (shrews) to a century (humans). Habitats range from the arctic, with temperatures of minus 68°C (polar bears, foxes) to deserts of up to 50°C (camels, kangaroo rats). Yaks graze on 6000m high altitude plateaux; sperm whales dive to 2000m underwater; grassland ruminants graze in herds and use speed and numbers to escape predators, forest mammals are usually solo, nocturnal and often camouflaged. Desert mammals have kidney functions adapted to retaining water; high altitude animals carry extra red blood cells to maximise oxygen capacity.

F) Recent advances in genetic science and improved technological tools have led to some controversy in the evolutionary theories of mammals. In the 1960s, evidence emerged to indicate that originally, earth had one single land mass, now called Pangaea. This split into two, about 225 million years ago, with each 'continent' carrying certain evolving species. Fossil finds indicated that mammals evolved in the northern land mass and later migrated south over land bridges that formed at various times. However, more recent finds in the south have found evidence of advanced mammals from a much earlier period than in the north, turning the 'north to south' model upside down. Additionally, geneticists can now compare fossil DNA with the genes of living mammals. They are finding that our classification of different groups of mammals is not accurate; that hippos and whales for example are not distant but very close relatives. Humans share around 95% of their genetic material with chimpanzees.

G) All mammals have four-chambered hearts, a muscular diaphragm separating their heart and lungs from their abdominal cavities, a lower jaw made of a single bone, and the same arrangement of tiny bones in the inner ears. Almost all mammals, including whales, mice, and giraffes, have seven vertebrae in their necks. The arms of humans, the flippers of seals, and the wings of bats all have the same number and arrangement of bones.

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