Paragraph D Salt-water crocodiles use the water, sun and shade to maintain their preferred body temperature of 30
to 33 degrees Celsius. When basking, they orientate their bodies to ensure the maximum surface area
is exposed to the sun. All crocodiles are unable to sweat. To avoid over-heating, they may return to the
water or lie with their jaws agape, allowing cool air to circulate over the skin in their mouths. This
process of heating and cooling their bodies is called thermoregulation and is crucial for many bodily
functions, including digestion and locomotion, and ultimately for their survival. Often observed
basking on the banks of watercourses where they are generally inactive, salt-water crocodiles are less
likely to be seen when they are in the water.
Paragraph E Salt-water crocodiles are one of the few reptiles to have a four-chambered heart, like mammals. They
can also stay underwater for extended periods of time, because they have the ability to slow their
heart rate, allowing them to hold their breath. A particular feature of salt-water crocodiles is their
inability to maintain strenuous activity for extended periods of time and they can easily become
exhausted while capturing prey or fighting other crocodiles. Extreme exertion is carried out
anaerobically and must be followed by a period of rest so that the oxygen debt can be repaid to their
muscles. One result of this anaerobic activity is a build-up of lactic acid in the blood. Although
crocodiles can withstand higher levels of blood acidity than other animals, it can sometimes be fatal.
Paragraph F Until 1974, salt-water crocodiles were hunted to the brink of extinction for their skins. In Australia’s
Northern Territory for example, as few as 3000 salt-water crocodiles were present when hunting
ceased. Today, the total Australian population is currently estimated to be approximately a hundred
thousand salt-water crocodiles, although some biologists estimate the population is even higher. It is
estimated that less than one per cent of eggs laid by salt-water crocodiles hatch and survive to
adulthood, and overheating, flooding and predation claim a high proportion of unhatched embryos.
From the small numbers that do hatch, more than half die in their first year of life, mainly from
predation by birds of prey, fish, snake-necked turtles and other crocodiles. Once they have reached
maturity, their only enemies are each other and humans. Habitat destruction by people is now
considered a major threat to Australian salt-water crocodiles.
Paragraph G Australia is one of the very few places in the world where salt-water crocodiles have a good chance of
survival in the wild. As apex predators, they are an important part of the food chain and as such cull
vulnerable prey, such as the old, injured, sick, or very young, leaving more food for the survival and
prosperity of healthy prey animals. This helps keep the Australian wetland environments healthy and
stable, and so protecting the ‘saltie’ is vitally important. This means Australians have a responsibility to
conserve and manage the country’s crocodile populations carefully.
IELTS 5 Practice Tests, Academic Set 5