Life on Earth

The label “cold -blooded”, so often applied to reptiles, is a very misleading one. They are much better described as ectotherms – that is to say creatures that gain their heat from their external surroundings, as contrasted with endotherms, like mammals and birds, which generate it internally.

Endothermy has many advantages. It makes possible the development of delicate and complex organs that would be damaged by fluctuations in temperature. It allows its possessors to remain active at night when the warming sun has disappeared. It even enables them to live permanently in cold parts of the world where no reptile can survive. But the price paid for such privileges is very high. Something like 80 per cent of calories in our food, for example, goes towards maintaining our body temperature at a constant level. The ectothermic reptiles, by taking their heat directly from the sun, can survive on 10 per cent of the nourishment that a mammal of a similar size would require. As a consequence, reptiles can live in deserts where a mammal would starve.

The reptiles not only survive in waterless places, they manage to breed there too, so their eggs, like their bodies, must be watertight. A gland in the lower part of the egg-producing organ secretes a parchment like shell around the egg as it passes down. The embryo must breathe, so the shell has to be slightly porous to enable oxygen to pass in the carbon dioxide to pass out.


1. Which of the following animals are NOT endotherma?
A. Cats.
B. Rabbits
C. Lizards
D. Wolves

2. Reptiles
A. do not absorb heat from the sun.
B. need to disperse heat to protect their bodies from overheating.
C. are threatened with extinction.
D. should not always be referred to as “cold-blooded”.

3. Endotherms
A. outnumber ectotherms.
B. produce heat inside their bodies
C. can live in deserts with very little food.
D. are affected by the sudden changes in the weather.

4. With reptiles, a gland in the lower part of the egg – producing organ
A. needs oxygen to function normally.
B. keeps body temperature at a constant level.
C. does not prevent the egg from drying out.
D. produces a substance which encapsulates the egg.

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