The Mammals

The mammals started in a quiet way. Some were living in the undergrowth of Jurassic jungles at the time the dinosaurs were common. These early mammals were tiny creatures just a few centimetres long. They were mammals, not reptiles, because of the way their teeth and jaws were built. A bony, hard palate separated the mouth of the mammal from its air passages, so that food could be chewed while the animal was breathing.

Warm-blooded mammals

Mammals were a great advance over reptiles. The typical reptiles were 'cold-blooded', which means that their body temperature was about the same as that of the air around them. When the temperature was low, the reptiles had to be inactive. If it was too hot they were in danger of being overheated. Mammals, on the other hand, are 'warm-blooded'. Their bodies stay at the same comfortable temperature. In cold weather their hair or fur helps to keep them warm, and they can also produce heat by rapid quivering of their muscles, or 'shivering'. In hot weather, mammals lose heat by sweating. Whatever the temperature, they can carry on with their lives.

Eggs retained in the female's body

Most young reptiles came from eggs laid in holes in the ground. They had to fend for themselves as soon as they were hatched. Eggs were sometimes disturbed, broken or eaten, and the young defenceless reptiles were often killed by larger animals. The vast majority of mammals overcame these problems by keeping the egg inside the female's body, where it was fertilized and allowed to develop out of danger.

The embryo develops inside the mother's uterus or womb to which it is joined at a region called the placenta. Through the placenta, food substances and oxygen pass from the mother's blood to that of the embryo, while the embryo's waste materials pass back into the mother's blood. When young mammals are finally born, their mothers feed them on milk from their own bodies until they are old enough to fend for themselves. Fewer young mammals die because of this long period of parental care and protection.

A calf developing inside the womb of a cow

Better brains and hearts

Mammals were more advanced than reptiles in other ways. Their brains were better developed, enabling them to learn more easily from past experience, and so change their behaviour. They also had more efficient hearts.