Robert Moreno, age 11, of Davenport, Iowa, for his question:
HOW MANY KINDS OF MAMMALS ARE THERE?
There are about 5,000 different kinds or species of mammals. They live in all parts of the world. Humans are mammals and so are dogs, cats, bats, dolphins, lions and even whales.
The name "mammal" comes from the Latin word for "mama," or "breast." A mammal is an animal with hair and mammary glands on its body. The mammary glands of female mammals produce milk that the females feed to their young. This feeding is called nursing. No other animals ¬such as birds, fishes, insects or snakes nurse their young.
The smallest mammal on earth is the shrew that weighs less than a dime. The largest mammal, which is also the largest animal that has ever lived, is the blue whale. It is larger than an elephant.
Scientists tell us that the first mammals probably developed from a group of reptiles about 180 million years ago. This was during what geologists call the Mesozoic Era.
In the olden days, reptiles were the dominant or ruling animals on the earth. Over the last 65 million years, mammals have slowly become the dominant land vertebrates, or animals with backbones.
During these millions of years, mammals have developed different ways to protect themselves against the weather and other forces of nature. Most mammals are covered with hair which helps to protect them from the elements.
All mammals are warm blooded. When the surrounding temperature gets hot, mammals can cool themselves off by panting or sweating. They can also warm themselves when the weather is cold by shivering or exercising.
Most mammals keep their body temperatures relatively constant, regardless of the air temperature. A human being's normal body temperature, as an example, is 98.6 degrees Fahrenheit. However, the body temperature may vary. It may go up when a mammal is sick and it will usually drop when a mammal hibernates.
Human beings, of course, have the most complex brains and nervous systems of all the mammals.
Mammals have keen senses of smell and they communicate with others by making sounds. Mammals can reason better and learn more easily than other animals. This is because a mammal thinks and remembers with a large and more complex brain than any other animal.
Mammals are classified into three different groups according to the way young mammals develop. Most young mammals develop inside the mother's body before birth. This group is called the "placental mammals." They receive nourishment through a special organ in the female body called the placenta. This group includes human beings, horses and most other mammals.
The second group of mammals are called the "pouched mammals" or marsupials. This group includes opossums and kangaroos. Most marsupials give birth to their young earlier than other mammals. The young then get nourishment from the mother's mammary glands inside a body pouch.
The third group are called monotremes or egg laying mammals. This group includes platypuses and spiny anteaters. The female lays eggs and then nurses the young after they hatch.