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 There are over 4,000 different types of mammals in the world that can all be identified by their unique differences.
The blue whale, which is the largest mammal on the earth weighing as much as 1500 humans, is obviously different than a bumblebee bat, the smallest mammal weighing less than a penny. Dogs don’t look like cats and mice aren’t elephants. So what in the world do they all have in common?
All mammals have hair or fur. Even whales have a few coarse hairs around their mouths. A sheep’s thick, curly wool and a porcupine’s sharp quills (yes, quills are hairs) feel and look very different from the hair of a gorilla or a dog, but they are all types of hair.
 Mammals are able to survive in almost any type of habitat from frigid tundra to a hot, humid, tropical rainforest. They can live almost anywhere because the temperature inside their bodies stays the same whether it’s cold or warm outside (they are homeothermic). They can also make their own body heat to keep themselves warm enough (they are endothermic).
Unlike most birds or most reptiles who lay eggs, most mammals give birth to live young. There are two exceptions to the rule. Can you name the two mammals that lay eggs? Once baby mammals are born, their mothers feed them with milk from special glands in their bodies called mammary glands. This milk nourishes them until they are mature enough to eat solid foods.
Most of these common characteristics shared by mammals may seem familiar to you. Have you noticed that people have hair and maintain a constant body temperature (98.6)? We also give birth to live young and feed them with milk. We are mammals too!
So… did you guess which mammals lay eggs? The duckbilled platypus and the spiny anteater are the only mammals that lay eggs.
Click here for a list of mammals
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