There are many different kids of animals. Insects, spiders, fishes, amphibians, reptiles, birds, and many others are animals. Another group of animals is the mammals. Let’s take a closer look at the mammals. What makes an animal a mammal? Any animal is a mammal if:

1. it has hair or fur;

2. it is warm-blooded (which means its body temperature is kept at the same level regardless of the outside temperature);

3. its young are born alive (not hatched from eggs). Before they are born, they develop inside the mother’s body in a special organ called a uterus. The time spent developing in the uterus before being born is called the gestation period and varies from mammal to mammal;

4. after birth the mother feeds the young with milk that is made in specialized organs called mammary glands; and

5. it has a larger and more complex brain than any of the other animal groups.

Source: Illinois Department of Natural Resources