An animal is classified as a mammal if it has the following key characteristics:
- Hair or Fur: Mammals have hair or fur on their bodies at least at some stage of life
- Warm-blooded: They maintain a constant internal body temperature regardless of the environment
- Live Birth: Most mammals give birth to live young (not hatched from eggs), with the young developing inside the mother’s uterus during a gestation period. There are exceptions like monotremes (e.g., platypus) that lay eggs
- Mammary Glands: Female mammals have specialized mammary glands that produce milk to feed their young after birth
- Complex Brain: Mammals have a larger and more complex brain compared to other animals, including a broad neocortex region
- Vertebrates: Mammals have a backbone, making them vertebrates
- Breathing with Lungs: Mammals breathe air through lungs
These features distinguish mammals from other animal groups such as birds, reptiles, amphibians, and fish
. While most mammals give live birth, the group also includes monotremes that lay eggs and marsupials that carry underdeveloped young in pouches
. In summary, an animal is a mammal if it is a warm-blooded vertebrate with hair or fur, gives birth to live young (or lays eggs in the case of monotremes), feeds its young with milk from mammary glands, and has a complex brain
