22 facts about mammals

1. Bats are the only mammal that can truly fly.
2. Mammals are the only animals with hair
3. Mammals have teeth that specialize in cutting and teeth that specialize in chewing and grinding.
4. Aside from monotremes, all mammals give birth to live young rather than lay eggs. The Platypus is an example of a montreme.
5. Mammals are one of the 6 main classes of animals.
6. The word mammal comes from the Latin mamma, meaning breast. This is derived from milk production in females for their young.
7. Mammals have a four-chambered heart.
8. Aquatic mammals (dolphins, whales) have fins evolved from legs.
   
9. Rodentia (mice, rats, etc.), Chiroptera (bats), Carnivora (dogs, cats, bears, meat-eaters), and Cetartiodactyla (deer, goats, sheep, herbivores) make up the four largest orders within the placentals.
10. Placentals, the largest group of mammals, use a placenta during gestation.
11. The Jurassic Period saw the first true mammals while modern mammals appeared in the Palaeogene Period
12. The sauropsid line split into what would become modern-day birds and reptiles with the synapsid branch leading to mammals.
13. Most mammals have seven cervical vertebrae.
14. Most mammals are endothermic; they produce their own body heat.
15. Mammalian lungs have a honeycombed structure.
16. The ability from young mammals to learn from experience is a primary reason for the evolutionary success of the mammal.
17. There are four mammalian groups that are fully aquatic.
18. Nearly all mammals have what are called "baby-teeth". Some exceptions include the anteater, which has no teeth at all.
19. The original purpose of lactation (milk production) is thought to have been to keep eggs moist.
20. All mammals are warm-blooded, which means they maintain a constant body temperature despite environmental conditions.
21. The largest mammal on land, and largest animal on land, is the African Elephant.
22. The tallest animal on the planet is also a mammal, the Giraffe.