What is a mammal?
Mammals are vertebrate animals that are endothermic, have hair on their bodies, and produce milk to feed their babies. Many mammals give birth to live young that are small and helpless. Producing milk to feed them allows them to develop more slowly and spend time with the adults as they grow up, learning the skills they need to survive. Regulating their own body temperature and having hair of various thicknesses for protection has also allowed mammals to live in almost every habitat on Earth.
Monotremes—unusual mammals
There are three classes, or main types, of mammals: monotremes, marsupials, and placental mammals. Monotremes are the most primitive and there are only three species: the duck-billed platypus and two species of echidna. These mammals have hair and produce milk, but they also lay eggs. The eggs are leathery, similar to reptile eggs, and hatch into tiny young that are not well developed. The young cling to the fur on the mother’s belly and suck at her milk, which comes from pores in the skin instead of from a nipple.
- See more at: http://animals.sandiegozoo.org/content/mammals#sthash.MUjt68AL.dpuf