Mammals are animals that have hair, are warm-blooded, and nourish their young with milk.
Some modern-day mammals include people, apes , cats , bats , dogs , tigers , mice , moose , aardvarks , beavers , elephants , gorillas , sloths , pandas , hamsters , horses , whales, and dolphins . The monotremes are primitive egg-laying mammals. Some marsupials include the koala , kangaroo , and the numbat . Before birth, the young are nourished through a placenta. The placenta is a specialized embryonic organ that is attached the mother's uterus and delivers oxygen and nutrients to the young.