Mammals excrete urea in the watery form of urine . Mammals are very intelligent , with complex behaviors, and often live in packs. Mammals can learn to adapt and change in response to experiences. They may also evolve complicated social structures within species populations. The gestation period after successful intercourse varies from 8 days in some marsupials to 22 months in elephants.
Marsupials have a very short gestation period and so the young are born relatively undeveloped.
Kangaroos, koalas, wombats, and opossums.