What are the three main groups of mammals?

The three main groups of mammals, based on how they give birth and raise their young, are:

1. Monotremes: These are the most primitive mammals, laying eggs instead of giving live birth. They include the platypus and echidnas.

2. Marsupials: These mammals give birth to relatively undeveloped young that continue to develop in a pouch on the mother's abdomen. This group includes kangaroos, koalas, opossums, and wombats.

3. Placentals: These mammals are the most diverse group, with most of the familiar mammal species falling into this category. They give birth to live young that have developed inside the mother's uterus, nourished by a placenta. This group includes humans, cats, dogs, whales, elephants, and many others.