Mice, bats, and humans are just a few of the many different kinds of modern mammals. Modern mammals of all shapes and sizes have four characteristics in common.
All mammals have hair, mammary glands, three bones in their middle ear, and the ability to chew. Let’s explore how scientists separate modern mammals into three groups.
Most modern mammals, including humans and other animals such as deer, cats, dogs, and whales, are eutherians.
Eutherian mammals give birth to live young that have completed fetal development inside the mother’s body.
Mammals that give birth to young that develop within the mother’s pouch are called marsupials. Kangaroos, Virginia opossums, and koalas are examples of marsupials.
Mammals that lay eggs are called monotremes. The eggs have soft, leathery shells, and development of the offspring occurs outside the mother's body.
The duckbill platypus and two species of echidna are the only examples of monotremes.
Click the Feature tab to learn more about the characteristics of modern mammals.