What Makes a Mammal

What Makes A Mammal?

Living mammals are easy to recognize: they’re usually furry, warm-blooded, produce milk, and don’t hatch from eggs. But while these familiar traits can help us recognize mammals, they don’t define what a mammal is. And they don’t always hold true: some mammals aren’t covered with hair, and others do lay eggs.

So how do scientists decide which fossils are really mammals and which aren’t?

By Features: One way to define mammals is by their features. But this is tricky, because some key traits are also found in non-mammals—or are missing in certain mammals—or evolved separately several times. So most scientists prefer to define evolutionary groups, including mammals, by their ancestry.

By Ancestry: All mammals alive today descended from a single, shared ancestor—the first species of mammal. All of its descendants, including extinct species within the group, were mammals. The first “true mammal” is defined as the last or most recent ancestor shared by all modern mammals.

Scientists at The Field Museum are working with dozens of other institutions in the U.S. and internationally on a Mammal Tree of Life project to determine how living and fossil mammal species are related to one another.