Competition has led to the males having the largest testes relative to their body weight for any known mammal, being 4.2%.

Repenomamus is the largest known mammal known from the Cretaceous period.

These prints were likely left by Arctotherium, the only known mammal of the age who could account for them.

Reindeer are the only known mammals that can see ultraviolet light.

Humans are the only known mammals to cry as an emotional reaction.

Only one known mammal has scales, the pangolin.

The first known mammals lived about 220 million years ago.

It is one of the smallest known mammals.

It is the most primitive known mammal to be preserved with hair, evidence for its evolution before the appearance of more complex mammals.