What is a common characteristic of all mammals?

Mammals are a distinct class of animals known for several specific traits. One of the defining characteristics of all mammals is that they possess hair or fur at some stage of their lives. This trait is significant because hair or fur serves important functions such as insulation, camouflage, and sensory perception.

The presence of mammary glands, which allow females to produce milk for feeding their young, is another key feature of mammals, but the question specifically addresses a physical characteristic. Whether or not mammals have hair or fur is consistent across the class, while other characteristics mentioned in the options do not apply universally to all mammals. For example, not all mammals lay eggs; this is a trait of monotremes, such as the platypus, but the majority of mammals give live birth. Similarly, while some mammals can fly, like bats, many do not have this ability, and while certain mammals live in water, such as whales and dolphins, many others inhabit terrestrial environments. Thus, hair or fur is the common characteristic that unifies all members of the mammalian class.