Definition: Mammals (Class Mammalia) belong to a group of vertebrates that includes approximately 5,400 extant species. Mammals display a remarkable array of adaptations that enable them to inhabit a wide range of habitats. Mammals range in size from the minute Bumblebee Bat which measures a mere three centimeters in length, to the magnificent blue whale, which can measure 33 metres head to tail—at such immense dimension, it is undoubtedly the largest animal alive today.
Hair is a defining characteristic of mammals: no other organisms possess true hair and all mammals have hair covering at least part of their body at some time during their life. Hair grows from skin cells known as follicles. The hair shaft is made of a protein called keratin. Hair serves numerous functions.
When present as a thick covering over the animal's body (known as pelage), it provides insulation. The pelage of most mammals is a mosaic of different kinds of individual hairs (such as guard hairs, barbs, bristles, awns, underfur, wool, fur, and velli; each of these has a different structure and serves a slightly different purpose).
