What is a Mammal?

Mammals are warm-blooded vertebrates that share six key characteristics that classify these animals as a mammal:

  • Mammals nourish their young with milk produced by a mammary gland in the female of the species. This gland is activated by hormonal changes that occur during pregnancy.
  • All mammals have hair or fur at some point during their development.
  • Mammals have a diaphragm: a thin muscular wall separating the upper and lower portions of the torso.
  • All mammals have three tiny bones in the middle portion of the ear.
  • All mammals have a single bone comprising the lower jaw.
  • Mammals have a heart with one primary artery bending to the left leaving the heart.

There are approximately 5400 species of mammals that range in size from 1.5 inches for the smallest bat species to the Blue whale which can grow to 108 feet in length.  Human exploitation and habitat loss has led to the decline of many mammal species in the wild, while others are quite adaptable and have healthy populations.