Vertebrates

• All mammals are vertebrates, meaning they have skeletal structures with a vertebral column, more commonly known as "spinal columns" and "backbones." The vertebral column is important for mammals since it serves as the foundation for the remainder of their skeletal systems. A mammal's vertebral column also hosts its spinal cord, a bundle of nerve tissue that are important in an animal's central nervous system.
  
  

Endothermic

• Mammals are endotherms, meaning they are warm-blooded animals that are able to regulate their own body temperature. These animals keep the inside of their bodies at constant temperature by cooling down in warm surroundings and warming up in cold environments. To warm their bodies, mammals create energy from the food they eat. As a result, mammals must eat more than cold-blooded animals, such as amphibians and reptiles.
  
  

Hair

• The only animals in the world with true hair are mammals. True hairs emerge from pits in the skin known as follicles. All mammals have hair on their bodies during at least one stage of their lives. Mammals in cold environments use hair to keep themselves warm. Also, hair prevents overexposure to the sun for animals in extremely hot habitats. Melanin, a group of proteins, determine the color of a mammal's hair.
  
  

Live Birth

• Live birth is a trait common to nearly all mammals. The majority of mammals are placental, meaning they give live birth to offspring that are ready to live freely from their mothers. Although they are nursed during the early parts of their lives, placental offspring are able to walk on their own. Certain female mammals, such as kangaroos, opossums and koala bears, are marsupials, which means they carry their young in a body pouch until their offspring mature.
  
  

Nursing

• Female mammals that give live birth nurse their offspring by using their mammary glands. Offspring suckle on their mother's teat, which connects to the mammary glands within the mother's body. Mammary glands secrete milk and deliver it to the teat through a duct system.
  
  

Lungs

• Mammals have lungs as a part of their respiratory system, which allows these animals to breath oxygen and exhale carbon dioxide. Even marine mammals, such as dolphins, whales and manatees, have blowholes on top of their bodies. Marine mammals periodically swim to the surface to breath through their blowholes. Land mammals use their nasal cavities for inhaling oxygen to their lungs.