Mammals are able to live in a variety of habitats due to a number of adaptations. There are characteristics that only mammals share such as hair and mammary glands. The mammal has the ability to control the body temperature with sweat glands that aid in cooling the body and the metabolism which controls the amount of heat produced. There are main classes of mammals. These are the placental mammals, marsupials and monotremes.
The monomtremes is the most primitive and there are only three species. Two enchida species and the duck billed platypus. These are egg laying mammals, when they hatch the young aren't completely developed. Milk in these mammals doesn't come from a nipple but from pores located in the skin.
Marsupials are mammals with a pouch that the young grow in. Then there are placental mammals that fully develop the young before birth. This is the largest of the mammal groups. There are over 4,000 mammal species that are put into groups by the body structure, teeth and bones.
Hair is something that only mammals have. At some point in their lives a mammal will have hair on a portion of their body. Follicles are the skin cells that hair grows out of, the shaft of the hair is made from keratin. There are a number of functions performed by hair.
While mammary glands are found on male and female mammals they only develop completely on the females. Milk is secreted from the nipple through glandular tissue and ducts in the glands. The mammals young nurse from the nipples to obtain milk that provides them with the required nutrients. Mammals give birth to babies that are fully formed.
Mammals have three bones in the middle ear which are the incus, malleus and stapes. These bones transfer sound vibrations from the eardrum to the inner ear and turn them into sounds. The incus and malleus were once a portion of the mammals lower jaw. The lower jaw of a mammal is now a single bone.
A layer of muscle found at the bottom of the ribcage separating the abdominal cavity from the thoracic cavity is known as the diaphram. In addition to mammals there are amphibians and reptiles that have diaphrams as well although the position and anatomy vary by species.
The teeth of a mammal is only replaced once in the species lifetime. The young mammal has small, weak teeth that are replaced by larger permanent teeth. Human's are the only mammal that walks upright the rest walk on four legs. Fins or flippers instead of legs are on aquatic mammals.