Mammals

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).

The adaptive benefits of mammalian hair are numerous. Hair provides insulation from extreme cold, enabling mammals to inhabit some of the harshest habitats on Earth. It also provides mammals with protection for their skin from abrasions and damaging UV rays. Many mammals have pelage that displays cryptic coloration, which helps to conceal them from predators (or in some cases prey).

Another characteristic unique to mammals is the presence of mammary glands. Mammary glands, like hair, are a uniquely mammalian trait. Though present in both males and females, mammary glands only fully develop in females. Mammary glands consist of ducts and glandular tissues that secrete milk through nipples. Young mammals obtain milk from their mother by feeding from her nipples. The milk provides the young with much needed protein, sugars, fat, vitamins, and salts.

A third characteristic unique to mammals is the presence of the presence of three middle ear bones. These ear bones (the malleus, incus, and stapes) transform sound vibrations into neural impulses.

Mammals are also characterized by having a diaphram, a four-chambered heart, and a large cereberal cortex. Additionally, some mammals hibernate during time periods when resources are scarce, such as during the winter season.
Classification:

* Kindom: Animalia
* Phylum: Chordata
* Class: Mammalia
The Class Mammalia includes the following subgroups:
o Order: Artiodactyla (even-toed ungulates)
o Order: Carnivora (carnivores)
o Order: Cetacea (cetaceans)
o Order: Chiroptera (bats)
o Order: Dermoptera (flying lemurs)
o Order: Hyracoidea (hyraxes)
o Order: Insectivora (insectivores)
o Order: Lagomorpha (rabbits, hares, and pikas)
o Order: Macroscelidea (elephant shrews)
o Order: Marsupialia (marsupials)
o Order: Montremata (monotremes)
o Order: Perissodactyla (odd-toed ungulates)
o Order: Pholidota (pangolins)
o Order: Pinnipedia (seals and sea lions)
o Order: Primates (primates)
o Order: Proboscidae (elephants)
o Order: Rodentia (rodents)
o Order: Scandentia (tree shrews)
o Order: Sirenia (dugong and manatees)
o Order: Tubulidentata (aardvark)
o Order: Xenarthra (anteaters and relatives)