Characteristics of Mammals

Characteristics of Mammals

Homologous Structure

A common mammalian skeletal structure is modified for each animal’s particular needs. Here, the bones of the forelimb are adapted to its mode of locomotion. From left, the elephant’s immense weight requires a sturdy support to carry it. In contrast, a bat’s wing stretches over a framework of thin, elongated finger bones. The forearms of aquatic mammals such as porpoises and whales form flippers, and the gibbon uses its jointed, flexible digits to grasp and swing through trees.

Oxford Scientific Films

Nursing Young

Unique to mammalian females are the milk-secreting, or mammary, glands that give the class its name. Milk provides complete nourishment, including water, for the young. A mother giraffe usually nurses her calf for 9 to 10 months, until its body can metabolize the leafy diet of its parents. Mammal babies lacking the sucking reflex at birth generally do not survive.

Oxford Scientific Films/Mike Birkhead

Four Limbs

Most mammals have four appendages, usually legs. These may be modified for walking upright, as in human beings, whose upper legs and knees articulate at an angle that supports their weight with most stability; swimming, as in whales, whose vestigial hind legs are still present in the skeleton; or flying, as in bats, whose metacarpal bones are webbed with skin.

Oxford Scientific Films/Kathie Atkinson

Helpless at Birth

Although this two-month-old parma wallaby joey successfully completed a blind journey from its mother’s birth canal to her pouch without help, it would not survive without her. All mammals, with the exception of egg-laying monotremes, are born alive, but they complete only partial development inside the womb. The young must be fed and protected sometimes for years before they can fend for themselves.

Dorling Kindersley

Large Brains

An interior view of the skull of a baboon reveals an enlarged brain case, sinus, and oral cavities. These large cavities are specialized for receiving and analysing sensory input from a variety of sources. The brain, the ultimate endpoint for all sensory signals, is proportionately larger in mammals than in any other group of animals.

Oxford Scientific Films/F. Schneidermeyer

Hair All Over

The bodies of mammals are covered with hairs, filaments of dead cells strengthened with keratin, that emerge from the skin. Closely spaced, they constitute a fur coat. Kinky and matted, they form wool. Hair serves as insulation, protection, water repellent, and, in the case of the porcupine, whose hairs are coarse, stiff, and pointed, as defence. Some mammals have lost their fur through evolution. A layer of insulating blubber, for example, takes the protective place of hair in whales.