Mammalian skeletons have become modified in many different ways as animals have adapted to different habitats and lifestyles. In primitive mammals, limbs tended to be relatively short, as for instance in the opossum. But adaptations, such as running faster, led to changes, like the elongation of limbs. In bats (A), the forelimbs have become wings, while in seals (B) the limbs form paddle-like flippers. Whales' limbs have a minimal steering function; in baleen whales (C), the jawbones are greatly enlarged and curved to accommodate the animal's huge, sieve-like plates of baleen (whalebone).
Mammalian specialized teeth.
A mammal is a member of the class Mammalia. Mammals are warm-blooded, usually hairy vertebrates whose offspring are fed with milk secreted by the mammary glands. All but a few are placental (see placenta). About 4,000 living species are known, including humans and their main domestic animals. The mammals are divided into 17 orders and three subclasses: the Monotremata (monotremes), Metatheria (marsupials), and the Eutheria (placental mammals). They range in size from shrews weighing a few grams, to the blue whale, which can weigh up to 150 tons. They probably first evolved about 180 million years ago from a group of warm-blooded reptiles.
In mammals, the bones and muscles of the legs carry the weight of the body, while the backbone is a girder between them from which the gut and other organs hand. The rib-cage protects the lungs and heart, but an important difference between mammals and birds is the separation of these organs from the organs in the lower part of the body by a thin sheet of muscle – the diaphragm.
The brain is more developed in mammals than in any other group, allowing for the variety of activities that is characteristic of them.
Most mammals give birth to their young alive (the monotremes, such as the platypus and spiny anteater, which lay eggs, are an exception); structures occur in the female therefore that are not found in birds and reptiles. In marsupials the young are born at a very early stage and suckled in a pouch. The latter is supported by the so-called epipubic bones. These are not found in other mammals.
Mammalian teeth are usually of several kinds – specialized for grinding, tearing flesh, and so on. Variety is greatest in the carnivores, which have biting incisors, tearing canine teeth, and sharp molars and premolars for crushing bones. Herbivores have more uniform teeth designed for grinding vegetation such as grass.
Adaptability of mammals
Some mammals with terrestrial ancestors have evolved to exploit the air and sea. The bats are highly successful, and the only vertebrates other than birds capable of sustained flight. Whales and dolphins have become so well adapted to the marine environment that they never come ashore. The baleen whales have evolved huge filters of baleen (whalebone) with which they sieve the vast shoals of krill that abound in the oceans, particularly in the Antarctic region.
In areas scarce of prey,predatory mammals tend to be solitary, like the polar bear, whereas where food is more plentiful, cooperative hunting techniques have evolved, where groups of predators such as hunting dogs or lions cooperate to bring down prey that may be larger than themselves. Many herbivorous mammals have evolved symbiotic relationships with cellulose-digesting bacteria so that they can take advantage of relatively indigestible plant food for which there is less competition. Bears and many primates are omnivorous, feeding on fruits, leaves, and berries, but hunting other creatures when they get the opportunity. Other mammals, such as raccoons and hyenas, are mainly scavengers.
Similarities among mammals
Despite geographical isolation, many mammals have adapted in corresponding ways to similar lifestyles in different continents; examples are the anteaters and armadillos of South America, the aardvark of South Africa, the pangolin of Asia, and the echidnas, or spiny-anteaters of Australian and New Guinea – these creatures are all anteaters with long, tough snouts, long tongues, and powerful claws for breaking into nests of ants and termites.