Mammals are a class of animals that is often considered to be the highest class of all animals. From a zoological point of view humans belong to this class.Mammals are four-limbed, warm-blooded, air-breathing animals.
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Mammals originated as terrestrial animals, and are the only class of animals that embrace all the environments available on Earth – terrestrial (carnivores, hoofed, elephants etc), underground (moles, gofers, marmots etc), air (bats), fresh water ecosystems (beavers, platypus, water rats), oceans (whales and dolphins, seals, dugongs and manatees), polar ice caps(seals, walruses, polar fox, polar bear).


Mammals have hair-covered skin (secondary naked in some) with external glands. One type of glands, milk glands, is another distinctive feature of mammals found in no one other group of animals (except for some fishes from Cichlidae family whose fry feed on mucous secretions from parents’ skin). Unlike other vertebrates, mammals have a single bone in lower jaw and three inner ear bones. Fertilisation is always internal. All mammals are viviparous with the exception of Monotremes. Forelimbs can be transformed into wings,(bats), or flippers, (dolphins, whales, dugongs, seals). Hind limbs can be absent in Ocean mammals, (whales, dugongs).

In spite of common major anatomical and physiological features, mammals achieved the greatest diversity of shapes and sizes. The largest animal which has ever lived on Earth is contemporary Blue Whale (Balaenoptera musculus). It can weigh more than 150 tonnes, while the smallest known mammal, the shrew, merely reaches 2.5 grams. This is a 60,000,000 times difference! The difference in weight between the largest and the smallest contemporary birds is just about 57,000 times.



These are:

1. Prototheria – egg laying animals , several species found only in Australia,(eg. the platypus and echidna)

2. Metatheria – marsupial animals found in Australia and South America, and (includes the marsupials)

3. Eutheria that includes majority of contemporary species and is distributed in all continents of our planet. (includes humans and other placental mammals)

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