What are mammals?This class includes the most advanced animals are completely different beasts: a huge elephant and a small mouse-vole, ravenous wolf and the meek sheep.They live in all poetry: on land, at sea (dolphins and whales) in the air (the bats).But what distinguishes them from other animals?Warm blood?The same feature is inherent and birds.The ability to give birth to live young?Well, firstly, not all mammals are viviparous.Some of them lay their eggs, as, for example, the platypus, while other young at birth are more like immature embryos and have to "ripen" in a special incubator-mother's pouch on his belly like a kangaroo.And secondly, there are viviparous and some reptiles, fish and even nasekomyh.Na question of who are mammals, what they differ from fish, the answer is simple: the presence of light.All animals breathe air.Therefore, the shark does not need to rise to the surface to take a breath, unlike like her appearance dolphin, which can simply drown without life-giving oxygen.But, again, the lungs have many primitive amphibians and all the birds.
Among the huge differences animals can be identified only two parameters that determine who are the mammals.It is the presence of hair and feeding calves milk.All the female animals of this group are the mammary glands on the body, which are filled with a liquid nutrient after birth.This property and gave the name of all mammals.Among other features characterizing the animals can be added presence of a lower jaw consisting of a bone, and a diaphragm which separates the heart from the lungs and digestive tract.
But the most important feature in determining who these mammals is their incredibly developed brain and flexible behavior.This means that two individuals of the same species caught in the same life circumstances, may behave differently.They are able to use their experience, observe and discuss.No wonder that in the class of mammals appeared view of Homo sapiens - Homo sapiens.Well, apart from him, the world is still home to about 5,000 species of other animals, which are grouped taxonomy 1,100 genera, 140 families and 18 orders and 3 groups.
No less interesting are the limbs of mammals.For all their apparent diversity (claws of predators, human and monkey fingers, hoof), all they end horny appendage - nail.In animals that live in water, these appendages atrophy.This feature is to adapt to the environmental conditions contributed to the fact that mammals have spread all over the planet and have taken all of life's niche - on land, in the air, at sea, under the ground.Thanks to the ability to maintain body temperature of warm-blooded, they can live in the Arctic (polar bears) and tropical (jerboa, camels) deserts.
question arises when there were the first animals?There is no doubt that the earliest mammals caught the era of dinosaurs.Then they were mainly small in stature and showed signs of "animal-like reptiles."As the scientists working with fossils, have at their disposal only the skeletons, it is difficult to determine at what stage in these animals appeared wool and the possibility of feeding calves milk.Assuming criterion edinokostnoy mandible, we can safely say that even 215 million years ago in South Africa, Europe, and China lived a small (15 cm) animal similar to a modern shrew.
