Who are mammals and how are they different from other animals?

Who are mammals? This class of the most developed animals includes absolutely different beasts: a huge elephant and a small mouse, a voles, a predatory wolf and a resigned lamb. They live in all the elements: on land, in the sea (dolphins and whales), in the air (bats). But what distinguishes them from other animals? Warm blood? The same feature is inherent in birds. The ability to produce live cubs? Well, firstly, not all mammals are viviparous. Some of them lay eggs, for example, the platypus, while other cubs at birth are more like immature embryos and forced to "ripen" in a special incubator bag on the mother's belly, like a kangaroo. And secondly, viviparous are found in some reptiles, fish and even insects. On the question of who such mammals are, what they differ from fish, you can answer simply: the presence of light. All animals breathe air. Therefore, the shark does not need to go up to the surface of the water to draw a breath, unlike a dolphin similar to it, which can simply be drowned without life-giving oxygen. But, again, many primitive amphibians and all birds have lungs.

Among all the huge differences between animals, we can distinguish only two parameters that determine who are mammals. This is the presence of the hairline and feeding the cubs with milk. All females of this group of animals have breast mammary glands on the body, which fill with the nutrient fluid after delivery. This property and gave the name to all mammals. Among other signs characterizing animals, you can add the presence of the lower jaw, consisting of one bone, and the diaphragm separating the lungs and the heart from the digestive tract.

But the most important sign in determining who these mammals are is their incredibly developed brain and a flexible system of behavior. This means that two individuals of the same species, having fallen into the same life circumstances, can behave differently. They are able to use the accumulated experience, observe and reason. Not surprisingly, it was in the class of mammals that Homo Sapiens appeared - the Intelligent Man. Well, besides him, there are still about 5000 other animals on the Earth, who were grouped in 1100 genera, 140 families and 18 detachments and 3 groups.

No less interesting are the finiteness of mammals. With all their visible diversity (the claws of predators, the fingers of man and monkeys, hooves) they all end with a horny process - a fingernail. In animals living in water, these processes atrophy. This feature of adapting to environmental conditions has contributed to the fact that mammals have spread throughout the planet and occupied all vital niches - on land, in the air, in the sea, under the earth. Thanks to the ability to maintain body temperature with warm blood, they can live in arctic (polar bears) and tropical (jerboa, camels) deserts.

The question arises: when did the first animals appear? There is no doubt that the oldest mammals found the era of dinosaurs. At that time they were mostly of small stature and showed signs of "animal-like reptiles". Since scientists working with fossils have only skeletons at their disposal, it is difficult to determine at what stage these animals had hair and the ability to feed young with milk. If we proceed from the criterion of a single-jawed mandible, then we can safely say that another 215 million years ago a small animal (15 cm) resembled a modern shrew in South Africa, Europe and China.