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The mammals (Mammalia) are vertebrate warm-blooded animals and they are classified into two large groups: viviparous , the platypus and the echidnas .
In most mammals fetuses are develop in the womb and, once they are illuminated, they feed with the milk that their mother produces for them and stores in their breasts ; However, there are exceptions, in the last two groups mentioned babies are fertilized in an egg and once they are born they feed on small larvae and seedlings that their mothers collect for them. It is important to mention that mammals belong to a monophyletic taxon , that is to say that all species share the same ancestor.

There are more than 5,410 species of mammals, which shows the great diversity of these types of animals. The largest mammal, the blue whale , it can weigh about 160 tons; he Kitti pig snout bat , on the other hand, it is the smallest mammal, with only two grams of weight. Humans, on the other hand, are also mammals.

The variety of mammals allows them to be present in such diverse environments as the desert, the jungle, rainforests and polar ice, for example.

Particularities of mammals

All mammals descend from a species known as primitive synapsids, which belonged to the group of amniotic tetrapods that inhabited the planet about 280 million years ago and that came to replace the predominance of reptiles and remained the majority until the beginning of the Triassic Period; At this time, dinosaurs emerged who, due to their strongest qualities, were banishing synapses from the throne. The few that survived (the multituberculates and the australosphenes) became the first mammals of the Middle Jurassic.

It is at this point possibly where it is easier to establish the classification of a species within this group; since despite the differences in the evolutionary development of each particular species, there are certain exclusive novelties known as synapomorphies that are only reflected in animals belonging to this group; these are certain anatomical and physiological characteristics. They are:
* A single tooth bone that differs in each species according to the type of feeding and that is contained in a jaw;

* A jaw articulated with the skull which allows them to feed using the tooth without it leaving the head;

* A hearing system formed by three bones in the middle ear (hammer, anvil and stirrup), characteristic that all mammals possess except monotremes, which have a reptilian ear;

* Full body hair throughout all stages of life and a bone chain;

* Mammary glands which allow them to secrete milk to feed their young in the first stage of growth.

On the other hand, the sex of the mammal is determined from the existence of two chromosomes at the time the zygote is formed. These chromosomes are known as X and AND .

In addition, it is necessary to point out that most mammals have a Central Nervous System which allows them to perceive the world and respond to external stimuli by processing information abstractly. It is thanks to that nervous system that perceive the pains in a certain part of the body and that, according to their experiences, can predict or anticipate certain dangers that they know will affect them negatively.

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