Class (Mammalia) of Vertebrates, the most evolved of the Cordati type, to which also man belongs.
Typical features of the class are:
- Viviparity, with the exception of the Monotremes, oviparous, with development of the placenta, perfected in the Euteri, but also present in the Marsupials;
- The nutrition of children through milk, secreted by the functional mammary glands only for females except in the Monotremes;
- The presence of cutaneous formations, the hairs, that in almost all the species cover the body, with the exceptions of the Cetaceans and the Sirenii that possess a very hairy hairy;
- The constant body temperature (homeothermia), present in all but in some lower groups.
Mammals occupy a dominant position due to high organization and extraordinary psychic development. Thanks to homeothermia and viviparity they colonized all terrestrial biomes. The behavior of Mammals is social in pairs or in small groups, often with a high structural organization. Sexual activity regulated by hormones is cyclic; females alternate periods of activity (estrus) with those of reproductive inactivity (anestro), with variable periods. Parental care is long and complex. You have offspring when the little ones become independent very soon (Ungulates), inept if it depends on the parents (Carnivores).
Their dimensions range from 3.5 cm of mustiolo, which weighs only 2 g, up to 33 m and 120 t of weight of the blue whale. The vast majority of the Mammals are provided with four limbs, five fingers: most of the species are terrestrial and live on the ground or on the trees (or both). Different ends in the locomotion allow further groupings between species such as the plantigrade (bear), whose limbs rest on the whole palm and plantar surface, the digitigrades (dog), which rest only on the fingers, and the unguligrade that rest on the ground only the ends of the fingers, usually reduced by number up to one (horse).
The function of hairs is perhaps the most important because, for almost all species, it generates fur and efficient thermal insulation. They may change periodically and may vary in color and length depending on the season, age, gender and geographical distribution of the animal. With secondary sexual characteristics, there may exist a non-uniform hairy mantle such as manes, beards, etc. which assumes maximum pigmentation in hot and humid climates. Some Mammals are provided with aculei or keratinized stem hair (hedgehog and porcupine), as well as bristles, less robust, and the vibrissae, on the snout, to tactile function. Scales (pangolins) and corned scales (armadillos) are special exceptions.
The skull of the Mammals shows a significant reduction in the number of bones compared to that of the Reptiles. Seven cervical vertebrae make up the vertebral column except for sloths and manates. To highlight in the skeleton the development of the sternum, a sturdy spine in the scapula, the fusion of the pelvis bones. In the Mammals there is a diaphragm, which divides the thoracic cavity from the abdominal cavity. Characteristic is also the jaw which is constituted for each branch by a single bone and articulated directly with the bone of the skull. Teeth heterodontes complete the jaws and differ in incisors, canines, premolars and molars. These represent important information about the diet of animals, as well as the digestive system that is well differentiated and specialized in relation to nutrition.
As for the teeth, even the nails contribute, and not a little, to the understanding of the gender of life of an individual. They are reduced in the Pinnipeds and the Sirenii, absent in the Cetaceans. In the Artiodactyls and in the Perissodactyls there are bony, cornea or both components, the horns, of different shape and nature, almost always in even numbers.
The senses are of fundamental importance in all and for all Mammals.
The sense of smell is of prevailing importance in the lower orders (macrosmatic Mammals, typical Insectivores), while in the most advanced orders the various senses are harmoniously balanced according to the type of life:
- Tact is widespread on the whole surface of the body;
- Hearing, sometimes of extraordinary acuity, is of primary importance;
- Sight is not one of the most developed senses, except in Primates, where it assumes primary importance: even the size of the eyes varies from the largest of the nocturnal species to the smallest of the aquatic ones;
- Peculiar is the ability of bats, Pinnipeds, Cetaceans and some Rodents to produce and perceive ultrasound (echolocation).