Birds and mammals are the only homeotherms on earth today and the only ones to have mammary glands.
The ability to maintain a constant and high body temperature was the main adaptive factor of this group's representatives to virtually any terrestrial environment.
Many mammals have returned to the aquatic environment (whales, seals, dolphins, manatees) and others have adapted to flight (bat) and share the air with birds and insects.
The characteristics of mammals
Some features differentiate mammals from all other vertebrates:
- mammary glands milk producers with nutritive substances for newborn feeding;
- body covered by hairkeratin-rich epidermal structures made of hair follicles;
- aorta artery facing the left side of the heart (in birds, the aorta is turned to the right side of the heart);
- skin containing sebaceous glandss, whose oily secretion lubricates the hair and the skin itself, and sweat glands, sweat producers (in fact, a water, salt and urea filter), homeothermic maintenance facility and excreta disposal pathway. Both glands have epidermal origin;
- diaphragm muscle, located between the chest and abdomen, used for pulmonary ventilation;
- placenta, an organ that regulates food exchange between maternal and fetal blood, present in most mammals called placentals.
Human and dolphin fetus surrounded by placenta
Breathing, excretion and circulation in mammals
Respiratory gas exchange occurs exclusively in the lungs, whose surface is enlarged by richly vascularized alveoli. Inspiratory and expiratory breathing movements occur thanks to the action of muscles located between the ribs (intercostal musculature) and also by the action of the diaphragm, important striated muscle that separates the thorax from the abdomen.
In mammals, the main product of nitrogen excretion is urea, a substance synthesized in the liver and filtered in the kidney.
The heart of mammals, like birds, has four cavities: two atria and two ventricles. There are no blood mixtures. The difference from the birds' hearts is that the aorta, which directs oxygenated blood to the body, is bent to the left side of the heart. The circulation is double and complete.
Mammals have a wide variety of teeth with specific functions. The incisors are flat and serve to cut; Canines are pointed and are used to shred meat. The molars are large and protruding and chewable. The number and type of teeth vary according to the diet of each species. Carnivores have very developed canines and molars; herbivores do not have canines as they do not need them to cut pasture.
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The mammalian brain has many circumvolutions or folds, which increase the surface of the organ and the number of nerve cells.
For this reason, mammals have developed complex behavior that can be perceived in attitudes such as hunting strategies, care for puppies, adaptation to any environment and the different communication systems established between individuals of the same species.
Reproduction: placenta appears
The sexes are separated. O sexual dimorphism is accentuatedthat is, females have external characteristics that differentiate them from males and vice versa. Fertilization is internal. For the most part, embryonic development occurs within the mother's body, in a muscular organ called the uterus. An organ of metabolic exchange arises, the placenta organized by maternal tissues and tissues of the embryo. Food, oxygen, antibodies, and hormones are passed from the mother's blood to the embryonic, which in turn transfers excreta and carbon dioxide to the mother.
The well-developed amniotic vesicle plays an important protective role in dampening shocks that strike the female's abdominal wall and also in providing an aquatic environment for embryonic development. The vitelline and allantoid vesicles lose their function, which is now performed by the placenta.