Marine mammals live in water together with fishes with whom they are confused sometimes, mollusks and Crustacea. Scientists consider that once this group of animals lived on the land, but on unknown so far to the reasons adapted to stay in the water environment.
Classification
There are two big groups of marine mammals. The first includes cetacea and sirens which carry out all life from the birth to death in water and never get out to the land. These are whales, dolphins, killer whales, sea pigs, a cachalot. The second group is presented by the Pinnipedia living both on the land, and to the sea. Seals, walruses, seals, seals, sea elephants, otters treat them.
Features
When marine animals lived only on the land, they had extremities. After change of the habitat their body adapted to new conditions. So animals gradually had fins, and tails changed that they could float and maintain balance in water.
From the ancient overland ancestors they kept lungs. Marine mammals can stay long time under water. However all of them are equally forced to climb periodically to the surface to inhale air which contains oxygen. As well as all mammals, these animals have constant body temperature.
More or less streamline, hydrodynamic shape of a body thanks to which they perfectly swim is characteristic of marine mammals. Their front extremities turned into fins, and back at some types disappeared, as at cetacea. At Pinnipedia they extended to edges and serve mainly for movement by land.
Cetacea
This group of marine mammals never leaves water elements. They conduct both a single, and gregarious way of life. The dolphin on freedom never is one. In bondage he can even die if to leave it alone. Cetacea are predators. They eat fish, squids, small Crustacea. Some types are capable to migrate, pursuing the production on thousands of kilometers. So, killer whales hunt, uniting in large packs. Communicating by means of whistle, they surround a fish school or pack of dolphins, and then attack them.
Pinnipedia
This group of marine mammals not completely adapted to life in water. Their body usually covers the fur helping to keep heat. It is also promoted by very thick layer of subcutaneous fat. It still is a power source in the conditions of the shortage of food.
Pinnipedia move in water not so quickly as cetacea. At the same time some types are capable to gather speed to 30 km/h on a surface. Pinnipedia very dexterous in water, and here on the land behave clumsily, hardly moving.
A large number of Pinnipedia is on the verge of disappearance. Since 19th century the number of walruses and seals was reduced to dangerously low level. They became subject to uncontrolled hunting because of the fur and tusks. Some types disappeared forever.