Dolphins feed on a variety of fish and squid. It depends on what part of the ocean they live, on the local prey and conditions. They need for about 20 – 30 kg of fish per day. Dolphins have perfect hydrolocating organs that allow them to discover their food in the distance of about 3-km, they dive to catch it to the depth of 200 - 300 meters. Dolphins interlock their teeth to catch fish. They do not chew, and only swallow their food whole. Dolphins are very active. Very often they just chase one another, roll over each other, and carry different sea objects. They chase fish actively, sometimes even playing with them.

Many studies have shown that the dolphins are quite smart, their intelligence is on a level with chimpanzees and dogs. They are able to do different tricky things and to learn to perform certain actions. They learn actions more effectively when actions are broken into steps and when they are given some rewards.

Phylogeny.

Recent palaeontological, morphological, and molecular studies {2} have suggested that the order Cetacea that is presented by whales, dolphins, and porpoises, might by closely related to the order Artiodactyla, whish representatives are cows, camels, and pigs.

It was also found that the order Cetacea is related to other orders of ungulates, such as Perissodactyla (horses), Proboscidea (elephants), and Sirenia (sea cows)

There also was found that Cetacea had some connection with the Hippopotamidae.

Hippopotamuses and cetaceans form one monophyletic group. It was revealed that they have the genome of a common ancestor. Some phylogenetic analyses support the hypothesis that camels represent the basal cetartiodactyl lineage. The Hippopotamidae and cetacea share several specialized aquatic adaptations. We can mention such adaptations as a lack of hair, sebaceous glands. These two species are also characterized by underwater vocalizations, that turn to be communicative ones. These specializations are considered to be examples of convergence, that is a result of adaptation to an aquatic existence.

The group of ungulates is believed to be the progenitors of cetaceans. Studies indicate that modern dolphins came from the extinct Archaeoceti, primitive cetaceans appeared 50 million years ago and which in turn arose from a group of land mammals – mesonychians. All results suggest that cetaceans are nested within the Artiodactyla {1}.