Pinnipeds and Cetacea are sea mammals that have evolved independently from each other. In evolutionary terms, sea mammals have been divided into two different groups. Firstly, there are the Cetacea (Whale-Likes), including porpoises, dolphins and whales, and secondly the Pinnipeds ("fin-feet"), including seals, sea lions and walruses.
The main characteristic of the Cetacea is that they always live in the water and never come on dry land. Their continual habitation in the water has caused a couple of changes to take place: They have lost their fur, but have kept a thick layer of fat, or blubber, around their bodies to keep warm in the cold seawater. During evolution, their noses have moved from the front of their head to the top of their head, and have turned into a blowhole through which they breathe. They have developed a vertical swimming stroke by moving their back and tail.
Seals, sea lions and walruses partially live in the water and partially on dry land. Generally, Pinnipeds come onto dry land for reproduction (mating) and are in the water to forage. Eared seals and true seals can be found all over the world. However, walruses are only to be found in the North Pole area. They have adjusted to the severe cold.There are a number of differences between true seals and eared seals (sea lions etc.) in the way they are built. Eared seals use their front flippers actively. On land they use them in combination with their flexible hind flippers so they are able to move about quickly, and they use them to climb onto the rocks. Eared seals are also able to tuck their hind flippers under their bodies, something that is impossible for true seals to do, because of their ankles. Eared seals actively use their front flippers whilst swimming by using them as paddles. True seals, however, swim by using their hind flippers in a horizontal swimming stroke, and the movement of their lower body and hind flippers is a very efficient way of moving along. On land, however, true seals are awkward. They are unable to move about very quickly, even though they use several techniques. Whilst on land, they prefer to stay close to the water.