About 260 million years ago, some land mammals began to return to the ocean and became the ancestors of today's marine mammals. Some scientists believe that the ancestor of marine mammals is otters; Others think it is a bear. But no matter who they are, after returning to the ocean, these mammals began to adapt to marine life-their bodies gradually became longer and became "webbed" or "fins". They still have hair, breasts and a constant body temperature.

Seals mainly live in the Antarctic and Arctic continents with bad weather. Every male seal has many wives.

Sea lions crawl on all fours on land, faster than seals. Every breeding season, groups of sea lions get together.

As mammals, whales cannot live on land like sea lions and seals. Their mating, reproduction and care for their young must be carried out in water.

Dolphins are intelligent and kind marine mammals. They like to live in groups. When their companions are injured or sick, they will not hesitate to lend a helping hand. They can also communicate with each other through language. Scientists are trying to decipher the language of dolphins. I believe that one day, humans can understand what dolphins say.