Read the text and answer the question!

    For many years people believed that the cleverest animals after man were the chimpanzees. Now, however, there is a proof that dolphins may be even cleverer than these big apes. Although a dolphin lives in the sea, it is not a fish. It is a mammal. It is in many ways, therefore, like a human being.

    Dolphins have a simple language. They are able to talk to one another. It may be possible for man to learn how to talk to dolphins. But, this will not be easy because dolphins cannot hear the kind of sounds man can make. If man wants to talk to dolphins, therefore, he will have to make a third language which both he and the dolphins can understand. 

    Dolphins are also very friendly toward man. They often follow ships. There are many stories about dolphins guiding ships through difficult and dangerous waters. 

What is the characteristic of dolphin according to the text?