Dolphins have become a popular attraction at zoos in recent years. They are more interesting than lions and tigers because they are livelier and perform tricks, like circus animals. But although they are more willing to cooperate with the trainer than other mammals in captivity, they get bored if they are asked to do the same trick twice. This is one reason for believing that they are very intelligent.

GLOSSARY:

captivity = cativeiro

In “They are more interesting than lions and tigers ...”, it means that