Dolphins are mammals, not fish. They breathe air through a hole on top of the head, so they must come to the surface often. They live in groups called pods, and the members help each other, for example by staying with an injured companion. Dolphins communicate with whistles and clicks. They also use clicks to find objects: the sound travels through the water, hits an object and returns as an echo, which tells the dolphin where the object is and how big it is. This ability is called echolocation. Dolphins eat fish and squid, and some groups work together to drive fish into a tight group before feeding. Scientists consider dolphins among the most intelligent animals, partly because they learn quickly and can solve problems. In some places, fishing nets accidentally catch dolphins, and protecting them is an important task.

Question: The word 'communicate' in the passage means