In this case, the whistling may help the dolphins keep track of where others are. Dolphins are capable of imitating certain sounds very accurately and often learn other dolphins' whistles. One reason for imitating another dolphin's whistle may be to make contact with that dolphin and get its attention within a large group. Preliminary research seems to bear this idea out although details of the exact social contexts under which dolphins imitate another's whistle are still under investigation. Dolphins may use other sounds besides whistles to communicate.
Courtship behavior can yield pulsed yelps.
When under duress dolphins emit pulsed squeaks. Aggressive confrontation can produce buzzing click-trains. The generally poorer visibility found underwater prevents dolphins from communicating visually over any distance. However, when close to each other, body language may play a role in dolphin communication. Below is a list of dolphin behaviors possibly used for visual communication.