Dolphins.
Dolphins are considered to be the most intelligent of animals and their friendly attitude have made them popular in human culture.
They really communicate with each other by means of a language, mainly whistling. Several researches have shown that some colphin species are capable of sending identity information to each other using a specific whistle.
Also, dolphins are a good way to help those people who has psychological problems, developmental disabilities and depression. But, there are professionals who have criticized it, because it is not known if dolphins are more effective than common pets.
That dolphins had been trained by the Us military is not checked. there are only rumors about this. The United States Navy denies that dolphins were trained to do harm, they only are still being trained as a part of the Navy Marine Mammal Program.
I have found this information on the Net.