Scientists have researched the whistles of dolphins for decades. They've long speculated that the complex series of whistles and clicks have specific meanings, but are only just beginning to realize that dolphins are able to call out to each other by name.
Vincent Janik from the University of St. Andrews in Scotland has spent years tracking dolphins with underwater microphones. He's made recordings of the dolphin pods that swim around in St. Andrews Bay and has surmised that all those squeaks and chirps that are unintelligible to human ears are actually used to differentiate one dolphin from another. Janik discovered that dolphins actually name themselves when they're babies, unlike humans, who need to be taught their names when first learning to speak. Once they've come up with a name, they introduce themselves to other members of their pod, who remember the name and then refer to them as such.
Not only does each dolphin have its own signature whistled name, there are also "words" for different types of interactions. A sequence of whistles may indicate that a dolphin is introducing itself to another dolphin in a friendly way; the marine equivalent to shaking hands. These types of communications are important when one pod of dolphins runs into another unfamiliar pod. They're a way of preemptively keeping the peace and avoiding West Side Story-style dolphin showdowns. Certain types of call-and-response whistles can also indicate that it's okay for two pods to swim together, or bro down, for a while.
It's not yet known just how complicated dolphin language can get. Researchers have observed that dolphins use certain whistles to refer to certain objects while in captivity. It's entirely possible that they have words for objects, places, and maybe even concepts in addition to having individual names. Dolphins are among the only animals to make up their own specific sounds instead of just relying on their voice tones to recognize each other. These marine mammals may be more linguistically creative than we had previously thought.