Did you know dolphins can recognize each other in almost the same way we humans can recognize each others voice? However, instead of using voices, they use whistles.
Dolphins are social mammals that have evolved to live in an aquatic environment. Like bats, they can use sonar to locate prey by emitting different frequencies of broadband sounds called clicks. But it wasn't until the 1990s that scientists discovered that they have another kind of sound that they use for communication. That sound is a pure narrow band tone that's frequency is modulated up and down. We call it a whistle.
Dolphins live in groups, usually mothers with calves or adult males with each other. Individuals in these groups help each other with hunting, scouting for food sources, or protecting each other. Since ocean water is difficult to see through, they have adopted whistles as a way to keep in touch with each other. But these whistles are not just a way to greet each other. Each individual dolphin has its own signature whistle, like I have a voice that identifies me. When individuals get separated from each other, they will whistle to let other dolphins know not only where they are, but who they are.
If you didn't know it already, you do now; dolphins are amazing animals. Like humans, they have large brains for their body size. We are just starting to understand how intelligent they really are.