Dolphins invent their own names and call out to specific friends

Within its first few months of life, a bottlenose dolphin invents a unique signature whistle that functions like a personal name. Researchers found dolphins copy a companion's signature whistle specifically to address that individual, effectively calling out to them by name, and can recognize old associates' whistles decades after last hearing them.

— Wikipedia contributors, Signature whistle — Wikipedia; based on King & Janik, PNAS (2013)