It turns out that there’s strong evidence to suggest that at least one kind of dolphin sound, studied extensively over the past decade, does function as a kind of referential symbol. Dolphins use distinct “signature whistles” to identify and call to one another. Each dolphin is thought to invent a unique name for itself as a calf and to keep it for life. Dolphins greet one another at sea by exchanging signature whistles and seem to remember the signature whistles of other dolphins for decades. Though other species, like vervet monkeys and prairie dogs, make sounds that refer to predators, no other animal, besides humans, is believed to have specific labels for individuals.