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It's no secret humans have an abiding fascination with dolphins. These marine mammals are cute and friendly. They're also smart and curious and have remarkably complex social lives. Well, a new study gives us one more reason to marvel at dolphins. As NPR's Rhitu Chatterjee reports, they have the longest known memory in animals.

RHITU CHATTERJEE, BYLINE: Scientists have known that dolphins recognize each other by the sound of their signature whistles.

JASON BRUCK: Those individually specific whistles that each dolphin generates before their first birthday, that, for them, functions like a name.

CHATTERJEE: That's Jason Bruck. He studies animal behavior at the Institute for Mind and Biology at the University of Chicago. He says if a dolphin wants to announce itself to other dolphins, it will let out its signature whistle. And dolphins who are acquainted with each other learn each other's whistle, just like we learn the names of other people in our lives. The whistles sound like this.

(SOUNDBITE OF WHISTLE)

CHATTERJEE: Or like this.

(SOUNDBITE OF WHISTLE)

CHATTERJEE: Bruck wanted to know if one dolphin could recognize the whistle of a former friend long after they'd last been in touch. To do that, he used dolphins housed in research facilities across the U.S. He says these dolphins get moved around a lot. When they're in one place, they make friends with other dolphins in the tank. When they're moved, they're separated from their former tank mates.

BRUCK: I knew exactly who was housed with who and for how long.

CHATTERJEE: So he collected the whistles of dolphins in different facilities and played them to animals that they were once familiar with. To his surprise, dolphins did recognize the call of a long lost friend, even if they hadn't seen that friend for years. And Bruck could tell this because when the dolphins heard a familiar whistle, they would swim eagerly towards the speaker and hover around to investigate.

BRUCK: Sometimes they'll whistle back, and the dolphin hears a familiar signature whistle, they're more likely to give their own signature whistle back.

CHATTERJEE: He says the most striking example was of two dolphins named Bailey and Allie.

BRUCK: Bailey and Allie were together at a facility in Florida where Bailey was 2 and Allie was 4.

CHATTERJEE: More than 20 years later, Bailey could still recognize Allie's whistle. Bruck says he was humbled by what he saw.

BRUCK: I looked and just said, I can't do this. I know I can't do this.

CHATTERJEE: He's published his findings in the latest issue of the British journal Proceedings of the Royal Society B. Janet Mann of Georgetown University has studied dolphin behavior for decades. She says the new findings are a big deal.

JANET MANN: It's one of the first to really demonstrate this for a long-lived socially complex mammal.

CHATTERJEE: Mann says the findings make sense given the social lives of dolphins. Each dolphin meets and gets to know hundreds of other dolphins over the course of its life. And like humans, they have strong relationships with families, friends and enemies.

MANN: It's obvious when you're watching dolphins in the wild, when you know them and you know who their associates are and who they're kind of friends with and not so friends with, that you see how much it means to them, certainly these social bonds, and how important they are in their lives.

CHATTERJEE: She says it's possible that other animals with complex social lives like elephants, chimpanzees, dogs and even parrots also have good memories. But scientists have yet to find a way to test it in those species. Rhitu Chatterjee, NPR News.

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