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Scientists have known for years that dolphins recognize each other by the sound of each animal's signature whistle. But it wasn't known for just how long dolphins could remember these whistle calls.
The individually specific whistle that each dolphin generates before its first birthday "for them functions like a name," says Jason Bruck, who studies animal behavior at the Institute for Mind and Biology at the University of Chicago.
Bruck says that if a dolphin wants to announce itself to other dolphins, it will let out its signature whistle. And dolphins that are acquainted with each other learn the other's whistle, just like we learn the names of other people in our lives.
The whistles sound like this:
Bruck wanted to see 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. These dolphins get moved around a lot, but when they're in one place, they make friends with their tank mates.
Bruck collected the whistles of dolphins in different facilities and played them to other dolphins 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.
He could tell this because when the dolphins heard a familiar whistle, which was played back from an underwater speaker, they would swim eagerly toward the speaker and hover around to investigate.
"Sometimes they'll whistle back," he says. "Oftentimes when a dolphin hears a signature whistle, they're more likely to give their own signature whistle back."
He says the most striking example was of two dolphins named Bailey and Allie.
"Bailey and Allie were together at a facility in Florida, where Bailey was 2 and Allie was 4," Bruck says. More than 20 years later, Bailey could still recognize Allie's whistle. Bruck says he was humbled by what he saw.
"I looked and said, 'I can't do this, I know I can't do this,' " he says. He published his findings in the latest issue of the British journal Proceedings of the Royal Society B.
Janet Mann, a professor of biology at Georgetown University who has studied dolphin behavior for decades, says the new findings are a big deal.
"It's one of the first studies to show this in a long-lived, socially complex mammal," she says, adding that 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.
"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 — these social bonds — and how important they are in their lives," Mann says.
She says it's possible that other animals with complex social lives, like elephants, chimpanzees, dogs and even parrots, may also have good memories. But scientists haven't yet found a way to test it in those species.