Sorry, elephants: Dolphins have taken the top spot for best memory, at least for now.

New experiments show that bottlenose dolphins can remember whistles of other dolphins they'd lived with after 20 years of separation. Each dolphin has a unique whistle that functions like a name, allowing the marine mammals to keep close social bonds.

The new research shows that dolphins have the longest memory yet known in any species other than people. Elephants and chimpanzees are thought to have similar abilities, but they haven't yet been tested, said study author Jason Bruck, an animal behaviorist at the University of Chicago. (Also see "Chimps, Orangutans Have Human-Like Memory.")

Bruck came up with the idea to study animal memory when his brother's dog, usually wary of male strangers, remembered and greeted him four years after last seeing him. "That got me thinking: How long do other animals remember each other?"

I Remember You!

Bruck studied dolphins because their social bonds are extremely important and because there are good records for captive dolphins (as opposed to wild ones).

So he collected data from 43 bottlenose dolphins at six facilities in the U.S. and Bermuda, members of a breeding consortium that has swapped dolphins for decades and kept careful records of each animal's social partners. (Watch video: "Dolphin Talk Decoded.")

He first played recordings of lots of unfamiliar whistles to the dolphins in the study until the subjects got bored and stopped inspecting the underwater speaker making the sounds.

At this point, he played the whistles of the listening dolphins' old friends.

When the dolphins heard these familiar whistles, they would perk up and approach the speakers, often whistling their own name and listening for a response.

Overall, the dolphins responded more to animals they'd known decades ago than to random animals—suggesting they recognized their former companions, according to the study, published recently in Proceedings of the Royal Society B.

Cheeky Dolphins

Working with animals as intelligent as dolphins was a challenge, Bruck added. The animals loved participating in the experiment so much that they'd often hover over the speaker, blocking the noise. (See "The Secret Language of Dolphins.")

Others would begin "whistling directly at me as if I could understand them," he said.

And one set of cheeky young dolphins swam up to Bruck and started whistling the names of the dominant males in their group in order of rank, perhaps suggesting the names they wanted to hear, Bruck said.

Memory Linked to Smarts?

Why dolphins—which live an average of 20 years in the wild—need long-term memory is still unknown. But it may have to do with maintaining relationships, since over time dolphin groups often break up and reorganize into new alliances.

This sort of social system is called "fission-fusion," and it's also seen in elephants and chimpanzees—two other highly intelligent and social beings. (See pictures of intelligent animals in National Geographic magazine.)