Bottlenose dolphins can remember each other's signature whistles for more than 20 years, a study said Wednesday -- the longest social memory ever observed in an animal.

Elephants have long been credited with the animal kingdom's most prodigious memory, but evidence for that was anecdotal, said the study's author Jason Bruck of the University of Chicago's Institute for Mind and Biology.

Bruck claims to have compiled the first study showing social recognition in an animal persisting beyond two decades -- possibly "the longest pure memory of any kind in a non-human species".

For the study published in the Proceedings of the Royal Society B: Biological Sciences, Bruck worked with 43 captive dolphins at six zoos and aquariums in the United States and Bermuda, collecting whistle recordings he played to them over speakers underwater.

Previous research had shown that dolphins design unique signature whistles to identify themselves, and answer when called by "name".

They use these whistles like we identify people by their faces.

Bruck played dolphins the recordings of signature whistles from those they had once shared a tank with, as well as the calls of complete strangers.

"Dolphins got bored quickly listening to signature whistles from dolphins they don't know," he said in a statement.

But familiar calls often elicited an immediate response.

"When they hear a dolphin they know, they often quickly approach the speaker playing the recording," said Bruck. "At times they will hover around, whistle at it, try to get it to whistle back."

In one case, Bruck played the whistle of a female dolphin named Allie, who lives at Brookfield Zoo in the US, for Bailey, a female now in Bermuda. The pair had last lived together in the Florida Keys when Allie was two and Bailey four -- but Bailey clearly recognised the call.

The reaction was not affected by how long the animals had lived together, nor their sex or kinship status, the biologist said.

"I was expecting five, maybe 10 years. Memory for over 20 years just has not been systematically shown in animals so I was amazed by what I found," he told AFP.

In the wild, bottlenose dolphins have an average life expectancy of about 20 years, though some can survive for 45 years or longer.

Bruck said the study revealed a social memory "very consistent" with that of humans.

It may even be more long-lasting than facial recognition abilities by humans whose faces change over time while dolphin whistles remain the same.

Long-term social memory may help animals assess threats and opportunities for hunting alliances while preventing inbreeding.