Dolphins can recognise the voice of an old friend even after being separated for 20 years, a study has shown.
They say elephants never forget, but dolphin social memory is said to be the longest of any non-human species. The creatures may even be better at remembering each other's whistles than humans are at remembering faces, US researchers claim.
Whereas human faces age and change over time, the signature whistle that identifies a dolphin remains unaltered for many decades. "This shows us an animal operating cognitively at a level that's very consistent with human social memory," said Dr Jason Bruck, from the University of Chicago, who led the research reported in the journal Proceedings of the Royal Society B.
Previous studies have shown that early in life every dolphin develops its own name-like signature whistle. Dolphins can learn and repeat signature whistles belonging to other individuals, and answer when they hear their "names" being called.
Dr Bruck's team studied 53 captive bottlenose dolphins at six different facilities that have swapped animals and kept records going back decades as part of a breeding programme.
The scientists played recordings of signature whistles to dolphins that had once lived with the animals making the calls. By observing the dolphins' reactions, it was possible to tell when they were listening to a whistle they remembered or one that was unfamiliar.
"Dolphins get bored quickly listening to signature whistles from dolphins they don't know," said Dr Bruck. "When they hear a dolphin they know, they often quickly approach the speaker playing the recording. At times they will hover around, whistle at it, try to get it to whistle back."
To make sure these were cases of true recognition, Dr Bruck would play a test recording of a stranger dolphin that was the same age and sex as the familiar animal. The data showed a clear pattern. Dolphins responded significantly more to whistles from animals they once knew, even if they had not heard them for decades.
One notable example involved two female dolphins called Allie and Bailey, who once lived together at Dolphin Connection in the Florida Keys. Twenty years and six months after their last contact, Bailey, now living in Bermuda, recognised the recording of Allie's signature whistle.
The research suggests bottlenose dolphins have a lifelong social memory, since the animals' average life expectancy in the wild is around 20 years.