Dolphins can recognise the voice of an old friend.
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.
Dolphins’ ability to recognise old acquaintances appears to supercede that of any other non-human species, said the scientists.
There have been anecdotal reports of elephants remembering a mother after 20 years, but no clear evidence of similar feats of recognition outside family relationships.
In the open ocean, dolphins may part company with one group and join other groups many times over. Such a complex social life may have driven the development of dolphin memory, the scientists believe. On the other hand, it could simply be the result of having a highly evolved brain.
“Why do they need this kind of memory? I’m not sure they do,” said Dr Bruck.
“The cognitive abilities of dolphins are really well developed, and sometimes things like this are carry-along traits. But to test whether this kind of social memory capacity is adaptive, we would need more demographic data from multiple populations in the wild to see if they experience 20-year separations.”
Another question the scientists would like answered is how similar dolphin signature calls are to human words and names.
No-one has yet been able to test how signature whistles are represented in a dolphin’s mind.
“We know they use these signatures like names, but we don’t know if the name stands for something in their minds the way a person’s name does for us,” said Dr Bruck said. “We don’t know yet if the name makes a dolphin picture another dolphin in its head.”
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