Dolphins are able to remember one another's signature calls for at least 20 years making it the longest memory for "faces" among animals - perhaps even surpassing the ability of people to remember one another from their appearances alone.

Every dolphin has a unique whistle which is used as a signature call and stays with them unchanged throughout life. These identifying noises are even more reliable than the facial features used by people to recognise each other, which notoriously change over time.

Now a study has shown that when dolphins have been separated for 20 years or more they are still able to distinguish the whistle call of a former close companion from a host of other calls emitted by complete strangers, scientists said.

This kind of "social memory" surpasses the recognition abilities of elephants, primates and all other intelligent animals that have been studied in this way, said Jason Bruck, who carried out the work whilst studying for is PhD at the University of Chicago.

"This research shows that dolphins have the potential for lifelong memory for each other regardless of relatedness, sex or duration of association. This is the first study to show that social recognition can last for at least 20 years in a nonhuman species," Dr Bruck said.

"This shows us an animal operating cognitively at a level that's very consistent with human social memory. This is the kind of study you can only do with captive groups where you know how long the animals have been apart. To do a similar study in the wild could be almost impossible," he said.

The study, published in the journal Proceedings of the Royal Society of London B, involved collecting the recorded signature whistles of 53 different bottlenose dolphins living at six different facilities, from the Brookfield Zoo near Chicago to Dolphin Quest in Bermuda.

The dolphins at these sites are part of a captive-breeding programme and they have periodically spent time together going back many years, complete with fully-verified records.

During the study, Dr Bruck would play a recording of signature whistles of a stranger that a dolphin had never heard before. After repeating the call many times, the dolphin would quickly show signs that it is bored with the sound of another dolphin it had never met, he said.

Dr Bruck then introduced the call of a dolphin they had once known, and measured its response and behaviour. "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," Dr Bruck said.

In one notable example, for instance, a female named Allie living at Brookfield Zoo recognised and responded to a recorded call of female called Bailey, now living in Bermuda, which had shared a pool with Allie 20 years and six months previously when they both lived at Dolphin Connection in Florida Keys, he said.

Dolphins are thought to live for about 20 years on average in the wild, although they are known to live as long as 45 years in captivity. It is possible that they social memory for signature whistles could last their lifetime, Dr Bruck said.

This is the longest, scientifically-tested social memory in animals, matched only by the anecdotal reports of elephants being able to recognise their mothers after 20 years of separation.

A study published earlier this year by researchers at the University of St Andrews found that dolphins not only recognised each other by their whistles but often mimic a close companion's unique whistle in an attempt to get them to respond.