University of Chicago researchers found that dolphins can recognize their old tank mates' whistles even after being separated for more than 20 years. (Agencies)
The remarkable memory feat is another indication that dolphins have a level of cognitive sophistication comparable to only a few other species, including humans, chimpanzees and elephants.
"This shows us an animal operating cognitively at a level that's very consistent with human social memory," said Jason Bruck, who conducted the study.
To establish how well dolphins could remember their former companions, Bruck collected data from 53 different bottlenose dolphins at six facilities, including Brookfield Zoo near Chicago and Dolphin Quest in Bermuda.
Bruck played recordings of signature whistles to dolphins that had once lived with the animals that made the calls.
First, Bruck would play recording after recording of signature whistles that the target dolphins had never heard before. His initial studies showed that these "dolphins get bored quickly listening to signature whistles from dolphins they don't know."
Once they were habituated to the unfamiliar calls, Bruck would play a recording of an animal with which the target dolphin had lived.
The familiar calls often would perk up the dolphins and elicit an immediate response.
"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, and try to get it to whistle back, "Bruck said.
To check that the response was the result of recognition, Bruck also would play a test recording of an unfamiliar bottlenose that was the same age and sex as the familiar animal.
A clear pattern emerged in the data: Compared with unfamiliar calls, dolphins responded significantly more to whistles from animals they once knew, even if they had not heard the calls in decades.
Researchers could not determine why dolphins' social memories persist so long. Dolphins exhibit sophisticated social connections that follow a 'fission-fusion' model. In the open ocean, dolphins may break apart from one group and 'fuse' with other groups many times over.
Such relationships could have required a growth in memory capacity. But it's also possible that memory is just one facet of the advanced mind that evolved in dolphins for other reasons.
The study was published in the journal Proceedings of the Royal Society of London B.
University of Chicago researchers found that dolphins can recognize their old tank mates' whistles even after being separated for more than 20 years.