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 author Dr Jason Bruck, University of Chicago.
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. The six sites were part of a breeding consortium that has rotated dolphins and kept records of which ones lived together.
“This is the kind of study you can only do with captive groups where you know how long the animals have been apart,” Bruck said, “to do a similar study in the wild would be almost impossible.”
In recent years, other studies have established that each dolphin develops its own unique signature whistle that appears to function as a name. Researchers Vincent M. Janik and Stephanie L. King at the University of St. Andrews reported earlier this year, also in Proceedings of the Royal Society B, that a wild bottlenose dolphin can learn and repeat signatures belonging to other individuals, and answer when another dolphin mimics its unique call.
Bruck played recordings of signature whistles to dolphins that had once lived with the animals that made the calls. 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,” Bruck said. “At times they will hover around, whistle at it, try to get it to whistle back.”
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. All the behaviour was scored according to how quickly and to what degree the animals responded.
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.
Exactly why dolphins’ social memories persist so long remains unclear. In the open ocean, dolphins may break apart from one group and join 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.