Dolphins can recognize their old tank mates’ whistles after being separated for more than 20 years—the longest social memory ever recorded for a non-human species.

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. Dolphins’ talent for social recognition may be even more long-lasting than facial recognition among humans, since human faces change over time, but the signature whistle that identifies a dolphin remains stable over many decades.

“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 and received his PhD in June 2013 from the University of Chicago’s Department of Comparative Human Development. His study is published in the current issue of the Proceedings of the Royal Society B.

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 on which ones lived together, going back decades.

“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.”

‘Signature whistles’ offer means to test memory

In recent years, other studies have established that each dolphin develops a unique signature whistle that appears to function as a name. Researchers Vincent M. Janik and Stephanie L. King at Scotland’s University of St. Andrews reported earlier this year that a wild bottlenose dolphin can learn and repeat signatures belonging to other individuals, and answer when another dolphin mimics its unique call.