Dolphins\' social memories can last a lifetime and are the longest in any non-human species, a researcher at the University of Chicago says. Dolphins in captivity can recognize the whistles of old tank companions after being separated for more than 20 years, a remarkable memory feat suggesting dolphins have a level of cognitive sophistication comparable to only a few other species, including humans, chimpanzees and elephants, a university release said Tuesday. Their social memory may be even more long-lasting than facial recognition among humans, the researchers said, since human faces change over time but the signature whistle that identifies a particular dolphin remains stable over many decades. \"This shows us an animal operating cognitively at a level that\'s very consistent with human social memory,\" researcher Jason Bruck said. His study, conducted while receiving his doctorate in the university\'s program in Comparative Human Development, has been published in the Proceedings of the Royal Society London B. Bruck collected data from 53 different bottlenose dolphins at six facilities that 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 when you know how long the animals have been apart,\" Bruck said. \"To do a similar study in the wild would be almost impossible.\" In the study, playback of familiar calls often would elicit an immediate response in the dolphins, Bruck said. \"When they hear a dolphin they know, they often quickly approach the speaker playing the recording,\" he said. \"At times they will hover around, whistle at it, try to get it to whistle back.\"