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Dolphins found to remember their friends at least 20 years

Aug. 7, 2013
Courtesy of The Royal Society
and World Science staff

Dol­phins can rec­og­nize their old friends’ whis­tles af­ter be­ing sep­a­rat­ed for more than 20 years, the longest so­cial mem­o­ry ev­er recorded for a non-hu­man spe­cies, ac­cord­ing to a stu­dy.

The feat is an­oth­er sign that dol­phins have cog­ni­tive so­phis­tica­t­ion com­pa­ra­ble to only a few oth­er spe­cies, in­clud­ing hu­mans, chim­panzees and ele­phants, bi­ol­o­gists say.

© Bas Kers (NL)


“This shows us an an­i­mal op­er­at­ing cog­ni­tively at a lev­el that’s very con­sist­ent with hu­man so­cial mem­o­ry,” said Ja­son Bruck of Uni­vers­ity of Chi­ca­go, au­thor of the study pub­lished Aug. 7 in the jour­nal Pro­ceed­ings of the Roy­al So­ci­e­ty B.

Bruck col­lect­ed da­ta from 53 dif­fer­ent bot­tle­nose dol­phins at six facil­i­ties, in­clud­ing Brook­field Zoo near Chi­ca­go and Dol­phin Quest in Ber­mu­da. The sites were part of a breed­ing con­sor­ti­um that has ro­tated dol­phins and kept records of which ones lived to­geth­er.

Oth­er re­cent stud­ies have found that each dol­phin de­vel­ops a un­ique sig­na­ture whis­tle that seems to func­tion as a name. Re­search­ers Vin­cent M. Janik and Steph­a­nie L. King at the Uni­vers­ity of St. An­drews, U.K., re­ported ear­li­er this year, in the same jour­nal, that a wild bot­tle­nose dol­phin can learn and re­peat sig­na­tures of its peers, and an­swer when an­oth­er dol­phin mim­ics its own.

Bruck played record­ings of sig­na­ture whis­tles to dol­phins that had once lived with those that made the calls. The fa­mil­iar calls of­ten would perk up the dol­phins and elic­it an im­me­di­ate re­sponse, he said. “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 of the same age and sex as the fa­mil­iar an­i­mal. Dol­phins re­sponded sig­nif­i­cantly more to whis­tles from an­i­mals they once knew, even if they had not heard the calls in dec­ades, the study found.

Just why these mem­o­ries per­sist so long re­mains un­clear. In the open ocean, dol­phins may break apart from one group and join with oth­er groups many times, Bruck said. Such rela­t­ion­ships could have re­quired a growth in mem­o­ry, he added, but it’s al­so pos­si­ble that mem­o­ry is just one fac­et of the ad­vanced mind that evolved in dol­phins for oth­er rea­sons.


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Dolphins can recognize their old friends’ whistles after being separated for more than 20 years, the longest social memory ever recorded for a non-human species, according to a study. The feat is another sign that dolphins have cognitive sophistication comparable to only a few other species, including humans, chimpanzees and elephants, biologists say. “This shows us an animal operating cognitively at a level that’s very consistent with human social memory,” said Jason Bruck of University of Chicago, author of the study published Aug. 7 in the journal Proceedings of the Royal Society B Bruck collected data from 53 different bottlenose dolphins at six facilities, including Brookfield Zoo near Chicago and Dolphin Quest in Bermuda. The sites were part of a breeding consortium that has rotated dolphins and kept records of which ones lived together. Other recent studies have found that each dolphin develops a unique signature whistle that seems to function as a name. Researchers Vincent M. Janik and Stephanie L. King at the University of St. Andrews, U.K., reported earlier this year, in the same journal, that a wild bottlenose dolphin can learn and repeat signatures of its peers, and answer when another dolphin mimics its own. Bruck played recordings of signature whistles to dolphins that had once lived with those that made the calls. The familiar calls often would perk up the dolphins and elicit an immediate response, he said. “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 of the same age and sex as the familiar animal. Dolphins responded significantly more to whistles from animals they once knew, even if they had not heard the calls in decades, the study found. Just why these memories persist so long remains unclear. In the open ocean, dolphins may break apart from one group and join with other groups many times, Bruck said. Such relationships could have required a growth in memory, he added, but it’s also possible that memory is just one facet of the advanced mind that evolved in dolphins for other reasons.