The calls enable the marine mammals to communicate over long distances—which is necessary because they live in “fission-fusion” societies, meaning that dolphins in one group split off to join other groups and later return. In this research paper, the dolphins were able to remember family members as well as strangers. To find out, Jason Bruck, a cognitive ethologist at the University of Chicago in Illinois, spent 5 years collecting 71 whistles from 43 dolphins at six captive facilities, including Brookfield Zoo near Chicago and Dolphin Quest in Bermuda. Total recall 
 
 The researchers then played recordings to the dolphins on underwater speakers of the signature whistles of animals they had once lived with, and measured their responses. To check that the dolphin's response was due to recognition of a former companion, Dr Bruck would also play a test recording of an unfamiliar bottlenose that was the same age and gender as the familiar animal. Dr Bruck says this type of response was typical. Compared to unfamiliar calls, there was a clear pattern in the data where dolphins responded significantly more to whistles from animals they had once known, even if they hadn't seen or heard them in decades. In one case, Bruck played the whistle of Allie, a female dolphin at the Brookfield Zoo, for Bailey, a female in Bermuda. Image caption The scientists say that the complicated nature of their social groupings impact their ability to recall whistles 
 
 Bailey now lives in Bermuda but when a recording of Allie was played, she instantly responded in even though it was was 20 years and six months since they had been in contact. The dolphins often responded as if they were picturing their long-ago social pals, Bruck reports online today in the Proceedings of the Royal Society B.
– Dolphins not only have names for each other, their memories are so good that they can remember the "signature whistles" of friends—and enemies—for at least 20 years, according to a new study. Researchers studied scores of captive bottlenose dolphins that had been shifted around the US and found that they responded much more readily to the sounds of dolphins they had once known—including family members, ex-tankmates, and former mates—than the calls of strangers, the BBC reports. The dolphins' "social memory" is the longest ever recorded in the animal kingdom. Researchers believe the ability to recognize familiar whistles, which stay the same while age changes their outward appearance, helps dolphins live in "fission-fusion" societies, where they join and leave different groups many times over a lifetime. "We know they have relationships in the wild that last decades," an animal behaviorist tells Science. "Remembering a particular individual—even in the absence of that individual—could help them navigate their current social milieu."