Ever been at a party where you recognize everyone’s faces but can’t think of their names? That wouldn’t happen if you were a bottlenose dolphin (Tursiops truncatus). The marine mammals can remember each other’s signature contact whistles—calls that function as names—for more than 20 years, the longest social memory ever recorded for a nonhuman animal, according to a new study. 
 
 “The ability to remember individuals is thought to be extremely important to the ‘social brain,’ ” says Janet Mann, a marine mammal biologist at Georgetown University in Washington, D.C., who was not involved in the research. Yet, she notes, no one has succeeded in designing a test for this talent in the great apes—our closest kin—let alone in dolphins. 
 
 Dolphins use their signature whistles to stay in touch. Each has its own unique whistle, and they learn and can repeat the whistles of other dolphins. A dolphin will answer when another dolphin mimics its whistle—just as we reply when someone calls our name. 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. By whistling, they’re able to find each other again. Scientists don’t know how long dolphins are separated in the wild, but they do know the animals can live almost 50 years. So how long do the dolphins remember the calls of their friends? 
 
 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. The six sites belong to a consortium that rotates the marine mammals for breeding and has decades-long records of which dolphins have lived together. The dolphins ranged in age from 4 months to 47 years and included males and females. Some of the animals had spent as little as 3 months together; others had been housed with each other for as long as 18.5 years before being separated and sent to another facility; and some had been apart for 20.5 years. 
 
 At each facility, Bruck placed a submerged speaker in the dolphins’ pool and waited for one of the animals to swim past. He then played a recording of a whistle that the dolphin had never heard before. “They don’t pay much attention to signature whistles of dolphins they don’t know,” he says. But when he played the whistle of a dolphin they had once lived with, the animals often swam immediately to the speaker. “They will hover around it, whistle at it, seemingly try to get a response,” he says. 
 
 Bruck also played recordings of an unfamiliar dolphin that was the same age and sex as the familiar animal—but these also did not elicit much of a response. “It was a striking pattern,” Bruck says. “They were potentially bored by unfamiliar calls but responded to whistles from the animals they’d known,” even if they had not heard the whistles in decades. “It seemed to be stimulating to them. In Bermuda, a mother dolphin even brought her calf over to listen to the whistles of dolphins she’d known,” Bruck says. Sometimes the dolphins got upset, slapping their tails in protest, when Bruck removed the speaker from the pool; but they quickly settled down again after he put it back in the water. 
 
 In one case, Bruck played the whistle of Allie, a female dolphin at the Brookfield Zoo, for Bailey, a female in Bermuda. They had lived together at the Dolphin Connection in the Florida Keys when Allie was 4 and Bailey was 2. Twenty years and 6 months had passed—yet Bailey instantly recognized Allie’s whistle, Bruck says, as evidenced by her close attentiveness to the speaker. 
 
 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. For instance, two younger dolphins, Kai and D.J., became watchful and alert when they heard the whistles of Lucky and Hastings, two dominant males they had spent time with at the Brookfield Zoo. “Their whistles elicit a certain vigor and spirit in males that hear them,” says Bruck about the responses of Kai and D.J. “It looked as if those whistles put the image of those two dominant males in the heads of Kai and D.J.,” although he adds this has yet to be shown experimentally. 
 
 The study demonstrates the “long-term stability of the dolphins’ whistles,” Mann says. “Even though dolphins may change in size and physical characteristics—getting scars and speckles—their whistles provide a reliable means of identification.” And that in turn enables them to “track relationships and connections between individuals,” she says. 
 
 “We know they have relationships in the wild that last decades,” adds Richard Connor, an animal behaviorist at the University of Massachusetts, Dartmouth. “Remembering a particular individual—even in the absence of that individual—could help them navigate their current social milieu.” 
 
 Bruck’s study, however, did not test whether the dolphins mentally picture the correct dolphin when they hear his or her signature whistle. So far, scientists have only been able to demonstrate this ability in horses. Researchers from the University of Sussex in the United Kingdom videoed individual horses while a herd member was led past them and out of view. The scientists then played the whinny of that horse or of a different horse. If the whinny was from the horse that had just walked by, the watching horse continued doing whatever it had been doing before; but if the whinny came from a different stable-mate, the watcher instantly turned to look in the direction of the call, as if saying, “that didn’t sound like you.” 
 
 A similar experiment now needs to be done with dolphins, says Stephanie King, a biologist at the University of St. Andrews in the United Kingdom. She wonders if the animals are paying attention to the whistles of their former pool-pals because the sounds are familiar—or because they “evoke a mental representation of the absent animal in the dolphin’s mind.” In other words, does Kai mentally picture Lucky when he hears the dominant male’s brassy whistle erupting from the speaker? Or does he merely register, “That call takes me right back to Chicago.” Stay tuned—Bruck has a test in the works to find out. ||||| Image caption Kai was one of the dolphins in the study, he was 16 years old when this picture was taken 
 
 Forget about elephants - scientists say that dolphins have the longest memories yet found in a non-human species. 
 
 Researchers in the US say that even after 20 years of separation, dolphins could recall the whistles of former companions. 
 
 The authors believe that these long-term memories are a product of the complex social connections that dolphins have evolved. 
 
 The research is published in the Proceedings of the Royal Society B. 
 
 In the study, the scientists used information on the relationships between 56 captive bottlenose dolphins that have been moved for breeding purposes between six different zoos and aquariums in the US and Bermuda. 
 
 It's unprecedented in the study of animal behaviour to find memories this long Dr Jason Bruck, University of Chicago 
 
 The records, dating back decades, showed which of the dolphins had been housed together. 
 
 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. 
 
 "When they are familiar with the call, the dolphins are more likely to approach the speakers for longer periods of time," said Dr Jason Bruck from the University of Chicago who carried out the study. 
 
 "They will maintain contact with the speaker - if they are unfamiliar with the call they are more likely to ignore what I am playing. It's unprecedented in the study of animal behaviour to find memories this long." 
 
 Dr Bruck highlighted the case of two female dolphins called Allie and Bailey. They had once lived together in the Florida keys when they were very young. 
 
 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. 
 
 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. 
 
 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. 
 
 Nuclear family 
 
 The researchers believe that the complex nature of dolphin social systems is behind the long term memory effect. 
 
 In the ocean, dolphins have a fluid social arrangement that scientists term a "fission-fusion" model. They may leave one group and join others many times in their lifetimes. 
 
 Intelligent cetacean behaviour Dolphins taking part in an experiment had to press one of two levers to distinguish between sounds, some of which were very similar. By pressing a third lever, they were able to tell the researchers they wanted to "pass" on a particular test because it was too hard. "When you place dolphins in a situation like that they respond in exactly the same way humans do," said Dr Lori Marino. "They are accessing their own minds and thinking their own thoughts." 
 
 A number of captive dolphins were rewarded with fish in return for tidying up their tank. One of them ripped up a large paper bag, hid away the pieces, and presented them one at a time to get multiple rewards. 
 
 "It is important for them to recall the calls of dolphins they have had previous encounters with, to decide whether or not that's someone they want to approach when they hear that whistle about a mile out, or whether they want to avoid that individual," said Dr Bruck. 
 
 "Having a long term social recognition for that ecological reason can be the difference between an animal having a very negative social interaction and a positive one." 
 
 According to the researchers, a dolphin's abilities to recall events indicate that the cetaceans have a level of cognitive sophistication comparable to humans, chimpanzees and elephants. 
 
 While elephants are also reputed to have extremely long memories of up to 20 years, there is little scientific evidence of their abilities outside of family relationships. 
 
 In this research paper, the dolphins were able to remember family members as well as strangers. 
 
 In recent weeks, another study has shown that dolphins have their own signature whistles that appear to have the same function as names do for humans. 
 
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– 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."