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Dolphins, which are one of the smartest animals alive, constantly amaze researchers. Their brains are the second most complex brains in the world right after humans and it is this brain that gives these creatures its intelligence. Several studies have researched into the dolphins' brains with the hopes of understanding how this organ contributes to certain mannerisms that dolphins have. In a new study, researchers discovered that dolphins have the ability to hold on to lifelong memories for decades, the longest in a non-human species.

"This shows us an animal operating cognitively at a level that's very consistent with human social memory," the lead investigator of the study, Jason Bruck, Ph.D. said according to a press release. Bruck received his Ph.D. from the University of Chicago's program in Comparative Human Development.

For this study, Bruck gathered data from 53 unique bottlenose dolphins that lived in six facilities, two of which were the Brookfield Zoo near Chicago and Dolphin Quest in Bermuda. All six facilities were a part of a breeding consortium, which was responsible for moving dolphins around while keeping track of which dolphins lived together at some point over the past decades. Based from this set of data, Bruck discovered that after more than two decades, dolphins were able to recognize the whistles of dolphins that they have lived with in the past.

Bruck was able to discover the dolphins' ability to maintain lifelong memories by analyzing how dolphins reacted to certain calls from other dolphins. Based on research done by Vincent M. Janik and Stephanie L. King from Scotland's University of St. Andrews, researchers now know that each dolphin has a unique whistle that appears to function as a personal name. Having this information, Bruck played recordings of mating calls that occurred over decades ago. Dolphins will react differently to familiar and unfamiliar calls. The researchers noted that dolphins that heard calls from other dolphins they used to live with became more alert.

"When they hear a dolphin they know, 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."

One particular case that the Bruck studied was between two female dolphins, Allie and Bailey. These two dolphins lived together at Florida Keys' Dolphin Connection when Allie was two and Bailey was four. Down the line, Allie was moved to the Brookfield Zoo while Bailey was relocated to Bermuda. Bruck played Allie's whistle to Bailey and after being a part from one another for 20 years and six months, Bailey was still able to recognize Allies' signature call.

When it came to unfamiliar calls, the dolphins were less interested and appeared to get bored with the whistles that came out of the speakers. Although this finding was interesting, the researchers are unsure as to how memory might play a role for dolphins. They believe that the ability to hold on to memory does not have to do with survival, but rather, with the fact that their brains are so complex that this particular trait might just be a carry-along trait. However, more research can be done to determine the role of memory for dolphins. Bruck acknowledges the fact that his finding is limited to dolphins in captivity and that conducting "a similar study in the wild would be almost impossible."

The study was published in the Proceedings of the Royal Society of London B.