Bottlenose dolphins can remember the whistles of old roommates — well technically "tank mates" — for up to 20 years, according to a new study in the Proceedings of the Royal Society of London B.

"This shows us an animal operating cognitively at a level that's very consistent with human social memory," said Dr.Jason Bruck, who conducted the study while earning his Ph.D. in Comparative Human Development from the University of Chicago.

Bruck's study relied upon the recent discovery that dolphins can whistle their own "names," or distinct signature that are recognized by others. Dolphins recorded or collected these whistle signatures from 53 different dolphins raised between six marine facilities, including Brookfield Zoo near Chicago, Dolphin Quest in Bermuda, and The Seas at Walt Disney World.

Over the course of several decades, some of the dolphins were rotated between these six sites so they could breed. Concise records were kept on which dolphins were housed together, which allowed Bruck to investigate whether they could remember former tank mates.

To test for social memory, Bruck started by playing a whistle he knew the dolphin had never heard before.

"Dolphins get bored quickly listening to signature whistles from dolphins they don't know", said Brock who followed this series of unfamiliar calls with a whistle from a former tank mate.

This signature whistle immediately caused the dolphins to perk up. This enlivened response was seven times as likely to happen with familiar whistles vs. unfamiliar ones.

"When they hear a dolphin they know, they often quickly approach the speaker playing the recording," said Bruck. "At times they will hover around, whistle at it, try to get it to whistle back."

I'll Remember You Forever

The pattern persisted across age groups, which ranged between four months to 47 years. The recognition even occurred between a pair of female dolphins — Allie and Bailey — who had been 2-year-old and 4-year-old pups, when Bailey moved from their home in Chicago to Bermuda.

At the time of the test, they had been separated for 20 years. The average lifespan for bottlenose dolphins is 20 years, although long-lived ones survive for up to 45. This means dolphins may remember former buddies for their entire lives.

Prior work has discovered that other mammals can remember non-relatives. Hyenas have a social memory that last about a year, while monkeys have longer lasting recollections for between 3-4 years. Elephant children can remember their mothers for nearly 20 years, but Bruck's research suggests dolphins have the longest kin-independent memory, second only to humans.

In the wild, dolphins grow up in packs, but the adolescent males often leave their original families to form bachelor pods of 2-3 males, according to Bruck. Being able to recognize old pals may help prevent dolphins from inbreeding.

"Why do they need this kind of memory? I'm not sure they do," Bruck said. "The cognitive abilities of dolphins are really well developed, and sometimes things like this are carry-along traits. But to test whether this kind of social memory capacity is adaptive, we would need more demographic data from multiple populations in the wild to see if they experience 20-year separations."

Source: Bruck JN. Decades-long social memory in bottlenose dolphins. Proceedings of the Royal Society of London B. 2013.