If You Insulted a Dolphin 20 Years Ago, He's Probably Still Bitter About It

A bottlenose has exhibited "the most durable social memory ever recorded for a non-human."

The guy on the right? He's cool. But do not mess with the dude on the left. (eZeePics Studio/Shutterstock)

Say that, in 1993, you were at a bar having some beers with a dolphin. Say that the dolphin said, "I've got the tab," and you said, "Okay, thanks -- I've got the next one." Say that you proceeded to forget about that promise, and that -- not because you didn't like the dolphin, or anything, just because life got in the way -- you never saw the dolphin again. Now say that, this morning, you ran into the dolphin, at the bank or in a tank or in the pool of some terrible resort in the Bahamas.

Here's the awkward thing: The dolphin may well remember you. And maybe the debt, too.

Yep. Dolphins, it turns out, have the longest social memories of any species besides humans. And we're learning more and more about how lengthy those memories can actually be. The researcher Jason Bruck, a biologist at the University of Chicago, wanted to test whether bottlenose dolphins in particular can, indeed, remember each other after a long stretch of separation.

So he took advantage of something else about dolphins: the fact that they seem to have something like names. Sometime between their first 4 months and their first year of life, dolphins will develop a distinct whistle -- one that will remain the same for the rest of its life. According to research published last month in the Proceedings of the National Academy of Sciences, dolphins use these whistles in pretty much the same way we humans use names: as ways both to identify themselves and to call each other.   

Bruck used that to test dolphins' memories. He studied dolphins from six breeding facilities that rotate the animals among themselves -- and, in the process, keep detailed records of which dolphins shared tanks, and when. There were more than 50 animals in all. Bruck first recorded each dolphin's unique whistle. And then he played it back to the dolphins' former tankmates using underwater speakers. (As controls, Bruck both habituated the dolphins to unfamiliar whistles -- thus eliminating the possibility that the animals were simply responding to the novelty of speaker-based sound signatures -- and alternated those whistles with the calls of unfamiliar dolphins.) The point? To test whether the dolphins would recognize each others' whistles, even after years-long stretches of separation.

The findings? The dolphins did, indeed, seem to remember each other. The whistles of the dolphins' former tankmates resulted in the dolphins doing things like purposely bumping into a speaker, or (even more sadly) whistling at it -- "trying," Nature notes, "to make it whistle back."

And that recognition behavior held, even more interestingly, no matter how long a pair had been separated. A dolphin named Bailey had been living apart from a former tankmate, Allie, for more than 20 years. Yet Bailey, in Bruck's experiment, seemed to recognize Allie's "name" -- her signature whistle -- despite the temporal distance. Bailey's recognition of Allie, Nature puts it, represents "the most durable social memory ever recorded for a non-human."

Bruck just published the results of his research in the Proceedings of the Royal Society B. And while it's unclear what his findings might mean about dolphins' memories overall -- he was testing name recognition, not circumstantial or emotional memories -- there's some reason to think that dolphins' memories stretch beyond rote recognition itself. In tests that broadcast the signature whistles of "extremely dominant males," for example, Bruck found that females responded with "exceptional interest." "There was also a lot of posturing from the males," Bruck noted. And "some young ones would just go ballistic."

In other words, dolphins may well have the capacity for relatively complex memories -- memories that associate individuals with actions. Memories that can last for decades. So, should you find yourself doing dealings with a dolphin, try your best to stay on its good side. Twenty years is a long time.