Last week, we looked at evidence that bottlenose dolphins use distinctive whistles to identify themselves, suggesting that these creatures, among the smartest in the animal kingdom, use the noises in a way that’s roughly analogous to our use of names to identify people.

Now, a separate study confirms dolphins’ ability to recognize these “names”—and indicates that they’re able to remember them over time far longer than we imagined. In tests of 43 dolphins kept in captivity around the United States, Jason Bruck of the University of Chicago found that the animals reacted differently upon hearing whistles that belonged to dolphins they’d shared tanks with up to 20 years previously, as compared with those of dolphins they’d never met.

The findings, published today in the Proceedings of the Royal Society B, could force us to reconsider what we imagine animals are capable of remembering over time. If they hold up, they’d represent the longest-held memories we’ve seen thus far in any non-human animals.

The basics: High-pitched whistles, or “chirps,” are made by bottlenose dolphins in friendly social settings. Acoustic analysis has shown that the whistles differ slightly from individual to individual, and that the whistle a particular dolphin makes is consistent over time.

Previous studies with this dolphin species have found that the animals are more likely to move towards a speaker emitting the whistle of of a relative than a random bottlenose and that mothers frequently emit the whistles of their calves when separated from them, suggesting that they’re calling out their names in hopes of finding them.

For this project, Bruck sought to test the animals’ ability to distinguish between whistles of dolphins they’d previously shared tanks with from those of others they’d never met. Relying on records kept by a consortium of six different aquatic facilities that frequently rotate dolphins for breeding purposes (the Brookfield Zoo in Chicago, the Indianapolis Zoo, the Minnesota Zoo, Dolphin Quest: Bermuda, the Texas State Aquarium and The Seas at Walt Disney World), he was able to sort out which of the 43 dolphins included in the study had previously lived together, and which had never met.

To test their memories, he used an underwater speaker to repeatedly play various dolphins’ recorded whistles and then observed their responses, specifically noting whether they largely ignored the noise, turned their head towards the speaker, swam towards it, or even made forceful contact with the gate protecting the acoustic equipment.

When they dolphins heard unfamiliar whistles, they tended to get bored quickly, showing little response. On the other hand, their reactions upon hearing whistles from animals they’d previously lived with were notably different. “When they hear a dolphin they know, they often quickly approach the speaker playing the recording,” Bruck said in a press statement. “At times they will hover around, whistle at it, try to get it to whistle back.” This held regardless of the age or sex of the animal, and was also true for both unrelated pairs of dolphins that had lived together and family members.

The time period for which the dolphins had been separated from others ranged widely, from 4 to 20 years. Interestingly, though, quantitative analysis of the reactions showed that the time apart made no difference: Whether the pairs had been separated for 5 or 15 years, the dolphins demonstrated a similar level of response upon hearing a familiar whistle.

In the most extreme example, Bailey (a female dolphin who now lives in Bermuda) recognized the whistle of Allie (who lives at the Brookfield Zoo). They most recently lived together at Dolphin Connection in the Florida Keys, 20 years and six months ago.

Previously, monkeys have demonstrated the ability to remember the faces of other monkeys after three years apart, while elephants have been shown to recognize others’ vocalizations ten years later. If these new findings are accurate—and the dolphins’ behavior truly reflects memories they’ve held for decades, rather than, say, reactions to some other aspect of the recordings—they’d be the longest-held memories by any animals species by a wide margin. Along with other recent research on the surprising distinctiveness of individual animals’ personalities, the findings reveal how, in many ways, the most intelligent animals differ less from humans than we long imagined.

They also prompt another question, ripe for further research: When the dolphins appear to react to the whistles, what exactly is going on in their minds? It’s easy to speculate that the noises correspond to names, it’s hard to say how far the analogy can be taken. “We don’t know yet if the name makes a dolphin picture another dolphin in its head,” Bruck said. “That’s my goal—to show whether the call evokes a representational mental image of that individual.”