According to researchers at the University of Chicago, dolphins can recognize whistles from old tank mates from decades ago, more than 20 years ago to be exact. Twenty years makes this the longest social memory ever documented for a non-human species.

The excellent memory accomplishment is another sign that dolphins have an extremely high level of intelligence, much like humans and monkeys. Dolphins’ ability to recognize other dolphins may be even longer lasting than facial recognition among humans, since human faces can alter drastically over time. Dolphin whistles, on the other hand, remain stable over multiple decades.

“This shows us an animal operating cognitively at a level that’s very consistent with human social memory,” said Jason Bruck, who recently received his Ph.D. from the University of Chicago’s program in Comparative Human Development.

To determine how well dolphins could recall their former friends, Bruck gathered data from 53 dissimilar bottlenose dolphins at six facilities. The six facilities were part of breeding association that has rotated dolphins and recorded which ones resided together for many years.

“This is the kind of study you can only do with captive groups when you know how long the animals have been apart,” Bruck said. “To do a similar study in the wild would be almost impossible.”

Previous studies have shown that each dolphin creates its own unique signature whistle that seems to act as a name. Bruck produced recordings of signature whistles to dolphins that had once resided with the animals that made the calls. Researchers compared how the dolphins reacted to familiar calls versus calls belonging to dolphins they had never known.

Bruck discovered that dolphins become bored quickly listening to signature whistles from dolphins they’ve never met. However, the familiar signature whistles would excite the dolphins and evoke an immediate reaction.

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

An obvious pattern soon emerged in the data: compared with strange signature whistles, dolphins reacted significantly more to whistles from animals they once met, even if they hadn’t heard the calls in two decades.

Researchers aren’t exactly sure why dolphins’ social memories last so long, but they hope to determine how similar dolphins’ signature whistles are to human words and names.

The study’s findings are described in greater detail in the journal Proceedings of the Royal Society of London B.