A scientists has discovered dolphins to have recognized a former tank mate's whistle after 20 years apart, making its social memory best among all non-human mammals, the Washington Post reported.

The study, published Tuesday in Proceedings of the Royal Society of London B, found it did not matter how much time had passed, but a dolphin would recognize a friend's call when it was played back to them. They also ignored unfamiliar ones.

University of Chicago scientists Jason Bruck, the only author, analyzed 56 bottlenose dolphins in six different places. Bruck visited the locations and its dolphins, including Chicago's Brookfield Zoo, the Minnesota Zoo and an aquarium at Orlando's Disney World, over a period of 20 years.

He recorded the dolphins' calls and used his iPod to play them back over underwater speakers.

"If you took our names and our faces, merged them into one thing, that would be the best way to describe a signature whistle," Bruck said. Dolphins choose this unique identity for themselves early in life, somewhere between four months and one year old.

"The main implications of such findings is that humans are not the only mammals that retain memories of others for long periods," said SUNY-Buffalo psychologist Eduardo Mercado III, who was not involved in the research.

To make sure the dolphins would not react to just any sound coming from the speakers, he played unfamiliar whistles for a little while. When they were used to the speakers, he began playing the familiar whistles. The dolphins who recognized the call would quickly turn and swim toward the speaker, even though they did not see an actual dolphin.

"Say you are walking along the street and someone projected a hologram of your grandmother in front of you," he said. "You'd turn and look."

Bruck told BBC News his findings were "unprecedented" in the field of animal behavior. He said we now know dolphins have social memories similar to humans.

"Having a long term social recognition for that ecological reason can be the difference between an animal having a very negative social interaction and a positive one," he said. "It is important for them to recall the calls of dolphins they have had previous encounters with, to decide whether or not that's someone they want to approach when they hear that whistle about a mile out, or whether they want to avoid that individual."