Not only do dolphins make friends, they also apparently remember them for decades.
Researchers noticed that dolphins in captivity that had been separated for as long as 20 years still recognize their former tank mates' calls, according to a study reported on LiveScience.com.
In the study, scientists were able to track dolphins who had met previously, and here's how they did it: Over five years, researchers followed 43 dolphins in six institutions in the United States and Bermuda. Dolphins are moved around a lot, but institutions keep careful records.
Also, each dolphin develops a unique whistle that is like a dolphin handle recognized by their fellow dolphins. Researchers were equipped with recordings of 1,200 bottle-nosed dolphin whistles that they played for the 43 dolphins over speakers.
When the dolphins recognized a whistle — even if it was one from a tank mate from years before — they would stop, swim up to the speaker and sometimes whistle back to their buddies. For those they had never heard, there was no response.
"It's mind-blowing; I know I can't do it," University of Chicago researcher Jason Bruck told The Associated Press. "Dolphins in fact have the longest social memory in all of the animal kingdom because their signature whistle doesn't change."
Other animals have been called out for long-term memories: Elephants supposedly never forget. Chimpanzees also are thought to have the trait. But neither have been tested.