Dolphins not only have names for each other, their memories are so good that they can remember the "signature whistles" of friends—and enemies—for at least 20 years, according to a new study. Researchers studied scores of captive bottlenose dolphins that had been shifted around the US and found that they responded much more readily to the sounds of dolphins they had once known—including family members, ex-tankmates, and former mates—than the calls of strangers, the BBC reports.
The dolphins' "social memory" is the longest ever recorded in the animal kingdom. Researchers believe the ability to recognize familiar whistles, which stay the same while age changes their outward appearance, helps dolphins live in "fission-fusion" societies, where they join and leave different groups many times over a lifetime. "We know they have relationships in the wild that last decades," an animal behaviorist tells Science. "Remembering a particular individual—even in the absence of that individual—could help them navigate their current social milieu." (Read more dolphins stories.)