Dolphins have long impressed people with their sharp minds and humanlike traits, such as calling each other by name, goofing off and even understanding numbers. Now a scientist has found that the mammals can recognize an old friend’s whistle, even after they have been apart for 20 years — the longest social memory ever recorded for a non-human.
In a study released Tuesday, dolphins largely ignored calls from unfamiliar dolphins but responded when an old tank mate’s signature whistle was played back to them. It didn’t matter how much time had passed since the two had last seen each other or whether they had been tank mates for only a few months: The dolphins appeared to remember a familiar whistle.
“The main implications of such findings is that humans are not the only mammals that retain memories of others for long periods,” said Eduardo Mercado III, a psychologist at the State University of New York at Buffalo, who was not involved in the research.
Before the new study, published online in the journal “Proceedings of the Royal Society of London B,” much of what had been known about dolphin memory was anecdotal. This recorded feat of long-term memory puts dolphins in the same field as other highly intelligent creatures, including some monkeys and elephants, both of which have been known to recognize unrelated members of their species after time apart.
University of Chicago scientist Jason Bruck studied 56 bottlenose dolphins that were moved between six institutions — including Chicago’s Brookfield Zoo, the Minnesota Zoo and an aquarium at Disney World in Orlando — over a period of 20 years. They were typically transferred for breeding purposes, which somewhat mimicked the shifts among pods of dolphins in the wild.
That approach gave Bruck a record of the animals’ social histories, which would be nearly impossible to collect for wild dolphins. The dolphins studied were separated for as long as 20 years.
Bruck stored all the dolphins’ signature whistles on his iPod and broadcast them through an underwater speaker, taking note of the animals’ reactions. A dolphin’s identity is encoded in its signature whistle, Bruck said.
“If you took our names and our faces, merged them into one thing, that would be the best way to describe a signature whistle,” he said. Dolphins choose this “name” for themselves while they are 4 months to a year old.
The dolphins vocalize their whistle when they find themselves isolated from others, but they also can mimic a friend’s whistle to call to another dolphin. The whistles, which can be heard up to a mile away, help dolphins discern friend from foe.