In a study released Tuesday, dolphins largely ignored calls from other 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 SUNY-Buffalo psychologist Eduardo Mercado III, who was not involved in the research.
Prior to 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 different 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 between the ages of 4 months and 1 year.
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.
To make sure that the dolphins weren't reacting to random noises, Bruck first played a set of unfamiliar whistles. Once the dolphins were accustomed to the speaker, he played a familiar whistle, which resulted in a dolphin quickly approaching the speaker, even if it realized another dolphin wasn't there.
"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."
One female dolphin named Allie, currently at the Brookfield Zoo, last lived with Bailey, a female now in Bermuda, more than 20 years ago. But upon hearing Allie's whistle, Bailey recognized the sound, according to the research.
Bruck had a light-bulb moment that led him to study dolphin recognition when he saw his brother's border collie, Abby, for the first time in four years.
"This dog absolutely hates men," Bruck said, but Abby immediately recognized him as a friend and not an enemy, even after a long absence. Bruck, an animal-behavior researcher, began to wonder whether other species could remember familiar faces long-term.
Heidi Harley, who researches dolphin cognitive processes at the New College of Florida, called the study "interesting," and the number of dolphins studied "impressive." But she wondered whether their responses were to a familiar sound rather than a connection to a dolphin they once knew.
"Is this really about the dolphins that produce these whistles, or is it just about the sounds themselves? It's a little hard to disentangle sometimes," Harley said.
Mercado III agreed: "It is in principle possible that a dolphin could find a whistle more 'interesting' without having any awareness of why."
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