Every bottlenose dolphin has its own whistle, a high-pitched, warbly "eeee" that tells the other dolphins that a particular individual is present. Dol
phins are excellent vocal mimics too, able to copy even quirky computer-generated sounds.
So, scientists have wondered if dolphins can copy each other's signature whistles — which would be very similar to people saying each others' names.
Now, an analysis of whistles recorded from hundreds of wild bottlenose dolphins has confirmed that they can indeed "name" each other and suggests why they do so — a discovery that may help researchers translate more of what these brainy marine mammals squeak, trill and click about.
"It's a wonderful study, really solid," said Peter Tyack, a marine mammal biologist at the University of St. Andrews in the United Kingdom, who was not involved in this project.
"Having the ability to learn another individual's name is… not what most animals do.
‘‘Monkeys have food calls and calls that identify predators, but these are inherited, not learned sounds."
The new work "opens the door to understanding the importance of naming".
Scientists discovered the dolphins' namelike whistles almost 50 years ago.
Since then, researchers have shown that infant dolphins learn their individual whistles from their mothers.
A 1986 paper by Tyack did show that a pair of captive male dolphins imitated each others' whistles, and in 2000, Vincent Janik, who is also at St. Andrews, succeeded in recording matching calls among 10 wild dolphins.
"But without more animals, you couldn't draw a conclusion about what was going on," said Richard Connor, a cetacean biologist at the University of Massachusetts, Dartmouth. Why, after all, would the dolphins need to copy another dolphin's whistle?
Unravelling dolphin calls was not easy; they produce their sounds underwater, often in murky conditions, making it difficult for scientists to identify which animal was emitting which call.
To get around this problem, Stephanie King, a marine biologist also at the University of St. Andrews, and her colleagues, analysed the acoustic recordings of dolphin signature whistles that scientists with the Sarasota Dolphin Research Program made of more than 250 wild bottlenose dolphins briefly captured between 1984 and 2009 in Sarasota Bay, Florida.She also recorded the signature whistles of four captive dolphins and made detailed observations of their behaviours while whistling.
The scientists with the Sarasota programme had captured pairs and groups of dolphins and held them separately in nets for 108 minutes on average.
During this time, the marine mammals could not see each other, but they could hear each other, and they whistled at a high rate, sometimes giving 5.3 calls per minute. In the wild too, dolphins use their signature whistle at a high rate, King noted; it was one of the most common sounds they made.
King and her colleagues compared the spectrograms or visual representations of the wild dolphins' whistles, and one pair of the captive dolphins, looking for evidence that the dolphin pairs copied each others’ sound.
They identified whistle-matching in 10 of the 179 dolphins recorded in the Sarasota study, and one of the captive pairs.
The males in two pairs had copied each others’ whistles, as had eight mother-and-calf pairs, the team reported in the Proceedings of the Royal Society B.
The dolphins did this quickly, too, repeating the others’ whistle within one second of hearing it.
Those dolphins that copied whistles also only imitated the call of their closest social partner.
"It means they were calling a specific individual," King said.
"They produce the copies when they are separated, which we think shows that they want to reunite with a particular individual."
It's what we do if we get separated from our friends at a fair or the mall, she added.
"You don't call out your name; you call the name of your friend. That's how you get back together."
Indeed, dolphins don't copy just anybody's signature whistle; they are very selective in how they use this talent, using it solely to maintain bonds between mother and calf and between allied males, King said.
The scientists recorded one pair of males twice, once at the beginning of the project and then 12 years later.
Even after all that time, each dolphin "copied the fine details of the others’ whistle," King said, an indication of the pair's especially tight bond.
"There's been some evidence before that dolphins are imitating each others’ whistles," Connor said
‘‘But this is the first really convincing paper. It's very exciting."
"It is fabulous," added Karl Berg, an ornithologist at the University of California, Berkeley, who specialises in parrot calls.
He noted that dolphin results were "strikingly similar to recent work on parrots," which also imitate each others’ vocalisations, sometimes as a way of negotiating foraging or roosting decisions.
King found no evidence that the dolphins used their whistle-copying to be aggressive or to deceive other dolphins, as some have hypothesized.
The dolphins that imitated another dolphin's whistle did so only to re-establish contact with a specific individual.
But in some cases, the dolphins added parts of their own signature whistle to the copy of their friend's, which suggested that they may also be including additional information — which would not surprise either Berg or Connor.
"The ability to imitate social companions can lead to more complicated exchanges of information," as well as disagreements, Berg said.
Connor noted that King's study focused on the signature whistles of captured dolphins that were separated from their social partners, and so may not be representative of how wild dolphins more typically use their calls.
"How do they use these calls in the wild? Can they add 'help' to someone's signature whistle?" he asked, noting that male dolphins come to each other's aid when battling over females.
"Can they add sounds that say they like or dislike someone? If you can say somebody's name, there's no reason why you can't talk behind his back."
King was tackling these questions with a new round of experiments.
- SCIENCE NOW
What will be the main motivation for humanity's future space endeavours?Related story: (See story)
The cost of losing nature