February 21st, 2013 by Michael Ricciardi
Dolphin’s are renowned for their intelligence and sociability and there have been many studies over the past 50 years of dolphin research documenting their remarkable range of abilities.One of these is the dolphins’ ability to imitate sounds, and, in particular, human vocalizations, and even computer-generated sounds.
This has led many researches to investigate the cetacean’s ability to communicate with members of their own species.
Nature, the argument goes, would not have selected for such an ability (sound imitating) if it did not have great survival value to the dolphin, or the pod.
Of the many dolphin species, the bottlenose dolphin (Tursiops truncatus) is perhaps most famous. Each bottlenose has its own “signature” whistle sound — a high-pitched, fluctuating, “eeee” sound. The unique whistle lets other dolphins in the vicinity know who is approaching the group. Researchers have known of this name-like whistling phenomenon for decades. But what all this whistling means has remained a bit of a mystery.
Recently, Marine biologists (Stephanie King and colleagues) at the University of St. Andrews, decided to test a hypothesis: do bottlenose dolphins ever imitate each other’s unique whistles — similar to how humans call each other by name? And, if so, why would they have need to do this?
Baby dolphins learn their unique whistles from their mothers. But at some point later, dolphins apparently learn the whistles of other individual dolphins, and, how to reproduce them. Though whistle-matching is common amongst these dolphins in the wild, this is no common trick in Nature; it is relatively rare in mammals. Monkeys, for example, use various vocalizations to announce discovery of food or to identify predators. These, however, are inherited, not learned abilities. Only in birds and humans, it seems, is this ability learned.*
There have been a few earlier studies showing indications of this “name calling”, such as a study back in 1986 (Tyack et al) describing a pair of captive male dolphins imitating each other’s whistles, while another study (Vincent Janik, 2000) recorded matching whistle calls in a group of about ten dolphins. But these studies were limited by small numbers of animals; without drawing upon data from more animals, scientists could not be sure what was really going on.
But now King et al have completed an analysis of hundreds of wild bottlenose whistles showing that these dolphins do indeed call each other by “name”, that is, they are imitating each other’s distinct whistles.
As dolphins typically engage in these behaviors under water, it has been difficult to study or analyze them. And so, to overcome this, King et al used acoustic recordings of some 250 wild bottlenose dolphins (briefly) captured over a 25 year period by the Sarasota Dolphin Research Program. Additionally, the researchers analyzed the whistles of four captive dolphins and observed their behavior as the whistling occurred.
The Sarasota project made the recordings from briefly captured pairs and groups who were kept apart (via nets) for 108 minutes on average.Though they could not see each other, the dolphins could still hear each other. They also began whistling at a “high rate” — calling up to 5.3 times per minute — similar to the rate recorded in the wild.
The researchers compared spectrograms of the wild and one of the captured dolphin pairs. They were able to identify whistle-matching in 10 out of 179 of the Sarasota dolphins and in one of the captive pairs. In two of the male pairs, the animals had learned to copy each other’s whistles; this was also seen in eight mother-and-calf pairs. This imitation of another’s vocal signature was learned very rapidly too; dolphins were able to successfully reproduce and repeat the other’s whistle within one second of first hearing it. And, as if to let others in the group know their “social status”, the imitating dolphins only repeated the whistles of their closest social partner.
[above] Spectrograms showing three examples each of the (i) signature whistle of the animal being copied, (ii) signature whistle copies and (iii) the signature whistle of the copier; sampling rate: 40 000 Hz, FFT (Fast Fourier Transform) length: 1024, Hanning window function. Numbers on the middle spectrograms give the mean human observer similarity scores between the original and the copy for each pair of whistles on a scale from 1 (not similar) to 5 (very similar). (a) Vocal interaction of a mother–calf pair. The mother, FB65, was the signature whistle owner (i) and the male calf, FB228, was the copier (iii). The male produced copies are in row a ii. (b) Vocal interaction of another mother–calf pair. The male calf, FB122, was the signature whistle owner (i) and the mother, FB90, was the copier (iii). The copies she produced are in row b ii. (c) Vocal interaction of a male–male pair from The Seas. The first adult male, Ranier, was the signature whistle owner (i) and the second adult male, Calvin, was the copier (iii). The copies he produced are in row c ii.
Commenting on the meaning of their findings, King stated:
“It means they were calling a specific individual. They produce the copies when they are separated, which we think shows that they want to reunite with a particular individual.”
King compared this to when members of a human group of family might get separated from each other.
“You don’t call out your name; you call the name of your friend. That’s how you get back together.” [quote source]
The dolphins did not used this whistle-copying ability indiscriminately either; the recordings show that they use it solely to maintain bonds between mother and calf and between allied males. These bonds are long-lasting too; one male pair — separated for 12 years — copied each other’s whistles in fine detail, which King and colleagues say is an indication of a tight social bond.
The researchers found no evidence to support alternative theories that whistle-matching is used aggressively or to deceive other dolphins (for some nefarious cetacean purpose). The addressing behavior was used only to reconnect with a specific individual. In some cases, however, the dolphins added segments of their own signature to the copy call, which may indicate the communication of additional information
Quoting from the paper’s abstract:
“This use of vocal copying is similar to its use in human language, where the maintenance of social bonds appears to be more important than the immediate defence of resources.”
* In birds, this learned imitation ability serves as an aggressive signal, whereas in humans and dolphins, it serves as an affiliative signal.
The Paper by King et al, entitled: ‘Vocal copying of individually distinctive signature whistles in bottlenose dolphins’, was published Feb. 20, 2013 in the journal Proceedings of the Royal Society B.
Some source material (including quotes) for this post came from the Science Magazine web article ‘Dolphins Can Call Each Other, Not by Name, But by Whistle’ by Virginia Morell
Top Photo: Credit: Jason Allen; (Inset) Sunnie Hart; (Both Photos) Sarasota Dolphin Research Program/NMFS Permit #15543
Chart:(spectrograms) The authors; CC
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