By Brian Behrens, Terri Gilmore, and Sharon Greaves.

Bottlenose dolphins (Tursiops truncatus) are well known for its use of echolocation and clicks for many things. Some of these uses include finding food, detecting predators, communication, and finding a mate. In this paper signature whistles, seeing inside objects, and sound as a weapon are examined.

A signature whistle is defined as a distinct narrow band high-pitched whistle. These whistles along with clicks play a central role in dolphin-to-dolphin communication (Chollar and Booth, 1989). According to research, as much as 90 percent of whistles produced may be signature whistles (Shane, 1980). These vocalizations are believed to be used for individual recognition, group cohesion, to show emotion or attract mates, or possibly to imitate others.

Dolphins are believed to have developed these skills to overcome problems of high background noise levels or alterations due to changing water pressures. To emit these sounds they tighten the sphincter muscles with in their blowhole. Each whistle lasts about fifteen seconds and ranges between 5-20 kHz. Each whistle has a distinctive contour or frequency variation overtime. The same whistle depending on pitch can mean several different things from hello to danger. Signature whistles have proved to remain stable for up to twelve years but are believed to last a whole lifetime.

Dolphins develop their own individual whistle between two months and one year of age. Each individual’s whistle is distinct and is thought to be used like their name. At birth a mother will repeat her whistle over and over so her calve will be able to identify her and eventually develop their own. Signature whistles do vary between sexes. A young male will develop a whistle similar to its mothers while a young female will develop one very different. This is believed to occur due to the females remaining close to their mother while; young males will leave their natal groups to form juvenile groups. This creates the distinction between the whistles.

Signature whistles also help a pod remain together and safe. A pod of dolphins is believed to continuously whistle as they swim, so individuals do not stray or fall behind. A change in pitch of a signature whistle can also alert a pod of danger. This danger could be from sharks to killer whales to fishing nets (Ola and D’Aulaire, 1992). By recognizing others whistles dolphins form of communication keeps them together and protected.

Many studies are also producing evidence of imitation. A dolphin will call out its whistle along with the whistle of another individual as if to say, " Hey Brian, its me, Terri, I am over here." By mimicking another’s whistle they can catch the attention of that individual. Other reasons for imitating whistles are not clear and easy to understand. One particular study of seven captive bottlenose dolphins revealed a whistle unfamiliar to the others. By reviewing old recordings, the whistle was discovered to be that of a young male who had died a year earlier (Shane, 1980).

Dolphins produce pings in the frequency range of 0.25 to 150 kHz for echolocation (Veinot & Calhoun, 1999). These echolocation pings can be used to distinguish between slight differences in size, texture, and even density (Gill & Gibson, 1997). These pings are believed to be produced by air moving in the diverticula, bounced forward by the parabolic shaped skull, and focussed by the melon.

Scientists believe that dolphins use their echolocation to "see inside objects". Herman, Pack, and Hoffmann-Kuhunt (1998) conducted an experiment to test a bottlenose dolphin’s ability to perceive shape only through echolocation, in doing this they also tested the ability to see through objects. To test this the scientists set up two experiments. In the first experiment the dolphin was to echolocate on an object suspended in a viewing box that the dolphin could not see through, with the front panel being opaque plexiglas 0.32 cm thick and the sides made of redwood. The plexiglas transmits sound very well (75%), while the redwood absorbs the sound. An object made of polyvinyl chloride (PVC) pipe filled with sand was suspended in the viewing box. The dolphin (Elele) was allowed to echolocate on it, and then she was given two choices in the air that she could see but not echolocate on. Elele would then choose the correct shape by remaining stationary in front of it for 3 seconds. In this test Elele was right 585 out of 600 trials (97.5%). In the second experiment Elele was visually shown an object above the water and then given two choices in viewing boxes. Elele would again remain stationary in front of her choice shape for 3 seconds. In this second test Elele was right 579 out of 600 trials (96.5%)

This study supports the theory that dolphins can see through objects, since the dolphin could echolocate through the plexiglas and was correct most of the time. The study also shows how good the dolphins shape perception is since that was the only real difference between the objects. What we think and what our results have thus far shown is best summed up by Pack and Herman's statement "… what a dolphin 'sees' through echolocation is functionally similar to what it sees through vision" (1995). Some people believe that dolphins may also use their echolocation ability to "peer inside one another", as well as other animals (Walter, 1999). This ability could be used to give the dolphin such information as health of the animal and even possibly what it last ate.

A study by Xitco and Roitblat (1996) sought to show that dolphins could use echolocation signals other dolphins send out to get information. This "eavesdropping" would allow dolphins to gather information just by interpreting the sound waves produced by a different dolphin, and thus increasing its knowledge of the surroundings. In the study two bottlenose dolphins (Bob and Toby) were the subjects used. One dolphin would echolocate on an object in a viewing box of 0.10mm black polyethylene while the second dolphin would wait with its melon out of the water, but it's lower jaw in the water so that it could "listen". After the first dolphin echolocated on the object, the second dolphin would have two viewing boxes, constructed of the same material, with the matching object in one box and a different object in the other. The dolphin would echolocate on each and then choose one. Bob was right 83% of the time , while Toby was correct 92% of the time.

The theory for sound as a weapon is known as the ‘Big Bang Theory’. This theory states that dolphins and whales generate ‘bangs’ that may stun prey making it easier to capture. Two soviet scientists first proposed it in 1963. They put forward the idea that the sperm whale might possess some sort of ultrasonic projector. However, at that early stage the theory was generally ignored as people thought it was too incredible to believe.

Years later, Kenneth Marten and his fellow worker Kenneth Norris, from the Long Marine Laboratory at the University of California at Santa Cruz, wondered whether dolphin echolocation clicks disturbed the sensitive lateral lines of fish. The lateral lines in fish are the organs that detect minute movements in the water. However, they too were unable to prove anything at that stage.

In 1982, Kenneth Norris teamed up with Bertel Mohl from Aarhus University in Denmark and together they published the big bang theory. They had carried out a small series of field observations of apparent unharmed prey that had lost escape reactions in the presence of porpoises or some of the larger toothed whales like the false killer whale. They were able to scoop fish up with a dip net & saw mahi-mahi lying immobile in the water with whales circling.

These were the experiments which led these two to believe that some of the toothed whales might be able to turn their sonar abilities into a weapon to stun their prey, making the victims easier to round up and kill.

Virginia L. Cass, who formerly worked at the La Jolla Southwest Fisheries Center of the National Marine Fisheries Service, found that wild bottlenose dolphins and killer whales produce banging noises while feeding. Recordings of the bangs show that they are much lower in frequency than clicks and so coincide with the hearing range of the prey. The bangs are also much louder and last approximately 1000 times longer. The recordings featured ascending trills of clicks followed by what sounded like a gun firing or a stun grenade exploding.

Another scientist, Whitlow Au, who works at the Naval Ocean System Center in Hawaii performed experiments with captive bottlenose dolphins in which he recorded and measured the high intensity sounds they emitted, audible to humans as a series of clicks. Norris using these measurements reproduced the sounds with blasting caps and played them back in tanks full of squid. The squid turned belly up.

Experiments in Europe found that both fish and squid can be immobilized by sounds near those most intense for toothed whales. It has also been observed that dolphins in threatening social interactions make similar noises. This suggests that for a dolphin a bang may be the equivalent of bared fangs for a dog.

There have been observations of other odontocetes other than bottlenose dolphins, which are believed to use sound as a weapon. Dusky dolphins were seen eating disorientated anchovies, which had balled up into a group making no attempt to escape. Divers apparently witnessed Atlantic Spotted dolphins focus their sonar on schools of razor fish that lost their vertical orientation therefore becoming passive targets.

This theory would also help to explain how Sperm whales which are not particularly well equipped to capture large, swift swimming squid, can use these prey as a major food resource.

However, this theory still hasn’t been proved. Bangs are not always produced when toothed whales and dolphins feed. Also captive dolphins have not been heard to make the sounds, but it is thought the bangs may be unbearably loud in a confined tank.

Beardsley, Tim. M, "Sonic Punch: Dolphins and Whales Generate Bangs that may stun Prey", Scientific American, Oct 1987, Vol. 257, p 36.

Chollar, S. and Booth, W. (1989). "Conversations with dolphins: these engaging and highly intelligent animals play major roles in language research’, Psychology Today. V23.n4.p52(6).

Ellis, R, "The Dolphins Secret Weapon", National Wildlife, Aug-Sep 1984, Vol. 22, p34.

Gill, Peter, and Linda Gibson. (1997). Whales dolphins and porpoises.

Veinot, Stacey and Jaime Calhoun. "Echolocation Mechanism in Dolphins." 15 Nov.1999 http://www.unb.ca/courses/bio4775/SPAGES/SPAGE9.HTM#Figure

Herman, Louis M., Adam A. Pack, and Matthias Hoffmann-Kuhnt. (1998) "Seeing through sound: dolphins (Tursiops truncatus) perceive the spatial structure of objects through echolocation". Journal of comparative psychology. p 292 (14).

Walter, George. Home page. 14 Nov. 1999. http://www.loop.com/~dacs/dolphin.htm

Ola, P. and D’ Aulaire, E. (1992). Playful Genius of the Sea. Readers Digest. March.p.54-9.

Xitco, Mark J. JR., and Herbert L. Roitblat. (1996). Object recognition through eavesdropping: Passive echolocation in bottlenose dolphins. Animal Learning & Behavior. 24 (4), 355-365.

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