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Every dolphin has it's own unique 'signature whistle'. Listening to these whistles is one way to identify specific dolphins and track their whereabouts.

Here's an extract from an article that appeared recently in 'Ear on the SEA' E-zine. It describes the work done at the Dolphin Study Group in Singapore:

"Bottlenose dolphins ( Tursiops truncatus ) have elaborate sound production and receiving systems. In the Tursiops, four major types of vocalisations have been identified: whistles, clicks, burst pulsed sounds and chirps."

"Whistles are used in communication. They are continuos, narrow-band, frequency-modulated pure tones, limited in frequency to the mid- and upper- range of the sonic spectrum, generally from 4 to 24 kHz, and of 0.5s in duration. Most of their energy is below 20 kHz.

"Clicks are directional and used for echolocation. These are short, broad-band pulses ranging from .2 to 150 kHz. Pulse duration is 50 to 80 ms. The interclick interval ranges from 10ms to ~160ms. Dolphins are able to click and whistle at the same time.

"Burst pulsed sounds occur commonly in social and emotional context and are thought to be used for communication. These are trains of clicks with repetition rates of up to 5000 clicks per second. The high repetition rate gives a tonal quality to the sounds.

"Chirps are thought to be used in communication. These sounds are pulsed, frequency-modulated, broad-band sounds. Frequency modulation occurs in different frequency bands"

All of the communications related sounds are well within the human hearing range. Even the echolocation clicks can have audible components within the nominal 20kHz upper limit of human hearing - some go down to 0.2 kHz as stated above.

Dolphin whistles were observed with both upward and downward sweeps. An up-sweep occurs when the whistle starts at a low frequency and the pitch increases, while in a down-sweep it starts high then decreases pitch. Sometimes a whistle starts, sweeps up then abruptly sweeps down. "Of the 20 down-sweep whistles mentioned, 16 do not have following vocalisation. This suggests that dolphin vocalisations may be produced in sequences that end with a down-sweep whistle. A down-sweep whistle may therefore be a termination signal at the end of a vocal sequence or a termination signal in a sequence of vocal exchange between bottlenose dolphins."

The study was based on a 63 minute tape made at Parc Asterix in France in 1996. The 'subjects' were 5 adult dolphins and one calf. The researchers did not see the dolphins nor were they able to correlate the animals actions with the sounds they produced. But, it gives us a good base to understand the kind of vocalisations we will hear in the wild.

This research project is also an excellent example of the quality of work that can be produced from a 'single encounter' using readily available tools. DolphinEar and the software included in the package can get you started in your own research. Casual encounters with dolphins lasting 20-30 minutes are not unusual in most areas of the world. If you've got your video camera handy (with the hydrophone plugged into the camera), the visual and audio record of the event will let you analyze dolphin sounds and correlate their actions with particular vocalisations. If you don't have time to analyze the tape yourself, there are many other researchers who would love to have access to it.



Spectrogram of dolphin whistles. Click below to listen.

Dolphin Vocalisations