The monopulsed nature of sperm whale clicks

Abstract

Traditionally, sperm whale clicks have been described as multipulsed, long duration, nondirectional signals of moderate intensity and with a spectrum peaking below 10 kHz. Such properties are counterindicative of a sonar function, and quite different from the properties of dolphin sonar clicks. Here, data are presented suggesting that the traditional view of sperm whale clicks is incomplete and derived from off-axis recordings of a highly directional source. A limited number of assumed on-axis clicks were recorded and found to be essentially monopulsed clicks, with durations of 100 micros, with a composite directionality index of 27 dB, with source levels up to 236 dB re: 1 microPa (rms), and with centroid frequencies of 15 kHz. Such clicks meet the requirements for long-range biosonar purposes. Data were obtained with a large-aperture, GPS-synchronized array in July 2000 in the Bleik Canyon off Vesterålen, Norway (69 degrees 28' N, 15 degrees 40' E). A total of 14 h of sound recordings was collected from five to ten independent, simultaneously operating recording units. The sound levels measured make sperm whale clicks by far the loudest of sounds recorded from any biological source. On-axis click properties support previous work proposing the nose of sperm whales to operate as a generator of sound.

Clicking for calamari: toothed whales can echolocate squid Loligo pealeii

“…The sperm whale generates the highest known sound levels in the animal kingdom, producing clicks with a source energy flux density of up to 193 dB re 1 μPa 2 s (Møhl et al 2003). Assuming the lowerfrequency sperm whale click gives rise to a similar TS of -41 dB from an individual Loligo pealeii, and using a detection threshold of 48 dB re 1 μPa 2 s (Au et al 2007), assuming a higher noise level in the deep sea at 15 kHz compared to 50 kHz (Urick 1983), we find that a sperm whale under noise-limiting conditions should be able to detect a 25 cm L. pealeii at ranges between 100 and 325 m depending on target orientation (Fig.…”

“…5). Such long detection ranges result from the extremely high source levels and low absorption of the 15 kHz sonar clicks produced by the hypertrophied nose of the sperm whale (Madsen et al 2002, Møhl et al 2003.…”

“…may have TS comparable to or greater than the L. pealeii measured here, and perhaps that the larger hard parts (cranium, beak, chitinous sucker rings, gladius) of these larger squid may also contribute to the acoustic backscatter. The large (1.2 m and 50 kg) and very muscular jumbo squid Dosidicus gigas in particular could have a TS of -30 dB or higher, thus rendering it a strong target for toothed whales with long-range sonars, such as sperm whales (Madsen et al 2002, Møhl et al 2003, which have been reported to prey upon these large squid (Davis et al 2007). Using a detection threshold of 48 dB re 1 μPa 2 s as in Fig.…”

Acoustics in water: synergies with marine biology

“…The impulses have energy up to 40 kHz, but this changes depending on the orientation between the animal and the hydrophone. Surface recordings often record off-axis clicks which have less energy at high frequencies [12]. The histogram Fig.…”

A comparison of model and non-model based time-frequency transforms for sperm whale click classification

“…We also explored the possibility of using these features as a biometric in order to uniquely identify a sperm whale. Recent papers suggest that the latter may be difficult due to the directionality of the click (Møhl et al, 2003) and the influence of hydrostatic pressure (i.e. the diving depth) on the whale (Thode et al, 2002), which both affect the frequency content.…”