Published online 5 November 1999 | Nature | doi:10.1038/news991111-1

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Sounding out the science of whale song

Whale song.Whale song.© NOAA

Whatare whales communicating when they sing? A new study suggests that whales may attune their songs to their surroundings.

Humpback whales are some of the most vocal denizens of the oceans. Their vocal prowess is enviable: while opera sopranos do well to produce frequencies above 1,200 hertz (cycles per second), male humpbacks can generate frequencies some three times greater, hundreds of times a day, sometimes for up to 20 hours without a break.

Lone humpback males ululate in rhythmic laments, while foraging whales sing 'feeding songs', and other groups create social sounds that might be territorial anthems. Some of these sounds, propagating through the water, can be heard 160 kilometres away from the singers.

Whatever the content, it seems clear that the whales intend their songs to be intelligible to others. But in the shallow waters in which they live, this is not easy to achieve. Sound bounces off both the water surface and the seabed, and is distorted by changes in water density, caused by differences in saltiness or temperature - which can be pronounced in coastal environments.

In the Journal of the Acoustical Society of America1, Eduardo Mercado of Rutgers University in New Jersey and Neil Frazer of the University of Hawaii consider what the whales might do to accommodate these environmental factors. In particular, they ask whether the whales choose their pitch to ensure optimal projection of their signals.

It has previously been believed, for instance, that in reflective, shallow-water settings, low-frequency sounds will travel farther than high-frequency ones, and therefore humpback whales might generally sing lower-pitched songs so as to be heard over greater distances.

Mercado and Frazer put this idea to the test by calculating how different frequencies propagate in the waters off Hawaii. They find that because of the complexities of sound reflection and distortion, lower frequencies actually travel worse than higher ones. So, they say, "humpback whales should produce higher frequencies rather than lower frequencies if they want their sounds to go farther".

But is this really what the whales want? The researchers found that the best frequencies for long-ranged propagation are not the ones actually produced by the whales, which seem to prefer singing rather lower than this. Yet the mammals are clearly capable of singing at this 'optimal' pitch - so why don't they?

Natural selection fine-tunes animal behaviour to maximize success at producing offspring. For singing male humpbacks, for instance, it will ensure that they sing the song best suited to attracting a mate and warding off rivals. But it seems that this might not amount to 'survival of the loudest'. Perhaps, say Mercado and Frazer, the lower frequencies might reduce the tendency of songs from different singers to interfere with, and obscure, one another.

Whether or not this is so, it seems that the formidable octave range of the humpback whale might be a consequence of environmental constraints on the propagation of their sounds. Because variations in these conditions - water depth, temperature and so on - will strongly affect the audibility of the singing, a wide vocal range should enable a whale to match its song to the prevailing conditions.

The researchers suggest that whales might be able to use echoes from the ocean floor or from schools of fish to judge how far their 'voices' are travelling - and to change the tune accordingly. The quality, rather than the loudness, of the reflected sound should convey the necessary 'feedback' information. Mercado and Frazer point out that humpbacks are good listeners, as they have been observed apparently mimicking the songs of others, and that they alter their songs when passing ships make the surroundings more noisy. 

  • References

    1. MercadoIII, E. & Frazer, L. N. Environmental constraints on sound transmission by humpback whales Journal of the Acoustical Society of America 105, 3004 - 3016 1999. | Article |