Whales and dolphins use sound for communication and for hunting. Some ocean mammals, such as the sperm whale, are capable of diving to enormous depths in pursuit of prey, which can have profound effects on the volume of air they have available to produce sound. New research by Peter Madsen, Roger Payne and colleagues from the University of Aarhus and Ocean Alliance, has revealed that the sperm whales' clicks are unaffected by the extremes of pressure which it commonly encounters (p. 1899).

As they dive, sperm whales produce clicks that are used to maintain their complex social relationships and to locate their prey, which include squid and fish. The sounds are generated by forcing air through a pair of `phonic lips',and transmitted into the surrounding water by connective tissues and spermaceti oil. These structures form a highly evolved `nasal complex' in the sperm whale's head, which may be up to one third its total body length and may be the largest sound generator of any animal.

Madsen also explains that with depth `pressure increases and air volumes in soft tissues are reduced'. A whale at 600m may only have around 2% of the volume of air it took in at the surface to produce sounds. Madsen, Payne and collaborators decided to study sperm whales in the Bismarck Sea, north east of Papua New Guinea, to find out how their clicks were affected by depth.

Using the considerable experience of the research organization, Ocean Alliance, and their research vessel Odyssey, the researchers were able to attach acoustic recording devices to sperm whales as they surfaced. The whales seemed relatively unconcerned by this and, according to Madsen`some of them even returned to the ship to get a good look at us, making you wonder who is studying who!' This technique allowed the team to record the clicks of the whale as it made a feeding dive in its natural habitat.

Analysing this data, it became clear that the sound pressure levels and frequency content of the sperm whale's clicks were unaffected by depth,suggesting that they are coping admirably with the limited air volumes available. Madsen believes that they are recycling air within their nasal complex, allowing them to re-use what little air they have available to produce the sounds they need to locate their prey at depth. The research also revealed that sperm whales produce two click types — one suited to sonar and another to communication — which might explain how the whales are able to use clicks for both functions.

Although not a highly endangered species, the sperm whale faces threats in the form of marine sound and toxin pollution. Madsen believes that his findings may have important implications in understanding the acoustic world of sperm whales. `As most marine mammals rely heavily on acoustic cues, it is important to understand how these animals produce, hear and use sounds in order to mitigate sound pollution in the world's oceans.' This collaborative research effort has taken the team a step closer to achieving these aims.