Reports of chemical pollution, oil slicks and the resulting habitat destruction are an all too familiar story in the press these days. However, an often overlooked, but no less-damaging form of pollution is noise; we're all familiar with the inconvenience of noisy neighbours. No matter how irritating these noises are for us, for animals that rely on echolocation to sense their environment, noise pollution could be deadly: whales and dolphins have been found stranded on beaches following sonar testing by the military. Examination of some of these whales revealed that they had gas lesions in their liver and kidneys due to rapid decompression similar to `the bends' that human divers suffer when they return to the surface too fast. Could the sonar used by the military lead to these mass strandings and, if so, how can they be prevented in the future? Intrigued by the possible threat posed to cetaceans by man's acoustic activities, Mark Johnson and a team of researchers from the USA,Italy and Spain decided to study echolocation in two species of beaked whale, Mesoplodon densirostris and Ziphius cavirostris.
A key step in understanding the dangers posed to whales by noise pollution is to study how echolocation works in some of the whale species that become stranded. However, many stranded whales belong to the beaked whale family,which are so elusive that new species have been discovered as recently as 2002. Fortunately these creatures' elusiveness did not deter Mark Johnson and his colleagues. By attaching a non-invasive acoustic recording tag to individuals from both species in the wild they were able to record the sounds generated and the echoes received by the whales for long periods of time (up to 15 hours) while the whales were behaving normally. These extended recordings revealed that although neither whale species produced clicks (short bursts of sound) when they were close to the surface (within 200 m) when the whales made deep dives (up to 1200 m) they clicked continuously and only stopped when they started their ascent to the surface. These clicks closely resembled those of dolphins during echolocation, which have been much more extensively studied. The clicks were also followed by echoes (that were also recorded by the tags) suggesting that they may be used in echolocation.
But could this echolocation be important during normal behaviour? The team calculated the time between the clicks and the echoes returning to the whales from an object and converted these into distances to the object just as the whales must do. By aligning the responses to successive clicks, their recordings of the beaked whales show the echoes returning more rapidly,suggesting that the distance to the object was getting smaller. As the whales get close to the object they switch from the distinctive pattern of clicks to a buzz and the whale starts to accelerate. This was interpreted by the team as showing individuals tracking and closing in on a prey item, suggesting that the whales use echolocation during hunting.
This study provides the first evidence that beaked whales, like dolphins and other whale species, hunt using echolocation. One possibility is that military sonar exercises could interfere with their echolocation, causing the whales to ascend rapidly to the surface and, therefore, depressurise. This work may provide information that could be crucial in future efforts to protect these whales against noise pollution.
