With light pollution all around, it's sometimes hard to appreciate just how dark the world can get. But for predators that hunt in the ocean's depths,darkness is no challenge; they have evolved an alternative sense:echolocation. Bats and dolphins perceive their dim environments by emitting squeaks and clicks. Yet both encounter different problems in their sonic world. While large dolphins cruise through empty wide-open oceans, bats tend to hunt in cluttered, echoey woods. But bats are not confused by these misleading echos; they have tuned their cries to match their prey and minimise distraction. Would the same be true for echolocating dolphin's clicks? Might different species have tuned their clicks to match their diet? Peter Madsen and Roger Payne decided to compare the clicks of similarly sized dolphin species, with very different tastes: Risso's dolphins dine on squid deep in the ocean, while false killer whales munch on pretty much anything, anywhere. Working as part of the Ocean Alliance's ocean pollution monitoring project,Madsen and his colleagues had the chance to record both animals' sounds in the Indian Ocean, and found that unlike bats, dolphin's clicks aren't tuned to match their prey (p. 1811).
But recording dolphin clicks in the ocean isn't as straightforward as it sounds. Neither species were particularly curious, so instead of waiting for the dolphins to come and investigate their ketch, the team knew they had to scout around the ocean until they happened upon a pod. Once they'd located the animals, Madsen had only a matter of minutes to record the dolphin's clicks before they lost interest in the research vessel Odyssey, and took off again.
Even then, Madsen's problems weren't over. Knowing that each click was projected as a beam of sound, he needed to be directly in line with a clicking dolphin to get a clear measurement of the click's spectrum. Easily done in captivity, but tricky out at sea. Madsen overcame the problem by rigging up an array of hydrophones. If one of the hydrophones picked up a loud click relative to the other two, it was likely to be directly inline with the click's emitter. And the arrangement also gave Madsen a convenient way to calculate the animal's position.
Over a period of months Madsen collected thousands of clicks from both species. But after painstakingly analysing the click's spectral properties, he realised that they were all very similar. Despite the dolphin's vastly different diets, the animals hadn't tuned their clicks to enhance their hunting skills.
Madsen suspects that the dolphin's click spectra depend more on the animals themselves, than the morsels they pursue. For echolocation to work well,dolphins generate a forward directed beam of sound by focusing the clicks through melon structures on their heads. Madsen explains that a click's directionality is related to the ratio of the size of the melon and the wavelength of the click. So small dolphins with small melons tend to produce high-pitched clicks to insure that their sonic beam is as well aimed as larger animals'. Madsen suspects that false killer whales' and Risso's dolphins'clicks are similar because of their similar body sizes, rather than the delicacies they dine on.