Working in the Atlantic, the Gulf of Mexico and the Ligurian Sea, scientists from Woods Hole Oceanographic Institution and the University of St Andrews attached acoustic recording tags to the dorsal surface of sperm whales with suction cups. The whales were then tracked acoustically with a towed hydrophone array.
The researchers used the tags to record the sounds that sperm whales produce while foraging. As sperm whales descended from the surface, they emitted a regular series of "clicks". When the whales reach the bottom of their dive, these clicks are emitted more often, eventually merging together to form "buzzes" of sound. This pattern reflects the whales homing in on cephalopods such as squid, with the buzzes reflecting the animals' final approach when detailed information on the squid's position and movement are required, the researchers believe.
Dr Stephanie Watwood and colleagues found that sperm whales produced buzzes on every deep dive they made, in all three locations, suggesting that they are highly successful at locating prey in the dark ocean depths.
The sperm whale is the world's largest deep-diving toothed whale, feeding mainly on squid, but until now little has been known about the timing of prey detection and capture during dives.
"Due to the difficulty of observing sperm whales during their long, deep dives, little has been known about their subsurface behaviour, giving rise to an array of speculations on how sperm whales find prey, including luring, touch, passive listening, echolocation and vision. Recording vocalisations of diving sperm whales presents a non-invasive opportunity to document feeding activity." says Watwood.