To find out, Wahlberg and his team ventured to the arctic waters north of Norway during summer 2000 to measure the sounds from a group of male sperm whales diving and clicking to feast on squid and fish.
"These males are like grumpy old men all eating at the same restaurant but not talking to each other," said co-leader Peter Madsen, a biologist and colleague of Wahlberg at Aarhus University and now a researcher at Woods Hole Oceanographic Institution in Massachusetts.
From seven boats, the scientists lowered hydrophones—underwater microphones—to a depth of about 1,500 feet (460 meters).
Using a global positioning system (GPS), the researchers synchronized their recordings and pinpointed the source of each click as well as its volume. Then they used the clicks to calculate length.
Acoustic ID Tags
Two years ago, Madsen, working with Ocean Alliance, a Lincoln, Massachusetts-based conservation and research organization, placed recording devices on individual sperm whales that surfaced next to his research vessel.
To do so, Madsen took a seat on the boat's 21-foot (6.4-meter) boom and swung out over a pod of sperm whales, wielding a 15-foot (4.5-meter) pole with a recorder on the end.
"The device had a suction cup that I would stick on the whale as soon as it surfaced—it was completely unnerving just sitting over the whale," said Madsen.
This summer Wahlberg presented the study at the Conference on Acoustic Communication by Animals at the University of Maryland in College Park. The data showed that the size of the animal, as based on acoustic readings, is accurate to 92 percent, or within 1.5 feet (50 centimeters).
"Sperm whales click for about 40 minutes out of every hour, and they only surface for minutes at a time—so your odds of hearing them are much better than seeing them," said Peter Tyack, a biologist at Woods Hole, who studies acoustic communication in whales and dolphins.
"With acoustic information we can build a much richer picture of the population," said Tyack. Adult males generate long-pulse intervals; adult females and adolescent males, medium-pulse; and calves, short-pulse.
A whale's length is also useful as an ID tag or fingerprint to identify individual whales when researchers are eavesdropping on them.
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