Sperm Whale "Voices" Used to Gauge Whales' Sizes

Bijal P. Trivedi
National Geographic On Assignment
November 3, 2003
The sperm whale belts out the loudest sounds in the animal
kingdom—booming clicks, quite different from the tonal songs of the
humpback whales, which can carry up to six miles (ten kilometers)
through the depths.

Sperm whales use different types of clicks for communication and for finding food. The loudest of the clicks work like biosonar to help identify squid and fish, anything from sardines to sharks.

These clicks can measure about 230 decibels underwater, equivalent to 170 decibels on land—"about as loud as a rifle shot three feet from your ear," said Magnus Wahlberg of Aarhus University in Denmark, a biologist and a co-leader of the sometimes frightening quest to listen in on the whales.

Wahlberg and his colleagues have found a way to use the clicks to gauge the size and possibly the age of the whales, teasing data from a population largely hidden from human eyes. The new research may also help reveal whether human noise pollution from sources like military and civilian sonar might affect the whales.

Not until the 1950s did scientists associate the big clicks with sperm whales.

"People heard all these knocking sounds but had no idea that these came from one of the biggest mammals on Earth," said Wahlberg. "There was no easy way to connect the sounds with what was down there."

As big as the sounds are, their intensity is difficult to measure. "[They're] emitted in a narrow beam like a flashlight, and you have to be directly in front of the animal to measure the full magnitude of the sound," said Wahlberg. "You have to listen for a long time and be very lucky."

From Clicks to Size

The team's research has conclusively established that sperm whales are indeed the loudest creatures on Earth.

The clicks are made up of sound pulses reflected between air sacs within the whales' nose, which can grow to about 15 feet (4.5 meters) long. Researchers can use the interval between pulses to calculate the length of the whale's nose, from which they derive the whale's length.

The theory behind the technique dates from the 1970s. In 1991, Jonathan Gordon, a zoologist with the University of St. Andrews, in Scotland, developed the equation to calculate length from click size.

But nobody knew whether the technique was accurate.

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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