VIMS

The Mouse that Roared

The blue whale Balaenoptera musculus is the largest animal to ever inhabit our planet, including dinosaurs. Adults can reach 110 feet and weigh up to 400,000 lbs. Even the babies are huge. At birth, a typical blue whale measures 25 feet long and weighs between 6,000 and 8,000 pounds.

The huge size of these animals belies their species name musculus, which means mouse in Latin. Some believe that Linnaeus gave these animals their diminutive name as a sort of taxonomic joke.

A blue whale's voice is commensurate with its bulk. Blues possess the loudest voice in the animal kingdom, emitting a low-frequency roar that can travel for hundreds of miles in deep water.

Allan Sauter of the Scripps Institute of Oceanography is aboard the Palmer to retrieve two acoustic moorings that were deployed last February to eavesdrop on blue whales in the Southern Ocean. The moorings are part of a multi-year project headed by Scripps researcher Dr. John Hildebrand and funded by the National Science Foundation. One mooring lies just off Cape Adare on the northwestern corner of the Ross Sea, the other in much deeper water about halfway between Antarctica and New Zealand.

Hildebrand's project is part of a worldwide effort to better understand the behavior, distribution, abundance, and vocalizations of these once-abundant and now uncommon leviathans. Researchers estimate that 20th-century whaling reduced global blue whale populations by 99 percent. The Southern Ocean once held the world's largest population of blue whales, but logbooks show that whalers took 360,000 blues from the Southern Hemisphere during the whaling era (harvesting of blue whales is now banned on a global basis). Today, the waters around Antarctica are thought to hold only a few hundred of these magnificent creatures, and knowledge about their natural history and ecology in the Southern Ocean is equally sparse.

Efforts to learn more about B. musculus in the Southern Ocean combine acoustic data from moorings with visual data from observers who sail aboard selected Southern Ocean research cruises, including previous IVARS legs. Observers use their knowledge of the size, shape, markings, and spout characteristics of different cetaceans to identify and count any individuals they might see en route. The acoustic moorings employ a low-frequency digital recorder to continually record sounds lower than 250 vibrations per second, or hertz. That's in the range of blue whale vocalizations, which typically vary between 20 and 40 hertz, but far below the range of human hearing, which is most acute at higher frequencies up to 20,000 hertz.

The sounds recorded by the moorings provide basic data on the number of whales in the area; their movements on annual (migratory), seasonal, and daily time scales; the relative number of males and females in the population (as their calls differ); and the calls' purpose.

The purpose of the blue whale's call is not completely clear. Some researchers think the calls are used for long-distance communication, others for imaging features in the environment such as seamounts. In a recent Scripps study in California's Channel Islands, blue whales were observed to repeat a series of "A" and "B" calls to communicate with each other, and a "D" call after feeding (as if to say "Yum!"). (Recordings of these different calls, and the vocalizations of other whales, are available on the web at http://www.cetus.ucsd.edu). Data from California also show that the acoustic frequency of calls has been declining over the last few years, as the blue whale population begins to recover from whaling and individuals once again have the opportunity to reach full adult size. Only the very largest blue whales have the bulk needed to vocalize at the extremely low frequencies that are now starting to be heard.

Data from Southern Ocean moorings have begun to help answer some of the basic questions concerning blue whale behavior in this part of the world. The data reveal that the animals stay in the Southern Ocean year round, and that they lag Minke whales in feeding on the spring plankton bloom. (Both minkes and blues are baleen whales that use long comb-like filters to sieve krill and other zooplankton from the water.)

But more complete knowledge of blue whale behavior in the Southern Ocean awaits many additional moorings and the patient work of many more observers.