The B call is simpler and easier to analyze. According to Bland, the B calls off Half Moon Bay are at a frequency of about 16 hertz, with no detectable variation between individuals. The researchers have not scrutinized the more complex A call in detail but say it too shows little variation.
In an attempt to get more detail on what blue whales are making the calls and why, John Calambokidis, a whale expert with Casacadia Research in Olympia, Washington, and colleagues with the Scripps Institution of Oceanography and the National Geographic Society are attaching acoustic and video tags to individual whales.
After recording data for several days, the tags detach from the whales and float to the surface, where they are collected by researchers and analyzed.
"One thing we find is that predominantly, if not solely, males are producing the calls," Calambokidis said. In addition, although there is no variation in the pitch of the calls among individuals in a population, the calls do vary among different populations around the globe.
"This may argue against this as some ideal frequency and argue maybe for males using it for advertising to other females or to other males related to reproduction," he said.
For example, males may all be trying to make the same low frequency call as a way to indicate their fitness. Any male that deviates from this frequency would not be considered a good mate and thus rejected, Calambokidis said.
Hildebrand and his colleagues also suggest that blue whale calls are related to mating. According to their theory, the whales make their calls as loud and at as low a frequency as they can. Loud calls travel further, and low frequency may be a desirable trait.
"Of any animal on the planet, the largestblue whaleswill surely have a mechanism to select large animals for breeding. Large animals have large lung volume and can therefore make high-intensity and low-frequency calls," Hildebrand said.
Interestingly, Hildebrand's research also suggests that the blue whale call has been getting lower in pitch over the past 20 to 30 years by about one-half hertz each year. "The cause is controversial, but I think it is a response to recovery from commercial whaling," he said.
Calambokidis said the available information on historical and current blue whale populations is imprecise, but that at the end of the 19th century, the global population was estimated at 300,000. By the time blue whale hunting was banned by the International Whaling Commission in 1966, that figure may have dropped to as low as 10,000.
Today some scientists believe blue whale populations are even lower. Conservationists are keen to keep close tabs on the cetaceans to determine if populations are recovering.
Bland's hope is that identifying individual blue whales from their calls could help conservationists with their monitoring activities. But since only males appear to be making the calls, trying to take a census of the population from calls alone is a difficult task, Calambokidis said.
Further complicating matters, only a small percent of the males in any group are responsible for the majority of the calls according to Calambokidis. In a group of ten whales out feeding, none might be calling whereas one, isolated male might call over and over.
"There's not a good correlation between the number of whales seen in an area and the number of calls being heard," Calambokidis said. "If you hear calls, there is at least 1, but trying to say 1 or 50 is not an easy thing to do."
Calambokidis added that if whale acoustic researchers can determine how to distinguish individuals from their calls, the information combined with better understanding of behavioral data may eventually yield a good method for tracking populations.
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