SAN FRANCISCO, Aug. 22 (Xinhua) -- New research may explain a gradual lowering of the frequency of blue whale calls documented by scientists for the past two decades, but hardly explains why the largest animals to have ever lived on Earth make the change.
In a study published this week in Scientific Reports, a group of acoustic researchers at Oregon State University (OSU) showed that whales can control the frequency of their calls by blowing air through their vocal cords at a faster or slower rate, in a much different way than previously thought.
Previously, it was thought that whales generated their calls mostly by resonating sound in large chambers or cavities in their upper respiratory system.
Made possible when the researchers at OSU's Hatfield Marine Science Center recorded a blue whale call, then created a model to replicate that sound based on a series of controlled air bursts from the animal's vocal cords, the new findings imply that the frequency of the whale's calls are dictated by the size of the animal - the lower the frequency, the bigger the animal.
In addition, according to Robert Dziak, an acoustics scientist with the National Oceanic and Atmospheric Administration (NOAA) and lead author on the study, the research shows that blue whales can change frequency in the middle of their call, by pulsing air through their vocal cords, and that the change in the frequency might be cognitive, meaning they are choosing to make it higher or lower in response to some sort of environmental stimulus.
Having created similar acoustic models to replicate the sounds of icebergs scraping across the seafloor as well as the explosions from undersea volcanic eruptions, Dziak and his colleagues began to recreate the sound of a blue whale by using a clear call from a blue whale off Yaquina Head near Newport, a city on the central coast of Oregon, a state in the U.S. Pacific Northwest, that they recorded using an undersea hydrophone that was part of a study to monitor the environmental impacts of wave energy.
The research group then developed acoustical models of the whale sound and incorporated anatomical respiratory system models of blue whales. "Only when we pulsed air through the process of opening and closing the vocal cords did we get a way to produce sounds that can change frequencies in mid-call as well as remove overtones," Dziak was quoted as saying in a news release from OSU on Tuesday. "And this method produced models that matched the natural Yaquina Head blue whale call very, very closely."
Why the change? Joe Haxel, an OSU acoustics specialist at Hatfield Marine Science Center, speculates that as blue whale populations recover from commercial whaling, there are more of them and the lowering of frequency and other unusual characteristics of the calls are related to changes in population, or that it may be possible the whales are modulating their vocalization frequency in response to an increase in human-generated noise, essentially trying to find "a radio channel" that has less static to communicate in.