Something curious is going on with the songs of blue whales in oceans all over the world. The whales are singing their same old songs, but year by year they're all shifting the frequency lower.
The blue whale is the largest creature ever to have lived on Earth, but it doesn't follow that everything about them is easy to detect and figure out. The theories on the change in their songs' frequency levels range from the effect of global warming to the increase in ship traffic to rising populations of the mighty creatures.
Then again, it might be something to do with boy meets girl. Only male blue whales sing. The ability to sing at a low frequency and make the song carry a long way is one indicator of large body size, something female whales may favor.
Mark McDonald, an oceanographer, discovered the change in whale frequency about eight years ago when he was working with restricted Navy data. McDonald runs Whale Acoustics, a small business that does research work on ocean noises and whales.
McDonald and his collaborators couldn't publish a report using the restricted data, so they spent several years gathering other blue whale recordings from all over the world.
Whales have regional song types. Within regions, whales maintain their songs over decades. The data showed that in all regions, the frequency was declining over time.
So the scientists started looking for an explanation.
McDonald and co-authors John Hildebrand, a professor of oceanography at the Scripps Institution of Oceanography in San Diego, and Sarah Mesnick, a biologist with the National Oceanic and Atmospheric Administration's Southwest Fisheries Science Center in La Jolla, Calif., describe their finding and the quest to explain it in a recent article in the journal Endangered Species Research.
"There are a number of things that first come to mind. One is that it's a cultural shift — that the whales are changing their song over time more or less randomly, but in a trend direction," McDonald said. "But then it got to be really problematic when we started digging all around the world and hey, they're going in the same direction all around the world and yet they're different song types."
The best records exist for the whales off California and they showed that the whales sing at a frequency 31 percent lower than they did in the 1960s, when blue whales around the world had been commercially hunted to the brink of extinction.
One possible explanation was warming ocean temperatures and changes in ocean chemistry as a result of emissions of greenhouse gases from burning fossil fuels. But the difference of warming temperature was much less than the differences whales encounter as they swim from polar to tropical areas.
"We can't see how climate change can do it. Maybe there's something we're missing," McDonald said.
Another was that the change in frequency was a response to the greater amount of noise in the ocean from shipping.
"The trouble is, every way we can look at it, increasing ocean noise should make them shift to higher frequency, not a lower one," McDonald said. "They're able to make louder sound at higher frequency, so that would get them more easily heard over the noise."
"The best guess we have as a technical physics answer is population, but I'm not at all convinced that's the correct answer," McDonald said. "I really like that it's somewhat mysterious."
The population hypothesis is that blue whale populations have increased since commercial hunting ended, and so blue whales don't have to project their songs as far to be heard.
Under very special circumstances and with two whales in optimum position, they theoretically should be able to detect each other's songs at more than 1,000 miles, McDonald said. The normal range, however, in the mid-latitude North Pacific is about 15 miles and in the Antarctic about 100 miles.
The article noted that blue whales seem to be increasing in numbers in most if not all regions, but that the data for most areas are sparse and uncertain.
The shift in the frequency of their songs still "begs explanation," the scientists concluded.
"We don't necessarily know what's important about it yet," McDonald said. "It could be very important. We have one piece of a much bigger puzzle. Maybe when the next piece of the puzzle gets plugged in, it will all make sense."
ON THE WEB
Video with John Hildebrand and audio of blue whale song
Information and photos from U.S. government scientists on blue whales
Hear a blue whale here, recorded by NOAA
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