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 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 blue whales, the largest creature ever to have lived on Earth.
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, runs Whale Acoustics, a business that does research on ocean noises and whales. McDonald and his collaborators spent several years gathering blue whale recordings from all over the world.
Whales have regional song types. The data showed that in all regions, the frequency was declining over time. So the scientists started looking for an explanation.
McDonald and his colleagues describe their quest in the journal Endangered Species Research.
The best records exist for the whales off California. They showed that 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. But whales encounter greater differences as they swim from polar to tropical areas. "We can't see how climate change can do it," McDonald said.
There also is more 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."
Another possibility is that blue whale populations have increased since commercial hunting ended, so blue whales don't have to project their songs as far to be heard.
"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."