The songs blue whales use to communicate and attract mates have been dropping in pitch worldwide for decades, and researchers think it might actually be a sign that an endangered population is recovering.
No one is completely sure what whale songs are used for – theories include mating calls, other forms of communication, and possibly a form of sonar. A group of researchers recently examined whale songs from several decades and from all the world's oceans. They found that the frequency, or pitch, of blue whale song has been steadily dropping for many years. Recently recorded whale songs are the lowest, while whale songs from the 1960s were higher in pitch.
The researchers don't know what's causing the change, but they have a theory based on a correlation with blue whale populations. When the songs were at their highest pitch, blue whales had been hunted to the brink of extinction. Since the International Whaling Commission banned blue whale hunting in the 60s, the worldwide blue whale population has been slowly but steadily increasing (though it's still a tiny fraction of what it once was). That seems to coincide with the pitch change.
It could be that whales used a higher frequency song when there were fewer whales because those songs traveled farther, hundreds of miles or more. With a sparse population, you'd need a long-distance call to find more mates or family members. With populations rebounding somewhat, the whales are able to use lower frequency songs with more success, since there's a greater chance another blue whale is nearby.
You may be wondering why higher frequency songs would travel farther, since generally low-frequency sounds are thought to be better for long-distance propagation. I asked the researchers about this, and scientist Mark McDonald explained that whales can sing louder at higher frequencies:
Across the frequencies of blue whale song, the underwater transmission losses are nearly the same regardless of frequency. It is absorption which is the primary cause of frequency dependent transmission losses, rather than dispersion in this case, and the absorption loss only begins to become significant when ranges reach thousands of kilometers. Theory tells us the whales can produce higher amplitude songs at higher frequencies, based on given lung volume.
I was also curious if this was an example of evolution in action, with subsequent generations of whales exhibiting a change in pitch due to natural selection, or if it was a behavioral change, with blue whales choosing to use a lower pitch song. He replied:
We presume it is a behavioral change, but we don't really know why the whales are changing their song frequency. We don't find our own best hypothesis entirely convincing.
Which is a pretty excellent example of science in progress. If only we could figure out what blue whales were singing about, so we could just ask them.