Why are Blue Whales singing deeper every year?

[Ignis]

All around the world, blue whales aren’t singing like they used to, and scientists have no idea why.

The largest animals on Earth are singing in ever-deeper voices every year. Among the suggested explanations are ocean noise pollution, changing population dynamics and new mating strategies. But none of them is entirely convincing.

McDonald and his collaborators first noticed the change eight years ago, when they kept needing to recalibrate the automated song detectors used to track blue whales off the California coast. The detectors are triggered by songs that match a particular waveform. Every year, McDonald had to set them lower.

Since then, he and Scripps Institution of Oceanography researchers Sarah Melnick and John Hildebrand have gathered thousands of blue whale recordings made since the 1960s, spanning populations from the North Atlantic to the South Pacific to the East Indian Ocean. Their analysis, published in October in Endangered Species Research, shows that the songs’ tonal frequency is falling every year by a few fractions of a hertz.

-- Wired @ http://www.wired.com/wiredscience/2009/12/blue-whale-song-mystery/

Here's a graph of the tonal frequency decrease since the 1960s:



The shift is appearing across all the geographical groups, but note that Blue Whales off the coast of California have decreased the pitch of their songs by 31 percent since the 1960s.
:wow:



Here's the abstract of the study:
http://www.int-res.com/abstracts/esr/v9/n1/p13-21/
And here's the fully study (PDF required) as published in ESR:
http://www.int-res.com/articles/esr2009/9/n009p013.pdf