A 2009 paper from the Scripps Institution of Oceanography, published in the journal Endangered Species Research (ESR) described the drop in musical pitch (31%) of songs of the Blue Whale (Balaenoptera musculus) over a period of 50 years or so.
Various suggestions have been put forward to explain the downward pitch-shift.
Any behavioral, ecological, oceanographic or anthropogenic change hypothesis seeking to explain the observed shifts should account for the worldwide occurrence of a nearly linear downward shift in the tonal frequencies of blue whale song. Hypotheses examined consider sexual selection, increasing ocean noise, increasing whale body size post whaling, global warming, interference from other animal sounds and post whaling increases in abundance. None of the commonly suggested hypotheses were found to provide a full explanation;
See: Worldwide decline in tonal frequencies of blue whale songs
Since the publication of the original paper, musical pitch declines have been observed in several other species ( e.g. Pygmy Blue Whale ).
The phenomenon remains unexplained.
Also see :
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