Experts say that the pitch level of the sounds whales send across the vast expanses of the ocean to attract potential mates has been steadily creeping downward for the past few decades.
Mark McDonald, of WhaleAcoustics in Bellvue, Colorado, and a team of researchers studied blue whale song data from around the world and discovered a downward curve in the pitch, or frequency, of the songs.
The decline was tracked in blue whales across the globe, from off the southern California coast to the Indian and Southern Oceans.
One of the team, John Hildebrand from Scripps Institute of Oceanography in California, said: “The basic style of singing is the same, the tones are there, but the animal is shifting the frequency down over time.
“The more recent it is, the lower the frequency the animal is singing in, and we have found that in every song we have data for.”
The researchers examined a list of possible causes for the frequency drop from climate change to a rise in human-produced ocean noise and believe it may be explained by the increase of blue whale numbers because of bans on commercial whaling activities.
The team said while the function of blue whale songs is not known and scientists have much more to learn, they do know that all singers are males and that the high-intensity, or loud, and low-frequency songs reach long distances across the ocean.
Blue whales are widely dispersed during the breeding season and it is likely that songs function to show which species is singing and the location of the singing whale, they added.
Mr Hildebrand added: “In the heyday of commercial whaling, as blue whale numbers plummeted, it may have been advantageous for males to sing higher frequency songs, in order to maximise their transmission distance and their ability to locate potential mates
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