The function of whale song, even the better-studied song of the humpback whale, has long baffled marine scientists. Songs of the blue whale, the planets largest living creature, can be divided into at least 10 types worldwide, each type retaining the same units and similar phrasing over decades, unlike humpback whale song which changes substantially from year to year. That is until recently with a worldwide occurrence of a nearly linear downward shift in the tonal frequencies of blue whale song.
“We don’t have the answer. We just have a lot of recordings,” Whale Acoustics President Mark McDonald explained to Wired. The company specializes in analyzing the cetaceans with sonic monitoring networks and ships. Their recordings are grist for experts hoping to unravel this mystery. “It’s a fascinating finding. It’s even more remarkable, given that the songs themselves differ in different oceans. There seem to be these distinct populations, yet they’re all showing this common shift,” Cascadia Research Collective blue-whale expert John Calombokidis adds.
Historical acoustic recordings dating back as faras the 1960s were examined, measuring the tonal frequencies of 1000s of blue whale songs. Within a given year, individuals match the song frequency (related to ‘pitch’ in musical nomenclature) to within less than 3%.
The best documented song type, that observed offshore of California, USA, now is sung at a frequency 31% lower than it was in the 1960s. Data available for 7 of the world’s 10 known song types show they are all shifting downward in frequency, though at different rates.
Limited observations of singing blue whales suggest usage patterns different from those of humpback whale song and are even more difficult to explain. All singers for which sex has been determined have been males, although both sexes produce non-song calls . Singers have always been found to be traveling at relatively high speed, whereas non-song calls are commonly produced by milling or feeding blue whales
Theories as to why this isa occuring include sexual selection, increasing ocean noise, increasing whale body size post whaling, global warming, interference from other animal sounds and post-whaling era increases in abundance. None of the commonly suggested hypotheses were found to provide a full explanation; however, increasing population size post whaling provides an intriguing and testable hypothesis that recovery is altering the sexually selected tradeoff for
singing males between song amplitude (the ability to be heard at a greater distance) and song
frequency (the ability to produce songs of lower pitch).