Scientists are baffled to why blue whales are changing their tune and singing in deeper voices. A study, published in the journal Endangered Species Research, has found male blue whales all over the world have dropped their tone, even though different populations sing different songs.

Blue whales in the northeast Pacific, including increasing numbers in waters off British Columbia have lowered their voices by 31 per cent between 1963 and 2008. The changes were noticed approx eight years ago when automated blue whale song detectors off the California coast had to be shifted to lower frequencies. If it dropped 50 per cent it would be down an octave.

Researchers have been comparing the songs in areas ranging from the Indian Ocean to the North Atlantic but the cause of the change remains a mystery. One theory is that with their numbers slowly increasing since whaling officially ended in 1966, the males who do all the singing do not have to call as loudly to be heard by females. Blue whale expert John Calambokidis, however doubts that populations have increased sufficiently to explain the change that way. Off the west coast of the U.S. and B.C., the population is believed to have risen slowly from about 2,000 in the 1980s to about 2,500. Blue whales continue to be listed as endangered and the worldwide population is about 10,000, compared with the pre-whaling population of 300,000.

Another theory is that females find the deeper tones more appealing and that there is something about the deeper songs that is attractive to females or deters potential rivals. At the moment however, there is no real answer.