In a puzzling development, blue whales around the world are singing in deeper tones. This finding came as researchers analyzed historical recordings of seven types of blue whale songs taken in the Pacific, Atlantic, Southern, and Indian Oceans. The most-extensively monitored type of song, recorded in the eastern North Pacific, dropped in tonal frequency by 31 percent between 1963 and 2008. The other six song types also have steadily moved to lower frequencies, the researchers found.
Why such a widespread shift? Changes in ocean temperature and acidity, which might modify sound propagation through the water, are too slight to explain the trend, the authors say. The team also considered the possibility that blue whales are responding to an increase in human-generated noise in the ocean. But if whales wanted to overcome the noise by singing more loudly, they would be more likely to shift their frequency up rather than down, says coauthor Mark McDonald of WhaleAcoustics.
Another potential explanation is the recovery of whale populations, prompted by the decline of commercial whaling. If whale numbers are rising, the team speculates, males might face more competition for mates. Since females may favor deeper tones, males could be lowering the frequency of their songs to improve their chances of success. McDonald also notes that louder sounds are easier to produce at higher frequencies. In denser populations, males may not need to sing so loudly to reach their listeners—and thus can sing more deeply. ❧
McDonald, M., J. Hildebrand, and S. Mesnick. 2009. Worldwide decline in tonal frequencies of blue whale songs. Endangered Species Research 9:13-21.
image: ©Amos Nachoum