For more than 40 years, researchers have been following a complex underwater song that is constantly shifting and reshaping as each season passes. This song, which can range anywhere from 10-30 minutes long, is performed solely by male humpback whales -- but for a reason that currently eludes scientists. During the song, humpbacks produce an intricate series of sounds ranging from high frequency squeals to deep, low frequency rumbles. The structure is rigid and predictable, and researchers have deconstructed its components into hierarchal elements. The base units (or notes) are singular units of sound which are linked together to form what is referred to as a sub-phrase. Sub-phrases contain 4-6 notes, and a pair of these groupings is called a phrase. Humpbacks tend to repeat phrases perfectly over and over for up to 4 minutes, and the repetition of a select phrases leads to a theme. The male humpback song is then composed of a collection of various themes, repeated in specific order, delivered with similar musical devices that we see in human created song: variation in tempo, crescendo, and emphasis for example. As far as we know, humpback whales are the only animal other than humans to create such complex, hierarchal patterns of sound.
What makes the song even more fascinating is its evolution between seasons. In any given area, in any given period of time, all singers will perform nearly identical versions of the song. It is most commonly sung during the mating season, but undergoes surprising transformations between years. Sometimes the song will only change subtlety – slight variations in tone or volume. Other years, the song is unrecognizable. Sections may completely disappear, and new themes incorporated. Regardless of the scale of change, however, all singers within the same geographical region will adopt the same adjustments.
Although researchers have managed to understand and monitor the basic song structure, there are many aspects of the song that continue to puzzle scientists. It is unclear why the same song evolves each year, or who initiates the changes. Why are some changes accepted while others ignored? And most importantly: what purpose the song serves. Since it is sung primarily during the mating season, it is presumed that it may be related to sexual selection, but performances may also act to strengthen bonds between male humpbacks. It may also convey information about the individual singer. But for now, the song is a mystery that scientists are trying to unravel, note by note.