Why Do Whales Sing? Long Structured Sequences That Change Every Year

The song is a hierarchical sequence rather than a call. Individual sounds are grouped into phrases, phrases repeat to form themes, and themes follow in a fixed order to form a song lasting from several minutes to half an hour, which is then repeated for hours. Every male in a population sings essentially the same song at any moment, which is the most striking feature. That shared version changes gradually through a season as small alterations are introduced and adopted, so the song at the end of a season differs substantially from the one at the beginning, and it never reverts. Singing occurs mainly on breeding grounds and mainly from males, which is the basis of most explanations.

Several functions have been suggested with uneven evidence:

• •Attracting females, which is the obvious proposal and which observation supports weakly
• •Competing with or signalling to other males, which some observations support better
• •Coordinating movement between individuals spread over long distances
• •Establishing spacing between males on a breeding ground
• •A combination, with the same signal serving different functions for different listeners
• •Females approaching singers has been observed rarely, which is a problem for the simplest account

The transmission of new versions between populations is documented and is remarkable. Recordings from populations across the South Pacific showed a song type originating off eastern Australia spreading progressively eastward across thousands of kilometres over several years, arriving at each population in turn and replacing whatever they had been singing. That is cultural transmission on a scale documented in no other non-human animal, since an entire population adopts a new version wholesale rather than gradually blending it. How the transmission occurs is not fully established, with contact on shared feeding grounds or at range boundaries being the likely route. Occasional revolutionary changes, where a completely novel song replaces the existing one rapidly, are distinguished from the gradual evolution that occurs within a season.

Song in this elaborate form is unusual and the vocal behaviour of other species is quite different. Blue and fin whales produce simple repeated low-frequency calls rather than structured songs, audible over enormous distances and used apparently for contact and for attracting mates. Sperm whales produce patterned clicks, with sequences that differ between social groups and that function as group identifiers, which is a form of cultural marker documented across ocean basins. Toothed whales generally use clicks for echolocation, producing and receiving them through specialised structures in the head, which is a sensing system rather than communication. Dolphins use individually distinctive whistles that function something like names, which other dolphins copy when addressing them.

The acoustics of the ocean are what make the whole thing possible. Sound travels roughly four and a half times faster in water than in air and is absorbed far less, particularly at low frequencies, so a loud low-frequency call carries enormous distances. A layer at a depth of around a kilometre, where sound speed reaches a minimum, acts as a channel trapping sound and guiding it with very little loss, and calls entering it can theoretically travel across ocean basins. Blue and fin whales produce calls at frequencies low enough to exploit that fully. Shipping noise occupies the same low frequencies and has raised ocean background levels substantially, which reduces the range over which whales can communicate and is a documented and growing concern.

The song is hierarchical, with sounds forming phrases, phrases forming themes and themes following in fixed order for minutes at a time. Every male in a population sings the same version, which changes through a season and never reverts. A song type has been documented spreading eastward across the South Pacific over years, which is cultural transmission on a scale seen in no other non-human animal.