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Figure 2. Track of a calling blue whale. This individual whale had a distinctive call that allowed US Navy acoustics expert Chuck Gagnon to track it as it swam >3,000 km in the North Atlantic over 43 days. Cour- tesy of Christopher Clark, Cornell University, and Clyde Nishimura, US Naval Research Laboratory.
km track of a blue whale whose distinctive call was localized over 43 days using Navy listening stations.
These studies of long-range propagation of whale calls were a technological tour de force, but we still do not know much about the effective range of communication when the whales are the receivers. Fin whales on the feeding ground have been observed from an airplane rapidly swimming toward a group of whales that were feeding 10 km away (Watkins and Schevill, 1979); they might have been responding to calls, but this could not be measured from the plane. Tyack and Whitehead (1983) used several boats, each following a dif- ferent group of humpback whales on the Hawaiian breeding grounds to show that one whale accelerated and made a bee- line for a large active group 9 km away. These observations represent the limit of our observations of potential call and response in whales. We still do not have good methods for determining whether a whale is hearing and responding to calls of another at ranges of tens or hundreds of kilometers.
Song of Humpback Whales
Unlike the low-frequency calls of fin and blue whales, humpback whales produce sounds with a broad frequency band, up to several kilohertz (http://www.dosits.org/files/ dosits/hump1.mp3). The high-frequency components of the humpback sounds have higher rates of absorption as they
pass through seawater. In addition, humpback whales tend to congregate in shallow coastal waters where propagation is not as advantageous as in the deep sound channel. These fac- tors limit the detection range of the full frequency bandwidth of humpback song to a few tens of kilometers compared with hundreds of kilometers for the fin and blue whale calls.
Humpback songs are complex not only in their broadband frequency structure but also in having a long complex se- quence of sounds that may continue for 5-15 min before re- peating (link to file http://acousticstoday.org/hbksong). An- other form of complexity is that the song is always changing throughout the singing season. At any one time, most sing- ers within a population sing very similar songs, but the song changes over time. Different parts of the song come and go at different times, but often the changes are clear enough to define new song types for different years. The only way this process of change can occur is if humpback males copy the songs they hear around them, all tracking the changes to- gether. This kind of vocal learning is common among song- birds but rare among mammals (Tyack, 2016).
Since the 1970s, bioacousticians have recorded the songs of humpback whales from local populations in tropical breed- ing grounds. Comparisons showed that songs were similar from different sites within an ocean basin, such as the West Indies and Cape Verde Islands in the North Atlantic or Ha- waii and the Pacific coast of Mexico, but songs differed across different ocean basins, such as the North Atlantic, North Pa- cific, or South Pacific. Not only are the songs similar within an ocean basin, but the trends of song change also are simi- lar (Cerchio et al., 2001). Because the full bandwidth of the songs does not propagate farther than tens of kilometers, the assumption was that whales must mix across breeding sites separated by thousands of kilometers within an ocean basin.
If humpback males learn their song by listening to those of other males, then it qualifies as a form of cultural transmis- sion, a trait shared through social learning. For nearly a cen- tury, anthropologists and other social scientists jealously re- served the word “culture” for humans alone, but evidence of tool making and other cultures in anthropoid apes have led to more acceptance of the idea of animal cultures. Learned vocal traditions such as song dialects qualify as forms of ani- mal culture, and the constant change of the songs of hump- back whales facilitates the study of cultural transmission of song. Interest in transmission of animal cultures led bioac- ousticians working in 6 humpback breeding grounds span- ning a distance of more than 6,000 km in the South Pacific to
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