Red shift, blue whale
| 16 Jan 2016Variation of whale-song pitch not fully explained by Doppler shift
Whale songs, the deep, reverberating vocalisations made by the largest mammal, have been the subject of research for decades. Recordings of the songs of the Antarctic blue whale show a pronounced drop in pitch between March and December before a “reset” over the following January and February. In addition to this intra-annual variation, the recordings also show an overall drop in pitch over several years. Several hypotheses have been proposed to explain both patterns. Marine biologists recently took to the oceans to test explanations for the intra-annual variation, including the idea that the changes could be caused by Doppler shifts.
The words “Doppler shift” bring to mind the siren of an ambulance, seemingly getting higher in pitch as it approaches you and then lower as it rushes past. This perceived change in sound is caused by the relative motion between the source of the sound and the listener: as the source and listener move closer to one another, the sound waves emitted get compressed, causing a rise in pitch, while relative motion away results in the emitted waves expanding and lowering the pitch. The phenomenon can also be observed in electromagnetic waves: stars that move towards us have their light shifted to the blue end of the spectrum (blue shift) and light from those stars that move away from us is shifted to the red end of the spectrum (red shift).
Antarctic blue whales are believed to migrate northwards to warmer waters during southern-hemisphere winters. Scientists have hypothesised that differences in whale-song pitch during a given year are caused by this movement of the mighty creatures. According to this hypothesis, as the whales travel towards sound-recording stations, the pitch of their calls appears to increase, with a corresponding decrease in pitch detected when their movement is away from the recorders.
A blue whale. Image courtesy of the NOAA Photo Library / Public domain (via Wikimedia Commons)
To test this hypothesis, members of the 2013 Antarctic Blue Whale Voyage of the Southern Ocean Research Partnership recorded the songs of blue whales using “sonobuoys” while also tracking their positions with special video cameras. By carefully measuring (or accurately estimating, when measurements were unavailable) the relative motion of the recording buoys and the whales, the scientists were able to determine how well the intra-annual whale-song-pitch variations could be attributed to the Doppler shift. However, although the variations followed the pattern predicted by the Doppler shift, they were also much greater than expected.
Environmental and biological explanations for the intra-annual variation were also considered. Measurements of the whale-song pitch from dispersed, static recording stations ruled out environmental factors (such as temperature and salinity of water), since varied locations provided similar data. Yet another possibility is that the changes in song pitch correspond to feeding patterns of the whales. The minimum and maximum pitches seem to coincide with the arrival and departure of the whales to their feeding grounds. Feeding habits would correspond to blubber thickness, and monthly measurements of this thickness correlated with the monthly change in pitch for whales less than 19 metres long but not for those more than 23 metres long. Combined with these data on blubber thickness, the Doppler-shift explanation for changes in whale-song pitch is certainly promising, though it doesn’t it tell the whole story.
Null results, or in this case inconclusive ones, are crucial to science. They help scientists refine their theories and discard unsatisfactory assumptions. Also, future researchers are better equipped to take advantage of this work and correct for Doppler shifts when measuring the pitch of whale songs.
Citation:
Miller, H.S., Leaper, R., Calderan, S., & Gedamke, J. (2014). Red shift, blue shift: investigating Doppler shifts, blubber thickness, and migration as explanations of seasonal variation in the tonality of Antarctic blue whale song. PLOS One, 9(9), e107740. DOI: 10.1371/journal.pone.0107740