| Focus 2/2 | Listening to cetaceans

The decline in the frequency of blue whale songs


MALIGE Franck, Doctor, Associate researcher at the CNRS Computer Science and Systems Laboratory, DYNI team, Mathematics teacher at the Victor Hugo high school, Marseille

Some of the sounds emitted by blue whales are complex and are repeated at regular intervals. They are called songs, in reference to the birds’ songs, often in a series of identical, structured and regularly spaced emissions over time. However, while bird songs are repeated after a few seconds, blue whale songs are composed of several infrasound parts that last around a minute and are repeated in series that can last for hours. The role of these songs for the species is not yet very clear but could be related to reproduction, since only males seem to sing [1].

In recent decades, the efforts of the marine bioacoustics community have led to the understanding that these sounds are not the same in different parts of the world [2] : there are about ten distinct types of songs, distributed in all oceans (except the Arctic Ocean). Each of these songs is related to a particular area, for example the “New Zealand” song, the “North Indian Ocean” song, or the “North-West Pacific” song. The question of whether the different groups corresponding to the different songs are of different species or have genetic and behavioural variations is being studied. These songs have an extremely stable structure that does not change much over the years.

Video: Capsule of an example of the “Chilean” song (SEP2, Song of the Southeast Pacific No. 2) recorded in Chañaral de Aceituno, Atacama region, Chile in early February 2017. Disclaimer: A computer’s internal speakers are often not sufficient to hear low-pitched sounds properly. In this case, use external speakers or good headphones.

In 2009, however, it was noted that these songs undergo a very regular and intriguing long-term variation: they are increasingly low-pitched [3]. This decrease in frequency is linear over time and concerns all types of songs around the world. On the other hand, the frequencies of all songs, and even of all parts of the same song, do not decrease at the same rate. This decrease is not at all anecdotal: the characteristic song of the northeast Pacific has seen its frequency decrease by about 30% in 45 years.

For the time being, no satisfactory explanation has been given for this phenomenon, although many hypotheses have been put forward.

The first hypothesis is related to the end of the hunt: records of blue whales begin around the time their hunt stopped (in the 1960s). It is possible that the return to a safer environment, or the trauma associated with this practice, or a gradual increase in the density of individuals may explain a change in vocal behaviour [3].

The second hypothesis is related to noise in the ocean: since the appearance of the first ship engines, the overall acoustic energy present in the ocean has only increased, making communication between marine animals more difficult. However, this hypothesis is not very well supported because the variation in sound volume in the ocean is very variable from one area to another (there are even areas where the background noise has decreased) and the phenomenon of decreasing frequencies is on the contrary quite homogeneous [4].

Other hypotheses have been put forward: ocean acidification, avoidance of vocalisation by other species, natural selection, increase in the average size of individuals after the end of the hunt, change in the depth of song emission. However, none of these hypotheses seems to be able to explain this phenomenon [4]. It is very possible that this decrease is the result of a compromise between two trends. A male may be more likely to attract a female by singing deeper (this is a known mechanism for other species). But there may also be a benefit to the species as a whole in having the same frequency for all: in this case, the whales could take advantage of the highly stereotyped broadcasts to extract information about the position of the long-range transmitter, which is crucial for finding their way in the ocean [5].

While the efforts of the marine bioacoustic community have provided many insights into their behaviour and biology, the reason for the decline in song frequency is one of the most important outstanding issues facing blue whales. Given the very high rate of this decline for certain dialects (e.g. North Pacific dialects, if they were still following this trend, should approach 0 Hz shortly after 2100), we may well witness a drastic change in this evolution within a few years. To be continued!

Notes and References

[1] Oleson, E. M. et al. Behavioral context of call production by eastern north pacific blue whales, Mar. Ecol. Prog. Ser 330, 269-284 (2007).

[2] McDonald et al. Biogeographic characterization of blue whale song worldwide: using song to identify populations, J. Cetacean Res. Manage., 55–65 (2006).

[3] McDonald et al. Worldwide decline in tonal frequencies of blue whale songs, Endangered species research 9, 13-21 (2009).

[4] Two unit analysis of Sri Lankan pygmy blue whale song over a decade, J. Acoust. Soc. Am. 144, 3618-3626 (2018).

[5] Malige et al. Inter-annual decrease in pulse rate and peak frequency of Southeast Pacific blue whale song types, Scientific reports, 10:8121, (2020)