Pygmy blue whale off Western Australia. Photo copyright: Micheline Jenner, Centre for Whale Research.
The pygmy blue whale is one of three recognized subspecies of the blue whale and is found in the waters of the Indian Ocean, Southwest Pacific, and eastern portions of the Southern Ocean near the Antarctic Convergence. The whales are thought to breed in the Indian and South Pacific Ocean Basins and travel to the Antarctic Convergence where they feed on krill and small fish. Reaching body lengths of up to 21-24 m (70-80 ft), the pygmy blue whale is slightly smaller in size than the blue whale. The pygmy blue whale also has a larger head, shorter tail, and shorter baleen plates when compared to the blue whale.
Pygmy blue whales produce stereotyped songs that may contain pulsed and tonal components. Songs are comprised of a series of sound “units.” Multiple units are referred to as a “theme.” Themes are repeated in semi-regular intervals to comprise a song. Pygmy blue whale songs can last from 10 minutes to several hours.
Underwater recordings from Comprehensive Nuclear Test Ban Treaty Hydrophone Stations and other underwater acoustic technologies have been used to study pygmy blue whale songs. These recordings provide insight into the acoustic behavior and geographic distribution of the pygmy blue whale. The frequency content and duration of pygmy blue whale units differ by region (Leroy et al. 2021, Leroy et al. 2018, Miksis-Olds et al. 2018, Stafford et al. 2011).
Pygmy blue whale underwater off Western Australia. Photo copyright: Micheline Jenner, Centre for Whale Research.
In the Eastern Indian Ocean pygmy blue whale population, themes with single, double, and triple units have been recorded. For 3-Unit themes in this population, the “introductory sound” is usually the least intense unit, but lasts the longest, up to 50 s. It begins at 20 Hz and increases in steps of about 1 Hz up to 80 Hz. The second sound is the most intense unit in a theme, with a source level estimated at 179 underwater dB. The fundamental frequency of this unit increases from 20 Hz to 24 Hz, with multiple harmonics, and lasts approximately 25 s. The third sound has a fundamental frequency of 19-20 Hz with prominent harmonics. Triple-unit themes repeat on an interval of 170-220 s. However, some songs omit the first unit, reducing the song repetition to 90-100 s. Single unit songs have also been recorded and are repeated every 45-49 s. (Gavrilov et al. 2011, Gavrilov et al. 2013).
The Sri Lankan population of pygmy blue whales in the northern Indian Ocean also produces a 3-unit theme for their song. It begins with a low frequency, pulsed unit, which contains energy at 20-40 Hz and lasts about 18 s. It is a low-level signal that is difficult to detect in recordings. The second unit in the Sri Lankan theme is a frequency-modulated upsweep from approximately 40 to 70 Hz that lasts about 14 s. The third unit, the most prominent part of the Sri Lankan pygmy whale song, consists of a long duration narrowband signal centered around 100 Hz, lasts 20-30 s, and is often used as an indicator of whale presence. (Miksis-Olds 2018, Stafford et al. 2011).
In Western Australia, non-song calls not repeated in regular intervals have also been described for pygmy blue whales. These include a pulsed call similar to the “D-call” for blue whale populations in the northern Pacific. These pulsed calls of the pygmy blue whales last about 2-4 s, with a down swept signal starting at a fundamental frequency of 70 to 100 Hz then rapidly decreasing to 20-50Hz. (Gavrilov et al. 2011, Recalde-Salas 2014)
Changes in song characteristics and inter-song-intervals have been documented for some populations of pygmy blue whales. Long term observations show a decrease in fundamental frequency of approximately 0.12 Hz per year in the second unit of pygmy blue whale song (Gavrilov et al. 2011 and 2013, Joliffe et al. 2019). This is similar to frequency shifts that have been documented for blue whales and other baleen whales since the early 2000s. The reason for these changes in whale song remains unclear.