linguistic:paraphrasing
Vocalizations

Blue whales produce some of the loudest and lowest frequency vocalizations in the animal kingdom, and their inner ears appear well adapted for detecting low-frequency sounds. The fundamental frequency for blue whale vocalizations ranges from 8 to 25 Hz. Blue whale songs vary between populations.

Vocalizations produced by the Eastern North Pacific population have been well studied. This population produces pulsed calls ("A") and tonal calls ("B"), upswept tones that precede type B calls ("C") and separate downswept tones ("D"). A and B calls are often produced in repeated co-occurring sequences and sung only by males, suggesting a reproductive function. D calls may have multiple functions. They are produced by both sexes during social interactions while feeding. and by males when competing for mates.

Blue whale calls recorded off Sri Lanka have a three-unit phrase. The first unit is a 19.8 to 43.5 Hz pulsive call, and is normally 17.9 ± 5.2 seconds long. The second unit is a 55.9 to 72.4 Hz FM upsweep that is 13.8 ± 1.1 seconds long. The final unit is 28.5 ± 1.6 seconds long with a tone of 108 to 104.7 Hz. A blue whale call recorded off Madagascar, a two-unit phrase, consists of 5–7 pulses with a center frequency of 35.1 ± 0.7 Hz lasting 4.4 ± 0.5 seconds proceeding a 35 ± 0 Hz tone that is 10.9 ± 1.1 seconds long. In the Southern Ocean, blue whales produce 18-second vocals which start with a 9-second-long, 27 Hz tone, and then a 1-second downsweep to 19 Hz, followed by a downsweep further to 18 Hz. Other vocalizations include 1–4 second long, frequency-modulated calls with a frequency of 80 and 38 Hz.

There is evidence that some blue whale songs have temporally declined in tonal frequency. The vocalization of blue whales in the Eastern North Pacific decreased in tonal frequency by 31% from the early 1960s to the early 21st century. The frequency of pygmy blue whales in the Antarctic has decreased by a few tenths of a hertz every year starting in 2002. It is possible that as blue whale populations recover from whaling, there is increasing sexual selection pressure (i.e., a lower frequency indicates a larger body size).