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Whale's Wails Are Out of Tune

Blue and fin whales are among the loudest animals in the oceans, as well as the largest. Only males sing, and their songs can travel more than 1000 km underwater, allowing them to communicate across vast oceans.

Blue whales have been dropping pitch incrementally over several decades, and now a study in the Journal of Geophysical Research: Oceans (https://goo.gl/dEGvqJ) has announced that this is also occurring with fin whales and Madagascan pygmy blue whales – and speculated why this might be occurring.

Lead author Dr Emmanuelle Leroy of the University of NSW analysed more than a million songs from three species of large baleen whale: fin, Antarctic blue and three acoustically distinct populations of pygmy blue whales. Six stationary underwater microphones recorded the calls over 6 years in the southern Indian Ocean, an area spanning 9 million km2.

The study measured the pitch of elements of each species’ song, which had fallen to about 25.6 Hz for the Antarctic blue whale by the end of 2015. In 2002, the pitch of the selected element of the blue whales’ call had been 27.5 Hz, a difference equivalent in Western music to a whole tone or major second interval (for a recording comparing the two see https://goo.gl/xijiPX).

Leroy found that the pitch of Antarctic blue whale calls is falling by 0.14 Hz/year. While fin whales, pygmy blue whales and Antarctic blue whale sing very different songs, the study observed similar trends in call pitch, falling by about 0.12–0.54 Hz/year depending on the species. Low pitches carry farther underwater, but the pitch change is likely to be too small to affect the whale calls over long distances in the ocean and too subtle for the whales to detect any changes.

Unlike most of the world’s oceans, the southern Indian Ocean has grown quieter in recent years, and its shipping traffic is limited. Because the long-term trends in pitch drop are steady around the global range of the whales, the data from the Indian Ocean indicate the ongoing drop cannot be explained as a response to human-generated noise. Instead, the study suggests that the pitch drop could be a by-product of lower call volume because the rebounding whale population doesn’t need to sing as loudly to reach other whales.

Recent population assessments estimate there are 10,000 to 25,000 blue whales globally, up from a few thousand when commercial whaling ceased in the 1970s. “Because the whaling stopped, the whale population is increasing,” Leroy said. “They can decrease their call intensity to keep in touch, because there are more whales.”

Alternatively, the whales may not need to be so loud because the speed and distance that sound travels are affected by the temperature, pressure and chemistry of the ocean. Sound travels further in ocean water that is increasingly acidic due to climate change.

Despite this long-term trend, blue whale calls in the southern Indian Ocean increase in pitch by 0.2–0.3 Hz during the summer, possibly because whales need to sing louder to overcome additional noise in the ocean at this time. “The noise is related to the increasing number of free icebergs in summer,” Leroy explains. “When the ice sometimes cracks, like when you put ice in your drink, it makes noise. This noise is really strong and will propagate over really long distances, so we can hear this noise at our northernmost site, up to 26° South.”

Iceberg crackles at a frequency range that overlaps the pitch of the Antarctic blue whales’ calls. To be heard over the noise, the whales may need to get a little louder, which makes the whales’ pitch rise in summer.

“What's surprising is the long-term and short-term changes could have the same reason, a change in call intensity, but the change responds to two different causes,” Leroy said.