Blue whales around the world are singing a little flat, according to a study which found that increase in their population and climate change may be to blame.
The study, published in the Journal of Geophysical Research: Oceans, found that there is a seasonal variation in the whales' pitch correlated with breaking sea ice in the southern Indian Ocean.
Researchers from the University of New South Wales in Australia also extended the mysterious long-term falling pitch to related baleen whales and rules out noise pollution as the cause of the global long-term trend.
Blue whales have been dropping pitch incrementally over several decades, but the cause has remained a mystery.
The study found the same mysterious long-term trend of falling pitch in fin whales and Madagascan pygmy blue whales.
Pitch, or the perception of how high or low a note sounds, is a result of the frequency of the sound wave, usually measured in hertz.
The study used data from the southern Indian Ocean to rule out noise pollution as the cause of the pitch change.
The research suggests the pitch drop is an anatomical consequence of singing less loudly.
The whales' calls could be quieter due to growing numbers of whales or changes in the ocean due to climate change, according to the study.
"We think it is something non-voluntary from the whale. Decrease the call intensity and it will decrease the call frequency, just because of the sound emission mechanism," said Emmanuelle Leroy, a research fellow at the University of New South Wales.
The research also uncovers a seasonal counterpoint in the calls of Antarctic blue whales, potentially related to the noise of melting sea ice.
The study found that blue whale calls in the southern Indian Ocean increase in pitch during the summer.
The pitch could be increasing as whales sing louder to be heard over breaking sea ice, according to the study.
"Our hypothesis is that the call frequency change is again linked to call intensity and that the whale will adapt the call intensity to the variation of noise level," Leroy said.
"The noise is related to the increasing number of free icebergs in summer. When the ice sometimes cracks, like when you put ice in your drink, it makes noise," Leroy said.
"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 degrees south, " he said.
The study analysed more than one 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 six years, from 2010 to 2015, in the southern Indian Ocean, an area spanning nine million square kilometers.
The stereotypical song of the Antarctic blue whale spans about 15 to 30 hertz, at the very bottom edge of human hearing, which ranges from about 20-20,000 hertz.
Blue whale songs are in the range of the lowest, longest pipes of large cathedral organs.
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