Antarctic blue whales are believed to sing louder to be heard over melting sea ice as a result of climate change, a new study suggests.
Scientists analysed more than a million recordings of songs from three species of large baleen whales, including the fin whale, the Antarctic blue whale, and three populations of pygmy blue whales.
The study, published in the American Geophysical Union's (AGU) Journal of Geophysical Research: Oceans, found Antarctic blue whale calls in the southern Indian Ocean increase in pitch during the summer.
Researchers believe the pitch could be increasing as whales sing louder to be heard over breaking sea ice.
"Our hypothesis is that… the whale will adapt the call intensity to the variation of noise level," said Dr Emmanuelle Leroy, a research fellow at the University of New South Wales.
"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.
"This noise is really strong and will propagate over really long distances."
Overall, the researchers noted how the past decades have shown "a progressive decrease in the frequencies of blue whale vocalisations" worldwide.
Antarctic blue whale's sing at a frequency between 15 to 30 hertz, right at the bottom of humans' hearing range.
The study measured the pitch of selected elements of each species of whales' song, which had measurably fallen to about 25.6 hertz for the Antarctic blue and 96 hertz for the fin whale by the end of 2015.
According to the team, in 2002 the blue whales' call was closer to 27.5 hertz, and the recordings show that it has been consistently falling at about 0.14 hertz per year.
"Though fin whales, pygmy blue whales and Antarctic blue whale sing very different songs, the new study observed similar trends in call pitch, falling about 0.12 to 0.54 hertz per year," according to the scientists.
The AGU has also published audio recorded by underwater microphones installed by the Comprehensive Nuclear-Test-Ban Treaty Organisation, where the difference can be heard by the human ear.
The songs can travel more than 1,000km (621 miles) underwater and allow whales to communicate across enormous expanses of ocean.
Scientists believe that the long-term trend could be a byproduct of the whale population growing globally, as they do not need to sing as loudly to reach other whales.
The most recently population estimates for blue whales suggest there could be as many as 25,000 globally.
This is up significantly from the few thousand there were when commercial whaling stopped at the end of the 1970s, although still less than 10% of the total population before the 20th century.
Dr Leroy said: "Because the whaling stopped, the whale population is increasing. They can decrease their call intensity to keep in touch, because there are more whales. These calls are long distance communication."
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But an alternate hypothesis regarding climate change may also explain the change, Dr Leroy added.
Sound travels further in water made increasingly acidic by climate change and as climate changes affects the chemistry of the ocean, the speed and distance that sound travels is also being affected too.