The songs of whales near the Antarctic have been changing over the past two decades and a new international study has suggested that climate change might be responsible.
The study, conducted by researchers at the French National Centre for Scientific Research (CNRS), recorded the songs of populations of fin whales, Antarctic blue whales, and pygmy blue whales in the southern Indian Ocean.
The researchers analysed more than a million vocalisations from these whales and found that the pitch of songs has decreased by a few tenths of a hertz every year since recording began in 2002.
The study also found that pitch had increased unnaturally in the warmer months from October to February.
Only male blue whales and fin whales sing and their songs can travel for more than 1,000 kilometres underwater. Blue whales usually sing at a frequency between 15 and 30 hertz, which is at the very bottom register of human hearing (which picks up sounds between 20 and 20,000 hertz).
Seasonal frequency shifts are common among these whales but the changes in the whale songs appeared to extend beyond normal seasonal variation.
Emmanuelle Leroy, who led the research and is now based at the University of New South Wales, told BuzzFeed News that the researchers were surprised to see growing differences in the pitch of the whale calls both in the long-term and a more obvious variance between the seasons.
Leroy said that the whale songs have dropped in pitch and loudness during the colder months and become louder and more high-pitched during summer.
The researchers believe this seasonal variance is most likely becoming more obvious because of climate change and its effects in the ocean environment.
When carbon from the atmosphere is absorbed into the ocean it dissolves to form carbonic acid and this increases the acidity of the water. Leroy believes this means the whales don't have to sing as loud and they produce deeper tones.
"When the ocean is more acidic, the propagation of low-frequency sounds like the whale song is increased and travels further," said Leroy.
However, during summer, the whales are increasingly being forced to compete with the sound of icebergs cracking and falling apart. According to the researchers, when icebergs melt and crack, they are the loudest sound in the marine environment.
Leroy said researchers have ruled out the possibility that noise pollution from human shipping traffic could be to blame because shipping in the southern Indian Ocean has actually become more restricted in recent years.
While it's possible that the change in calls is simply a consequence of increasing whale populations following a steep decrease from destructive whaling practices in the early 20th century, Leroy said that the seasonal fluctuations make it more likely that the song changes are associated with the warming ocean.