A team of researchers from Australia and New Zealand has tracked scores of blue whales off Antarctica, eavesdropping as the world's largest animals began their rumbling song, which can be detected up to 750km away.
During the six-week Australia-New Zealand Antarctic Ecosystem Voyage to the Southern Ocean the Antarctic marine scientists listened for the low moans of the endangered whales, and then tracked them to their feeding grounds.
The scientists were able to identify 58 individual blue whales and record more than 40,000 calls during the voyage, which aimed to help estimate the population size of the endangered animals and their movements.
But at first there was very little of the song, making those onboard anxious about whether they would be able to study the animals.
"What actually was remarkable this time was that very early on in the voyage there were very few calls and that made us nervous," the Australian Antarctic Division's Mike Double said.
"And then it was like flicking a switch. We didn't change any of our equipment and then suddenly the whales were calling. And then we were getting large numbers of calls and that allowed us to detect them," he added.
Mr Double thinks the animals were in the area all along, just not making any noise: "All of a sudden it (the song) is really ramped up and we don't really know why that occurs -- it may be linked to the start of the breeding season or it might be associated with foraging."
He said the expedition had been hoping to see a small number of the blue whales and was thrilled when they came across about 80 in a small area after travelling a long way with no sightings.
He said it was possible there were more than 100 in the 100km by 150km area, adding that researchers were intrigued as to what had brought the animals there.
They had noted their foodstuff krill was "really tightly aggregated into very dense swarms".
"And we didn't really see that in other areas throughout the voyage," Mr Double said.
The voyage was a collaboration between Antarctica New Zealand, the New Zealand National Institute of Water and Atmospheric Research and the Australian Antarctic Division.
"It's not just about finding whales," voyage leader Richard O'Driscoll said of the mission, which also gathered oceanographic and atmospheric data. "We now have a much greater understanding of the big picture -- the ecosystem -- on which the top predators depend."