Humpback whales are famous for their songs, rich in melodic trills and grunts. But new research shows they may ignore warnings from military ships designed to encourage them to swim away from potentially deafening sonar.

Key points: 'Ramp-up' sonar, which gradually increases in volume, is used to warn whales to swim away before it becomes deafening

'Ramp-up' sonar, which gradually increases in volume, is used to warn whales to swim away before it becomes deafening Only five out of 11 tagged and tracked humpbacks swam away when first approached by ramp-up sonar

Only five out of 11 tagged and tracked humpbacks swam away when first approached by ramp-up sonar Whales may become distracted by food or get used to the noise

The findings come from European researchers who tagged wild humpback whales (Megaptera novaeangliae) in the Arctic and tracked them while steering a sonar-blaring ship towards their path.

In some cases, the ship used a 'ramp-up' strategy to warn the whales — instead of turning on their sonar full blast, its volume was gradually increased.

But the study found fewer than half of the whales took evasive action in response to their first warning.

And of those that did, all but one ignored the sonic warning when it was played a second time, suggesting the animals may get used to the noise.

The work, published in the Journal of Experimental Biology today, shows how underwater noise can elicit a range of responses from humpback whales, commented Michael Noad, a University of Queensland marine biologist specialising in whales.

With this in mind, we need to be more mindful of the amount of noise we produce, he added.

"It really is an insidious, invisible form of pollution, and people just don't realise that we make so much noise in the ocean."

Biologists think loud noises from human activity can deafen whales, just like how listening to blaring music can cause hearing loss in us.

Apart from the military, fossil fuel exploration companies also use sound waves — to look for gas and oil reservoirs beneath the ocean floor.

Then there are the thunderous rumbles from pile drivers, drilling foundations for off-shore wind farms, which can also blast through the ocean.

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Although there is very little direct data on what whales hear, damage to their hearing can be estimated by extrapolating what is known from smaller species to the whale's ear anatomy.

"By seeing how whales behave when confronted with naval sonar, we get a much better idea of other potential problems that these sounds could be causing," said Dr Noad.

The 'ramp-up' strategy is designed to mitigate whale hearing loss by warning animals in its path to veer away before the sound reaches deafening levels.

But this is the first study to test its effectiveness in naval sonar specifically, said Paul Wensveen, marine biologist at the Universities of Iceland and St Andrews and lead author of the new study.

Tagging and tracking wild whales

Humpback whales are found all over the world and migrate tens of thousands of kilometres each year.

They feed and mate in inshore waters and those around continental selves — areas that often overlap with naval activity.

So to see how ships and whales interact, Dr Wensveen and his colleagues sailed into the Barents Sea, north of Norway, during June 2011 and 2012 aboard the research vessel H. U. Sverdrup II.

He and the crew were part of a larger project to study the behaviour of different whales in the Arctic.

Norwegian Institute for Research's vessel M/S H.U. Sverdrup II. ( Flickr: Kystverket / Norwegian Coastal Administration )

When they spotted a humpback whale coming up for air, a small boat whizzed out and the crew stuck a tag to the whale's back, secured by a suction cup.

The tag measured sound, depth and movement.

After a few hours, the research ship was then steered to intersect with the tagged whale's path three times — the first time without sonar, then second and third times emitting ramp-up sonar — to see how the animal reacted.

Only five of 11 whales veered away on the first occasion of hearing the ramped-up sonar. Of those five, four of them stayed on course during a second dose of warning sonar.

The only tagged whale to veer away during both sonar sessions was a female. She had a young calf and when she heard the sonar, she dove more than 100 metres.

Of 11 humpback whales tracked, only a mother and her young calf avoided both sonar warnings. ( Getty Images: Kerstin Meyer )

According to the researchers, this is presumably because mothers are extra protective of very young offspring or small calves have a quiet voice and the mother had to flee to deep waters so she could hear her baby above the sonar noise.

Sonar ramp-up still worth it

This doesn't mean sonar ramp-up isn't worth doing, Dr Wensveen said.

Starting sonar softly still means that any nearby animals aren't exposed to full power straight away, which might send them into panic.

Still, exactly how much sonar is bouncing around the oceans is unknown. The navy has traditionally been tight-lipped about their activities, Dr Wensveen added.