New research suggests male baleen whales looking for love sing a different tune when attracting a mate, and it all depends on if they are more likely to fight or flee from a predator.

Killer whales are the only natural predator of baleen whales 鈥� those that eat using a system in their mouths to sieve their plankton diet from the water.

Experts have divided baleen whales into two categories based on how they are likely to respond to killer whales.

The whales that travel in groups and are more likely to fight a killer whale, include right, bowhead, grey and humpback, while those that travel solo and are more prone to flight include blue, fin, sei, Bryde鈥檚 and minke whales.

Trevor Branch, a professor of aquatic and fishery sciences at the University of Washington, reviewed more than 250 research papers last summer on whales and found that most of the baleen whales more prone to flee from predators will sing at a bass level low enough to avoid being heard by killer whales.

鈥淚 think the astonishing thing for me is that you can have animals that produce some of the loudest sounds in nature, and yet their predators can鈥檛 hear them,鈥� he said.

鈥淎 whale you can hear for hundreds of kilometres away, and yet a killer whale could be right next to it and couldn鈥檛 hear it. They could hear, probably, the sound of the blow, or the sound of them splashing, but as soon as you get beyond the range of that it鈥檚 like they鈥檙e completely acoustically invisible. Like ghosts in the water.鈥�

Branch said most killer whales have poor hearing below 1500 Hz and can鈥檛 hear at all below 100 Hz.

He found only 24 per cent of flight species call above 1500 Hz, and generally very quietly, so they can鈥檛 be heard further than one kilometre away. Blue whales, for example, don鈥檛 call above 100 Hz.

Branch said it makes sense that females that are part of a flight species would be attracted to a male with a call that does not attract predators.

鈥淏lue whales everywhere in the world, every population, over any time period you look at, their calls have been getting deeper and deeper over time and this is a completely unexplained mystery,鈥� he said.

鈥淪o, maybe the females are basically forcing the males, through sexual selection, to sing a bit lower every year because the females find it more attractive to go toward a male that can sing deeply. And maybe one part of that is the deeper you sing, the less likely you attract killer whales.鈥�

About 89 per cent of the whales that will fight against killer whales call above 1500 Hz.

Whales in the 鈥渇ight club鈥� travel in groups and the males sing highly varied, beautiful, intricate songs that change every few years, Branch said.

鈥淚f you鈥檙e in a big aggregation with lots of males competing, then maybe singing low and monotonously is not going to get you a mate. You want to be singing in a way that shows off your abilities to sing. And maybe that鈥檚 a good indication of your fitness,鈥� he said.

Branch said his findings are just the latest in a long line of facts about baleen whale life linked to how they respond to killer whales.

鈥淜iller whale predation drives all kinds of aspects of whale ecology, where they move, where they stay, how they mate, why they sing, how they sing, where they go to breed, how long they suckle their calves until they wean them,鈥� he said.

鈥淎ll these aspects of life of baleen whales seem to be driven, at least in part, by killer whale predation, or at least the fear of killer whale predation.鈥�