By Paul Rincon
BBC News Online science staff
A species of whale that ignores most man-made noises - including passing boats - is attracted to sounds like the sirens used by emergency services.
Is it too late for the right whale?
It is the first time such a strong response to a man-made signal has been reported in whales, researchers claim.
The alarm could be used to check waters for the endangered northern right whales before conducting military exercises which could harm the animals.
Details of the study are published in Proceedings of the Royal Society B.
Northern right whales (Eubalaena glacialis) are particularly prone to collisions with ships. About two of the marine mammals are killed each year from injuries sustained through these so-called "ship strikes".
No one quite knows why this is, but there are a few theories.
The whales feed near the surface - usually in the top 10 metres - sifting the water for plankton. This means they are often invisible from above, but are still vulnerable to being hit by the submerged hulls of passing ships.
They also frequently get tangled in fishing gear.
Northern right whales also tend to swim slowly, though they speed up when chasing females during the mating season.
Because the sound of a ship builds up as it approaches, the whales may be lulled into a false sense of security. They may also get used to the sounds of ships through a process of habituation.
About 15 metres long
100 tonnes in weight
3 years between births
"Vessels are so common where right whales live that if they responded every time a vessel came by, they'd never get anything done," said Dr Peter Tyack of Woods Hole Oceanographic Institution in Massachusetts, US, and a co-author of the study.
"By the time a vessel gets to a kilometre away or so, it's just too late."
The team developed the "whale alarm" by looking at the kinds of sounds used to put humans on alert, particularly those used in hospitals.
They tagged five different right whales and then measured their responses as they played three different siren sounds back-to-back to them underwater. One of these sounds much like a police siren.
Four out of five whales powered to the surface at an astonishing speed rarely recorded in the animals when the sirens were played, said Dr Douglas Nowacek, of Florida State University, a co-author of the research.
"The whales were responding to sounds two orders of magnitude lower than the safe exposure limits in underwater sonar trials. The signal itself was the important thing for the whales," Dr Nowacek told BBC News Online.
The findings suggest the whales are put on alert by the same sounds as those that make humans jump to attention.
Dr Nowacek said the sirens could be used to check if whales were in the vicinity when the US Navy carries out military exercises in water used by the animals, or when demolition work is carried out on bridges over water.
But by luring the whales to the surface, the alarms also make them vulnerable to being hit by ships.
There are only about 300 northern right whales left, making them one of the most endangered whale species on Earth. Based on current demographic data for the species, they are expected to go extinct within 200 years.
Recently, Dr Andrew Pershing of Cornell University developed a mathematical model that apparently predicts 65% of the variability in right whale birth rates.
"What that told us is that there's not a lot we can do about their reproduction," said Dr Peter Tyack, senior scientist at Woods Hole Oceanographic Institution in Massachusetts, US.
"Which is why we have to concentrate on the human causes of mortality to right whales," he added.
The whales often collide with ships
The US National Marine Fisheries Service sponsored a report, released in 2001, that strongly urged the placement of route and speed restrictions for ships in areas inhabited by right whales.
The agency is currently deciding whether to make the recommendations part of its policy.
Bruce Russell, a maritime consultant who contributed to the report, said the estimated cost to the shipping industry of adopting the recommendations in the report would be up to $20m (£11.6m) but probably nearer $10m (£5.8m).
"This really is a minor amount for the shipping industry. The aggregate impact is small, but the impact on specific ports might be large," said Mr Russell.
"The proposals had caused concern from the operators of two ports: Boston in Massachusetts and Jacksonville in Florida, which is near to the right whales' calving area," he added.
The operators in question are said to be concerned they will lose business to other ports.