Image copyright J Tennessen Image caption Only an estimated few hundred North Atlantic right whales remain in the wild

US researchers say that they can distinguish individual whales based on the sound of the animals' voices.

Using a large set of recordings of North Atlantic right whales, they found that detailed analysis of one particular type of call allowed them to single out individual whales.

The biologists want to explore whether acoustic identification could be useful for monitoring whales in the wild.

Practically, however, this idea remains very difficult to put into practice.

The findings were presented at the spring conference of the Acoustical Society of America, in Pittsburgh.

Current estimates suggest there are only around 450 North Atlantic right whales left in the wild. They feed on plankton off the east coast of the US and Canada, which is where the recordings were made that were used in the new study.

Suction-cap sensors were attached to the backs of 13 right whales and used to record their vocalisations, over a period of more than a decade.

Masters student Jessica McCordic and her supervisor Prof Susan Parks analysed these recordings in various ways to see if they could identify sonic signatures of the individual animals.

They concentrated on "upcalls", one of the most common noises made by this species. These are fairly low-pitched vocalisations that rise over about one or two seconds.

Image copyright SPL Image caption Southern right whales (pictured) are more abundant than their North Atlantic cousins, but still endangered

Because they are one of the most useful signatures for differentiating human voices, the researchers first looked at whether different whales made upcalls with obviously different "formants". Formants are the loudest frequencies in the stack of harmonics within a sound. It is formants that create the different vowel sounds in human speech.

"What I found was that there actually wasn't much difference in the formants, but one of the variables that came out as most important in discriminating the individuals was the duration of the call," Ms McCordic said.

It was a combined analysis of several properties, including the rate that the upcalls went up in pitch, their duration, as well as their formants, that eventually allowed successful identification.

"The analysis classified the whales well above chance levels, so that was really exciting," Ms McCordic added.

She and Prof Parks want to see if they can achieve the same feat using fixed microphones to record whale noises at a distance, in the wild. That could be a great help to conservationists monitoring this critically endangered species.

Dr Denise Risch, a postdoctoral researcher at the Scottish Association for Marine Science, was impressed by the findings. She said the work was "very valuable and interesting" but that it was limited in terms of application - for now.

"I think it's a bit of a stretch to say that we'll be able to apply this to passive acoustic monitoring," Dr Risch told the BBC.

"It would be great, but the problem is noise. You need really clear and close recordings in order to use data from stationary acoustics to tell individuals.

"We're still working on really good detectors, just to hear the calls themselves - our detectors are not that great yet.

"So in the future, it's definitely something that would be good to aim for. But it's difficult."