Image copyright SPL Image caption The rorqual family of whales includes some of the biggest animals alive

Scientists have stumbled upon one of the secrets behind the big gulps of the world's biggest whales: the nerves in their jaws are stretchy.

Rorquals, a family that includes blue and humpback whales, feed by engulfing huge volumes of water and food, sometimes bigger than themselves.

Researchers made the discovery by inadvertently stretching a thick cable they found in the jaw of a fin whale.

Most nerves are fragile and inelastic, so this find is a first in vertebrates.

The work is reported in the journal Current Biology.

A Canadian research team had travelled to Iceland to investigate some of these whales' other anatomical adaptations to "lunge feeding" - things like their muscles, or the remarkable sensory organ in their jaws, discovered in 2012. They were working with specimens in collaboration with commercial whalers.

"It's probably one of the only places in the world where you can do this sort of work, because these animals are so huge that even getting in through the skin is something you can't do without having heavy machinery around," said Prof Wayne Vogl, an anatomist at the University of British Columbia and the study's first author.

When you are working with a 20m fin whale, it's important to have the right equipment, he said. "If a heart falls on you, it could kill you."

'Like bungee cords'

It was Prof Vogl's co-author Robert Shadwick, a zoologist at the same university, who sparked the serendipitous discovery.

"We were looking at the muscle in the floor of the mouth and there were these long white cords," Prof Vogl told the BBC. "Bob picked one up - about 3ft of it - grabbed each end and stretched it. He turned to me and said, 'Hey, look at this!'

"We thought it was a blood vessel."

Image copyright Vogl et al./Current Biology Image caption Nerves are essential for the whale's mouth muscles to contract and sieve its food

This thick, white cord could stretch to twice its length and repeatedly sprang back to its original size. But when the team cut it open, it did not have a hollow inside like a blood vessel; instead there was a small, yellowish core running through the middle.

"I realised this was a nerve, and it was very different from any other nerve I've ever seen," Prof Vogl said.

Stretchiness is a very odd property for a nerve. These are the brain's cables for carrying messages around the body and in most cases if one of them is pulled and gets longer, it is bad news - as anyone with a nerve stretch injury, common in many sports, will testify.

Where flexibility and a lot of movement are required, most animals simply have nerves that are a good safe length, with extra slack to be taken up if needed.

So how do the whale's big jaw nerves manage to stretch "like bungee cords", as Prof Vogl and his team found?

Image copyright Vogl et al./Current Biology Image caption The thick, stretchy nerves were a big surprise to the research team

When they looked at the nerves using a microscope, the team saw that the nerve fibres themselves were wound tightly into the central core and "unfold" as the cable stretches. The thick white layer around the outside, meanwhile, is full of tough, stretchy fibres of a protein called elastin - also found in human skin and blood vessels.

Along with the stretchy components, this specialised coating has coiled, rope-like collagen fibres that can unwind up to a point, but will then be strong enough to stop the nerve from over-stretching.

"They've used building blocks that are present in other animals but they've used them in different ways to produce this stretchy nerve," Dr Vogl explained.

Dr Guy Bewick, a neuroscientist at the University of Aberdeen in Scotland, said he was surprised and impressed by the finding.

"I've studied nerves and muscles all my career, and one of the things that you really try and avoid - in the clinical situation or just personally - is stretching your nerves," Dr Bewick told BBC News.

"Nerves are generally really quite fragile things; they're quite well protected and they don't withstand stretch well at all.

"This looks like a really neat trick to get around that problem, which the whale obviously needs to do because its mouth inflates so much."

Image copyright Thinkstock Image caption Some rorqual whales can gulp more than their own volume into their balloon-like mouths

Having functioning nerves is crucial to the whale's survival, Dr Bewick added. After these beasts take a gulp, they strain it through their baleen plates to keep the fish or krill, but let go of the water.

"To squeeze the water out again, it needs to contract those huge muscles in the floor of the mouth. The muscles won't contract unless the nerves tell them to."

In fact, evolutionary biologists have suggested that lunge feeding is the development that allowed whales to reach their colossal sizes. So stretchy nerves may be one of the key adaptations that produced these giants of the deep.

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