Nerves aren't known for being stretchy. In fact, "nerve stretch injury" is a common form of trauma in humans. But researchers reporting in the Cell Press journal Current Biology on May 4 have discovered that nerves in the mouths and tongues of rorqual whales can more than double their length with no trouble at all.
"These large nerves actually stretch and recoil like bungee cords," says A. Wayne Vogl of the University of British Columbia. "This is unlike other nerves in vertebrates, where the nerve is of a more fixed length that has enough slack in it to accommodate changes in position of the structures the nerves are supplying."
Those stretchy nerves support the animals' unique and extreme lunge feeding strategy, which helps to support the whales' gigantic bodies.
Rorqual whales represent the largest group among baleen whales, weighing in at an impressive 40 to 80 tons. To eat, the whales open their mouths and lunge while their tongues invert and their mouths fill like giant water balloons full of floating prey. Those prey are concentrated by slowly expelling the water through baleen plates. The volume of water brought in with a single gulp can exceed the volume of the whale itself.
"Rorqual whales attained large body size with the evolution of a bulk filter feeding mechanism based on engulfing huge volumes of prey-laden water," Vogl says. "This required major changes in anatomy of the tongue and ventral blubber to allow large deformation, and now we recognize that this also required major modifications in the structure of nerves in these tissues so they could withstand the tissue deformation."
Vogl and his team hadn't expected this at all. They made the discovery after a member of the lab picked up a dull white cord-like structure and stretched it. Vogl recalls him saying something like, "Wow, look at this."
It looked like a blood vessel, which ought to be stretchy. But they realized upon closer inspection that it was a nerve -- one unlike any they'd ever seen before. The nerves of other species are generally surrounded by a thin collagen wall, and any stretch can pull and damage the nerves.
What the researchers see in these whales is something entirely different. The nerve fibers themselves are packaged inside a central core; they don't stretch so much as unfold. A very thick and stretchy outer wall full of elastin fibers that Vogl likens to elastic bands surrounds that nerve core.
The researchers don't know yet whether anything similar will turn up in other animals -- the ballooning throats of frogs, for example, or the long and fast tongues of chameleons. They plan to keep studying the whales' nerves in greater detail, in hopes of understanding better how the nerve core is folded in such a way to allow its rapid unpacking and re-packing as the entire structure is stretched and then relaxed again.
Current Biology, Vogl et al.: "Stretchy nerves are an essential component of the extreme feeding mechanism of rorqual whales" http://dx.doi.org/10.1016/j.cub.2015.03.007
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