Two Levels of Waviness Are Necessary to Package the Highly Extensible Nerves in Rorqual Whales.

Peripheral nerves are susceptible to stretch injury [1-4] and incorporate structural waviness at the level of the axons, fascicles, and nerve trunk to accommodate physiological increases in length [5, 6]. It is unknown whether there are limits to the amount of deformation that waviness can accommodate. In rorqual whales, a sub-group of baleen whales, nerves running through the ventral groove blubber (VGB) associated with the floor of the mouth routinely experience dramatically large deformations. In fact, some of these nerves more than double their length during lunge feeding and then recoil to a short, compressed state after each lunge [7-9]. It is unknown how these nerves have adapted to operate in both extended and recoiled states. Using micro-CT and mechanics, we have discovered that the VGB nerves from fin whales require two levels of waviness to prevent stretch damage in both extended and recoiled states. The entire nerve core itself is highly folded when recoiled and appears buckled. This folding provides slack for extension but unavoidably generates large stretches at the bends that could damage nerve fascicles within the core. The strain at the bends is minimized by the specific waveform adopted by the core [10, 11], while the existing bending strains are accommodated by a second level of waviness in the individual fascicles that avoids stretch of the fascicle itself. Structural hierarchy partitions the waviness between the two length scales, providing a mechanism to maintain total elongation while preventing the stretching of fascicles at the bends when recoiled.