Head injuries sustained in car crashes and sports collisions commonly cause serious brain injury or death, but woodpeckers repeatedly bash their heads against trees without so much as a headache. This interesting observation became the focus of a study published in PLoS ONE, in which Lizhen Wang, of Beihang University, and colleagues set out to investigate exactly how woodpeckers protect their brain from impact injury.
There are a number of suggestions as to how woodpeckers resist the high impacts associated with their characteristic rapid pecking, but Wang and colleagues wanted to find out the extent to which all of these factors really play a role. To this end they analysed the movements and detailed anatomical features of great spotted woodpeckers (Dendrocopos major), which could hold the key to their unique shock absorption system.
First, the team used high-speed video cameras to record the 3D movements of woodpeckers pecking at a sensor that recorded their pecking force. What they observed were pecking speeds of over 7ms–1 and high decelerations on impact. Next, they used micro-CT scanning and microscopy to examine the bone and structures in the skull, and mechanically tested specimens of bone to obtain the exact mechanical properties of the tissue. Putting these data together, the team developed a finite element model of the woodpecker’s head that simulated the impacts experienced by the bird by reproducing the recorded pecking motions. They then modified various anatomical features, such as beak length, to examine the effects these had on how the force is transferred at impact.
Analysing the results, the researchers saw that the woodpecker’s shock absorption arrangement consists of a number of features. From the micro-CT scan, they observed an uneven distribution of ‘spongy’ bone, primarily at the forehead, which they think plays a role in shock absorption. Also, they suggest that the bird’s hyoid apparatus (a cartilage and bone structure that in woodpeckers reaches up to the top of the head and into the nasal cavity) may act as a ‘safety belt’ for the head, absorbing shocks and bearing high stresses. Finally, from the 3D model, the researchers found that the outer tissue layer of the upper beak is longer than that on the bottom but, conversely, the bone structure is longer on the bottom than on the top. They think that these unequal lengths allow impact to be distributed away from the brain, via the lower beak.
Although some of these adaptations are only useful if you are a woodpecker, scientists are keen to develop new woodpecker-inspired safety devices to keep us safe from impacts. Knowledge of the material properties and distribution of the shock-absorbing spongy bone can be incorporated into the design of new safety helmets. It could well be that nature has revealed a potentially life-saving mechanism to prevent head impact injury.