Concussions: What do woodpeckers have to do with it?

July 25^th 2022
Jeremy D. Bailoo

I remember quite clearly watching the movie, Concussion, and thinking how amazingly accessible the material was to the public. It took complex principles of Biology and Physics, and highlighted how dangerous one American pastime, football, was to the brains of the individuals who played it. It also was a good example about how basic research in animals could lend insight not only into the consequences of concussion but into strategies to reduce their impact.

So what do woodpeckers have to do with concussions? Consider, woodpeckers hammer (or headbang) wood with a force that is at least 1000 times that of gravity! Yet, woodpeckers suffer little harm as a consequence. If the same force was experienced by a human, they would likely die. So what are the evolutionary adaptations that allow woodpeckers’ brains to withstand such force¹?

• First, compared to humans, woodpeckers have a thick bony skull that is made of spongy bone—the thick skull protects the brain while the spongy bone distributes the forces experienced by the brain.
• Secondly, there is virtually no cerebrospinal fluid (CSF) in the extremely small subarachnoid space—CSF cushions the brain and the spinal cord, but in woodpeckers there is virtually no need for CSF because the space between the brain and the skull is extremely small. When a human hits an object with its head, in contrast, the head stops, but the brain continues forward (because of the large subarachnoid space)—compressing it in the front and stretching it in the back.
• Thirdly, woodpeckers strike objects in a perpendicular fashion with their beaks, eliminating the force (torsional shear force) that would otherwise cause concussions.
• Fourthly, the force of a woodpeckers’ strike would be strong enough to lead to detachment of the eye—woodpeckers overcome this by closing their thick eyelid which acts as a ”seatbelt” for the eye, while its nictitating membrane protects the eye from flying debris.
• Fifthly, the upper part of a woodpecker’s beak is longer than the lower part—this results in forces being transmitted from the upper part of the beak to the lower part of the beak and then to the body rather than the brain.
• Sixth, the woodpecker has an unusual tongue—it originates near the nostril, splits into two, encircles/encases the brain and then rejoins at the mouth. This unique tongue structure is thought to act as a shock absorber for the brain.

Decades of research into all of these characteristics have provided a deep understanding of how a woodpecker’s brain is able to withstand these forces. Moreover, such research has informed the design characteristics of sport safety devices, such as helmets based on such research.

Recently, however, new research has questioned some of the above conclusions considered to be fact.

Such research highlights a key aspect of science by use of the scientific method—that of falsification. Something can only be considered fact when it withstands years of scientific inquiry and attempts at replication and elaboration. Science is, by design, self-correcting.

Here, researchers employed a new approach to the evaluation of the striking forces that were exerted by the woodpecker’s head. Using high speed video (which can be played in slow motion while maintaining visual acuity) and tracking software (which allowed them to map specific regions of the head and to measure their movement with precision), these researchers evaluated the fifth point above—that is whether there was distribution of forces between the upper and lower beak due to their differences in length. If the woodpeckers were indeed cushioning the blows to the head, then the heads should come to a halt quicker than the beak. They instead found that both the head and the beak came to a halt at the same time. Moreover, using another technique called simulation (based on their video recordings) they found that shock absorption would not actually help protect the brains of woodpeckers—in order to remove the same amount of wood with shock absorption, the woodpeckers would have to peck with a greater force, which in turn would counteract built in protection in the previously mentioned points.

So what does this mean for our knowledge of woodpeckers and more broadly of science? Well, first of all, like all scientific endeavors, this new research needs to be repeated at least a few more times before it can replace the current way of thinking. Moreover, it highlights an often understated and overlooked fact about the process of science—that it occurs within a context. Such contexts involve the culture or spirit of the times in terms of scientific inquiry but also includes that of technological capability. In the present example, the new and innovative technology applied to this specific problem was not present (or at least, not sufficiently refined) in the previous decades and so researchers could not have pursued similar questions. Some may see this as an opportunity to disparage the process of science (we can never truly know anything) but for me, it is one of the more exciting aspects—our knowledge can always be refined.

Media outlets can, in particular, do better with respect to describing the process of science—scientists are not flip-flopping between conclusions. Rather, as new evidence is obtained, it is replicated and then placed within the appropriate context of the existing information. We saw a lot (and continue to see) these types of reporting during the COVID-19 pandemic but it equally applies to all scientific discoveries.

^{1. Cure for a headache. Schwab (2002). https://bjo.bmj.com/content/bjophthalmol/86/8/843.full.pdf}