Brain injury researchers have long been enthralled by woodpeckers. These hardy birds hurl their heads at massive trees a hundred times a minute. Yet, they evidently experience no brain damage. How do they do it?
Woodpeckers have some remarkable bodily adaptations that are effective at reducing the impact of pecks against tree trunks (1). These are illustrated by the pileated woodpecker. Two toes extend forward so as better to absorb the impact. At the other end, the tail feathers are braced against the tree using specially hardened feathers that absorb the impact. Its neck is stiffened by specialized muscles that also help to absorb impact. An inner eyelid helps to keep the eyeballs in place. The brain is also surrounded by thick spongy bone that is believed to absorb shocks rather like a helmet.
In addition to these bodily shock absorbers, the woodpecker deploys various other adaptations of brain and behavior that shield its brain from traumatic injury.
Compared to humans, woodpeckers and other birds have relatively small brains, or lower encephalization ratios (that relate brain size to body size). One of the many problems of having a very large brain is that it increases the force of impacts and risk of brain injury.
Force is also related to the size of the surface on which it acts. One of the elegant adaptations of the woodpecker brain is rotation of the brain within the skull so as to maximize the surface area over which the impact of pecking is distributed.
Conversely human athletes are disadvantaged because a blow to the forehead, common in contact sports, strikes the brain edge-on, so to speak. Given the comparatively smaller area receiving the impact, the possibility of injury is greater.
Thanks to this engineering perspective, we are beginning to understand both why woodpeckers are protected and why humans are so vulnerable. Hopefully these insights can be applied to preventing human brain injuries associated with contact sports like boxing, wrestling, soccer, and football (2).
The Engineer's Perspective
One way of thinking about brain injury is as an engineering problem where the impact of a collision is too great for the brain to withstand injury. In the woodpecker's case, there are many variables that protect the animal by reducing the impact of its pecks (1).
How the bill strikes its target is of considerable importance. First, the bird minimizes twisting forces (torque) within the brain by always striking the target exactly at a 90-degree angle. Such blows produce no turning force (thanks to the cosine rule used by engineers to estimate forces in various directions).
In addition to directing its blows precisely, the woodpecker minimizes impact by keeping the contact time very short.
In considering the small size of the woodpecker's brain, brain orientation, and short duration of impact, engineer Lorna Gibson estimates that a pileated woodpecker can withstand 64 times as much impact as a human brain.
From an engineering perspective, the woodpecker brain is well protected from injury by numerous adaptations that serve to reduce impacts within the brain.
One of the major considerations is the relatively small size of the woodpecker brain, so it is useful to look at a larger-brained animal that can withstand repeated blows to the head. Bighorn sheep provide a closer analogy. These wild sheep butt heads in high-speed impacts as a test of dominance during the mating season. Ironically, they are a common emblem for football teams.
Research on bighorns identified a very different mechanism through which protection against brain injury may be achieved.
The Hydraulic Theory of Brain Protection
One source of vulnerability for humans is that the brain is not perfectly fitted into the cranial cavity, leaving the brain more vulnerable to injury.
Some insight into this issue is provided by evidence that football players sustain fewer concussions when playing at high altitudes. This may be attributable to increased volume of blood in the brain, which prevents brain tissue from moving around inside the skull.
Further study revealed that both the woodpecker and the bighorn sheep have mechanisms for using rebreathed air , which increases carbon dioxide in the brain and is thought to have a “bubble wrap” effect.
This research has led to the introduction of a collar device that increases blood volume of the brain by pressing gently on the carotid artery.
Whether this can help athletes to minimize brain injuries remains to be seen.
How Does This Help Athletes?
One key benefit of the woodpecker analogy is that it highlights the extreme vulnerability of human brains to traumatic injury.
New helmets could be designed that are more effective at absorbing impact in specific sports based upon detailed scientific research that is now likely to be funded. The woodpecker analogy also encourages equipment designers to think outside the box. Based on adaptations of the woodpecker's feet, one could speculate that footwear might have more of a role to play in absorbing bodily collisions.
The woodpecker analogy helps us to understand the problem better. Understanding the problem well is always the first step to finding solutions.
1 Gibson, L. J. (2006). Woodpecker pecking: How woodpeckers avoid brain injury. Journal of Zoology, 270(3), 462-465.
2 DeKosky, S. T., Ikonomovic, M. D., and Gandy, S.(2010). Traumatic brain injury – football, warfare, and long-term effects. New England Journal of Medicine, 363, 1293-12996.