A well-built skull

Why don’t woodpeckers hurt their heads when they bang them into trees?

That question popped to mind the other day as I was working outside in my garden. We have a woodpecker that has taken to visiting one of our neighbor’s trees. Stay outside long enough on a decent afternoon, and soon you will hear the distinctive “rat-a-tat-a-tat-a-tat” of the woodpecker’s beak, as it rams over and over again into the trunk. These birds are tenacious when it comes to drilling holes in things! Not only are they tenacious, they are powerful. In fact, North America’s largest woodpecker (the Pileated woodpecker) can hit a tree 20 times a second, up to 12,000 times a day, with forces as high as 1,200 times the force of gravity with each hit. That’s roughly the same as hitting a wall with your face at 16 miles an hour – over and over and over again. Ouch. That makes my head hurt just thinking about it!

So how come they don’t hurt themselves?

Woodpeckers are highly adapted to this punishing lifestyle based on the design of their skulls and brains. First, the skull: a woodpecker’s skull is very thick and bony, but the bone itself is fairly spongy. There is also a think layer of cartilage at the bottom of the lower jaw bone that serves as a cushion. This cartilage and bone structure enables to force of the blow to be distributed to the base and back of the skull, rather than the brain.

Next, the brain: a woodpecker’s brain actually has several features that protect it from impact. First, it is small and packaged very tightly inside the skull cavity. Because of its tight packing, the brain does not move much more than the skull does. When a human hits his head, the brain can jostle around within the skull cavity, actually bumping against the bones. That bump causes much of the brain injury associated with a head trauma, such as a concussion. But woodpecker’s brains don’t move that way, so they can’t knock themselves out, get concussions, or incur brain bleeding. Second, there is not a lot of cerebrospinal fluid, either, because there is simply no room for it. This fluid helps transmit shock waves from the skull to the brain, which can also cause injury to the brain. Little fluid – few shock waves. Third, a woodpecker’s brain has a smooth surface, with a high surface area to weight ratio. This is unlike a human brain, which has ridges, folds and bumps all over the surface. We actually have a lot of surface area on our brains! That’s good for thinking, but bad for a head trauma. It means that any impact to the brain is felt over a small surface area. For a woodpecker, a blow to the brain is spread out over a larger surface area, which means that the impact on any given spot is smaller.

Finally, there is another very important fact that helps explain why woodpeckers don’t hurt their heads when they peck – it has to do with their pecking technique. Woodpeckers will always peck at a surface in a straight line, with no side-to-side twisting or torque. This prevents a specific kind of stress on the nerve fibers of the brain called “diffuse axonal injury” or DAI. DAI is one of the most common and dangerous forms of traumatic brain injury. DAI occurs when a sudden deceleration is coupled with some form of rotation, which can twist nerves apart like a lid coming off a jar. A brain impact that occurs straight-on will not cause DAI, because there is no rotational stress being applied to the brain. So it is vital that a woodpecker hit its target dead center and straight on with every hit. And, from all indications, they do!

Incidentally, an Ig Nobel prize was awarded in 2006 in opthamology to Dr. Ivan Schwab and Dr. Phillip R.A. May, for their work on woodpecker head trauma. In particular, they studied why woodpeckers are immune to retinal detachments, brain damage and spinal cord injuries with their repeated head banging. And in addition to the stuff I’ve talked about above, they did learn one additional thing. Woodpeckers always close their eyes with every bang of their beak.

Dr. Schwab did admit, however, that whether they do it to keep their eyeballs in or the wood chips out was an open question.