Clipart courtesy FCIT
In any case, a quick internet search found that I was definitely not the first person to ask this question. There are lots of good responses.
Tetrapod Zoology (This post is delightfully detailed. Did you know woodpeckers "have an incredibly long, protrusible tongue anchored to hypertrophied hyoid bones that have grown backwards around the skull base and then up and over the skull roof"? And just because his blog is called Tetrapod his interests wanders well beyond.)
How are woodpeckers able to withstand rapid, repeated percussion without sustaining brain injury? This was looked at by Gibson (2006), and the rather disappointing conclusions were that the small size of the brain and short duration of the impacts helped the brain withstand high deceleration, as did the shape of the brain (it's longest axis is arranged dorsoventrally rather than anteroposteriorly). I say that these conclusions were 'disappointing' as I imagined that woodpeckers had evolved some sort of unique, shock-absorbing, brain-cushioning specialisations. It has in fact been suggested that the muscles at the tongue base might serve this function, but this can't be true as these muscles wrap around the back and top of the skull and don't have any contact with the brain itself.
Chris, at The Naked Scientists gives another good, overlapping explanation in answer to someone's question. Citing a 1979 study by Philip May, Joaquin Fuster, Jochen Haber and Ada Hirschman, he gives a number of interesting facts:
- the impact deceleration when a woodpecker's beak slams into a surface can exceed one thousand times the force of gravity (1200g).
- a tame acorn woodpecker, did take the precaution of closing his eyes prior to each strike !
- woodpeckers have relatively small brains which, in contrast to a human, are packed fairly tightly inside their skull cavity. This prevents the excessive movement of the brain inside the skull which causes so-called 'contre-coup' injuries (literally brain bruising) in humans. These occur when the brain bashes into the skull following a knock on the head. In other words the head stops, but the brain keeps on moving.
- because the brain is small it has a high surface area to weight ratio, meaning that the impact force is spread over a much larger area, relatively speaking, compared with a human. Again, this minimises the applied trauma.
- Finally, the woodpecker always ensures that he strikes his target in a dead straight line. This approach avoids placing rotational or sheering stresses on the nerve fibres in the brain. Humans involved in car accidents frequently develop the symptoms of 'diffuse axonal injury' where sudden deceleration coupled with rotation literally twists the different parts of the brain off each other like a lid coming off a jar. By hammering in a dead straight line woody woodpecker avoids giving himself DAI, further minimising the risk of brain damage. Such an approach may have implications for the design of protective head gear - such as crash helmets - which could be modified to prevent rotational injuries.
Even the Wikipedia entry talks about this, and adds to why they close their eyes:
Many of the foraging, breeding and signaling behaviors of woodpeckers involve drumming and hammering using the bill. To prevent brain damage from the rapid and repeated decelerations, woodpeckers have evolved a number of adaptations to protect the brain. These include small brain size, the orientation of the brain within the skull (which maximises the area of contact between the brain and the skull) and the short duration of contact. The millisecond before contact with wood a thickened nictitating membrane closes, protecting the eye from flying debris. The nostrils are also protected; they are often slit-like and have special feathers to cover them.
Chris at The Naked Scientists warned that humans don't have these adaptations, so don't bang your head against the wall.
A Post Script: The Tetrapod Zoology writer, Darren Naish, had an wonderfully informative and detailed post, and I just wanted to note his varied interests (which fascinated me, if not you), so I'm adding here his bio:
Darren Naish is a science writer, technical editor and palaeozoologist (affiliated with the University of Portsmouth, UK) who mostly works on Cretaceous dinosaurs and pterosaurs. He also studies such things as the swimming abilities of giraffes and fossil marine reptiles. An avid interest in modern wildlife and conservation has resulted in many adventures in lizard-chasing, bird-watching and litter-collecting. I've been blogging since 2006 and a compilation of early Tet Zoo articles is now available in book form as Tetrapod Zoology Book One. Additional recent books include The Great Dinosaur Discoveries and Dinosaurs Life Size. For more biographical info go here. I can be contacted intermittently at eotyrannus (at) gmail dot com.
Post Post Script: I tried to get permission from another blogger to use a picture of a woodpecker skull, but didn't know how long I'd have to wait to hear back, so I made this video from a youtube video called The Tree Medic. [As I wrote this I got back the permission. Well, I should have looked more carefully because the blogger had given credit to the Florida Educational Technology Clearinghouse. But since the video is already embedded I'll leave it too. And I thought this would be a ten minute post. Dream on.]