The soundtrack to my walk in the woods last weekend was incredible. The forest was alive with creatures big and small, busy with the tasks of a new season: the spring peepers were cheerfully peeping, the partridges were drumming on hollow logs, tom turkeys were gobbling at full throat. And, above it all, the incredibly loud bursts of hammering from pileated woodpeckers carried through the treetops.
When I stopped to listen, I was struck by the frequency and the volume of this hammering. A pileated woodpecker can hit a tree up to 20 times a second, experiencing accelerations greater than 1000g! (For comparison, humans pass out if we experience anything greater than about 5g.) How does this little creature bash its head against a tree trunk repeatedly, with such force, without sustaining significant brain damage? I’m not the first to ask this question – Philip May and colleagues wondered “why the countryside is not littered with dazed and dying woodpeckers” back in 1976 in an article published in the Lancet.
May and colleagues examined frozen sections of the heads of woodpeckers, comparing them with that of a toucan, a similar-sized bird that doesn’t exhibit headbanging (unless at a Metallica concert). In the woodpecker’s noggin, there is little space between the skull and the brain so, unlike toucans, the woodpecker brain won’t slosh around with sudden motion and slam into the inside of the skull. Also, their skull is made up of dense and spongy bone, especially right at the front. The toucan’s skull bone is “light, almost frothy” in comparison.
The woodpecker’s lower jaw muscles were observed to be more powerful than those in the toucan, suggesting that they play a role in absorbing and distributing shocks, like a boxer tensing up to protect against an opponent’s blow. Perhaps most unique, however, is the woodpecker’s tongue. The base runs from the floor of the mouth to the back of the head, then up and over the top, ending at the front at the right nostril (the red line in the drawing from Jimfbleak here). May and colleagues suggested that this is a muscular protective sling to minimize motion during hammering.
Image by Jimfbleak CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=55897150
The story doesn’t end with that Lancet paper in 1976. Others have noted that there is a thick eyelid that closes just before the woodpecker strikes, both to protect from flying debris but also to keep the bird’s eyeballs from “popping out of its head” (Schwab in the British Journal of Ophthalmology, 2002). A lovely 2006 paper from L. J. Gibson in the Journal of Zoology explores the role of the size of the brain on its likelihood of injury, concluding that the smaller brain corresponds to a lower mass to surface area, meaning that the impact force is spread over a relatively large region.
More recently, function studies have added to the work done to date on the unique structures observed. Jung and colleagues reported on the mechanical function of the woodpecker’s skull in a 2019 article in Advanced Theory and Simulation. As part of their investigations, Jung’s team built a tower in which they repeatedly dropped their 3D printed woodpecker skulls, beak first, onto a metal plate to observe the effects. They concluded that the main stress wave from impact at the beak travels through the lower jaw towards the neck and spine, away from the brain cavity – a built-in natural stress deflector.
And scientific advancement marches on! From the structural analysis of May and colleagues in 1976 to functional studies by Jung and team in 2019, we are still learning how these birds can subject themselves to such trauma, seemingly without adverse consequences. Maybe some day the brains of athletes will be protected in contact sports with woodpecker-skull-inspired helmets and collars.
P. R. May et al, 1976 Lancet pgs 454-455
I. R. Schwab 2002 Br. J. Ophthalmol. 86 pg 843 https://bjo.bmj.com/content/86/8/843.short
L. J. Gibson 2006 Journal of Zoology 270 pgs 462-465
J-Y Jung et al, 2019 Adv. Theory Simul. 2 DOI: 10.1002/adts.201800152
G. D. Myer et al, 2016 British Journal of Sports Medicine 50 pgs 1276–1285 https://bjsm.bmj.com/content/50/20/1276.full