Most woodpeckers use their beaks to drill into tree trunks in search of tasty bugs and their larvae. One group of 'peckers, the sapsuckers, lick the sap out of the holes they drill using their long, brush-like tongues. The acorn woodpecker drills holes to store individual acorns. Colonies of acorn woodpeckers can store more than 60,000 acorns, each in its own hole, in a dead tree trunk.
Woodpeckers also carve out even larger holes in tree trunks to lay their eggs, a process known as excavation. They also use their pecking ability as a means of communication, drumming on hollow branches to attract a mate or to establish their territory.
With each peck, their heads move at a speed of around 6 meters per second, while the force of deceleration is between 1000 and 1600 g. To put that figure into context, fighter jets pull a paltry 9 to 12 g when they bank.
"Simple reasoning would indicate that if the woodpeckers got headaches," wrote researcher Lizhen Wang in 2011, "they would stop pecking." It's a wonder, with all the drilling, excavation, and drumming combined, that the woodpeckers never seem to sustain head or brain injuries. Here's how they do it.
Woodpeckers tend to have a bit of an overbite. That is, the upper half of the beak is a bit longer than the lower half. At the same time, the bottom part of the beak is stronger and more securely connected to the lower part of the skull; the upper part slides around a bit more, being less securely attached to the skull. Combined, this allows the top part of the beak to inflict more damage on the tree trunk without passing on too much of that energy onto the more fragile top half of the skull. The lower beak passes its own energy onto the stronger lower half of the skull.
A Seatbelt For A Spongy Skull
Speaking of skulls, those inside of woodpeckers' heads are spongy. As Matt Soniak explained at Mental Floss, "the bone that surrounds the brain is thick and spongy, and loaded with trabeculae, microscopic beam-like bits of tissue that give the bone a tightly woven 'mesh' for support and protection." The spongiest parts of the skull are in the forehead and at the back of the head, places where it can act most optimally as a shock absorber.
Then there's the hyoid bone. Humans also have the horseshoe shaped bones, located just above the Adam's apple, though for us it's fairly small and disconnected from the rest of the skeleton. Woodpeckers' hyoids aren't only larger; they're also connected to the lower side of the skull. And they're also shaped differently. Rather than being horseshoe-shaped, the two ends connect, but only after looping through the nostrils and around the skull. Wang writes that the unique properties of the hyoid bone allow it to act like a seatbelt, keeping the skull in place during all that hammering.
(A) the head of a Great Spotted woodpecker, with the hyoid visibly looping over the top. (B) The hyoid of a Great Spotted woodpecker; compared with (C), the hyoid of a different bird, a Eurasian hoopoe. (via PLoS ONE)
A Well-Packed Brain
Keeping the skull from flapping around is important, but the brain could still sustain a lot of damage by smashing into the skull. That's because the brain floats around in fluid, which is enclosed by a thin layer of tissue called the dura.
But the woodpecker's brain evolved to account for this. The space between the dura and the brain is very small, containing very little cerebrospinal fluid. By being packed in so tightly, the brain has less of a chance to jiggle around. The brain is also oriented within the skull such that the side with the largest surface area faces the front of the skull. When it does move, the force of the brain pressing against the skull's interior is therefore spread out across a larger surface, softening the impact.
Keeping The Rhythm
Together, the unique anatomical adaptations of the skeleton, beak, and brain allow the woodpecker to hammer away to its heart's content without worrying about head injuries. But those features only account by 99.7% of the energy produced by the impact. The remaining 0.3% of the energy makes its way to the brain. That might not seem like much, but woodpeckers evolved a strategy to account for that too.
Spend any time watching or listening to woodpeckers while they peck at tree trunks, and you'll notice that they don't constantly hammer away at the wood, but do it with a rhythm. They peck perhaps two, or three, or four times, and then they take a short break, before hammering out a second set of pecks. In a study published last week in the journal Science China Technological Sciences, researcher ZhaoDan Zhu discovered that those breaks might be critical to preserving the health of the brain. That's because that extra 0.3% of the energy that gets absorbed the brain gets converted into heat. With each successive strike, the woodpecker's brain heats up, just a bit. By pausing between sets of pecks, the woodpecker may be allowing its brain a chance to cool down and avoid overheating.
Is there anything that woodpeckers can teach us to help create stronger, more impact-resistant materials? These researchers hope so. There are some 300,000 sports-related concussions in the United States each year. One way to address that problem might be woodpecker-inspired helmets.
Header image: Nuttall's woodpecker via Jamie Chavez/Flickr.