Why Don't Woodpeckers Get Brain Damage?

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Hit your head really hard on something, and it’ll smart for a while. In worse cases, you might get a concussion, fracture your skull, or receive a brain injury that leaves you impaired or kills you (traumatic brain injuries account for nearly one third of injury-related deaths in the US).

Good thing you’re not a woodpecker, then. The lives and livelihoods of these birds revolve around slamming their heads into things. Whether it wants to get at an insect hiding in bark, excavate a space to build a nest, claim a bit of territory, or attract a mate, the woodpecker has one simple solution: bang its head against a tree trunk at speeds reaching 13 to 15 miles per hour. In an average day, a woodpecker does this around 12,000 times, and yet they don’t seem to hurt themselves or be the least bit bothered by it. This is because, after millions of years of this type of behavior, they’ve evolved some specialized headgear to prevent injuries to their heads, brains, and eyes.

To figure out what goes into woodpecker head trauma prevention, a team of Chinese scientists took a look at the birds’ skulls and brains and their pecking behavior. They watched as woodpeckers pecked at force sensors while recording them with high-speed cameras so they could see the strikes in slow motion and know how hard each blow was. They also scanned the birds’ heads with x-rays and an electron microscope to get a better look at their bone structure. Finally, they squished a few preserved woodpecker skulls in a material testing machine and, using their scans, built 3D computer models of the birds’ heads to smash in a simulation.

When all was said and done and both the virtual and actual woodpeckers' heads had taken a sound beating, the researchers found that there are a few anatomical features and other factors that come together to keep a woodpecker safe and healthy while it rat-a-tat-tats the day away.

First, a woodpecker’s skull is built to absorb shock and minimize damage. The bone that surrounds the brain is thick and spongy, and loaded with trabeculae, microscopic beam-like bits of bone that form a tightly woven “mesh” for support and protection. On their scans, the scientists found that this spongy bone is unevenly distributed in woodpeckers, and it is concentrated around the forehead and the back of the skull, where it could act as a shock absorber.

Woodpeckers' hyoid bones act as additional support structures. In humans, the horseshoe-shaped hyoid is an attachment site for certain throat and tongue muscles. Woodpeckers’ hyoids do the same job, but they’re much larger and are differently shaped. The ends of the “horseshoe” wrap all the way around the skull and, in some species, even around the eye socket or into the nasal cavity, eventually meeting to form a sort of sling shape. This bizarre-looking bone, the researchers think, acts like a safety harness for the woodpecker’s skull, absorbing shock stress and keeping it from shaking, rattling and rolling with each peck.

Inside the skull, the brain has its own defenses. It’s small and smooth, and is positioned in a tight space with its largest surface pointing towards the front of the skull. It doesn’t move around too much, and when it does collide with the skull, the force is spread out over a larger area. This makes it more resistant to concussions, the researchers say.

A woodpecker’s beak helps prevent trauma, too. The outer tissue layer of its upper beak is longer than the lower beak, creating a kind of overbite, and the bone structure of the lower beak is longer and stronger than the upper one. The researchers think that the uneven build diverts impact stress away from the brain and distributes it to the lower beak and bottom parts of the skull instead.

The woodpecker’s anatomy doesn’t just prevent injuries to the brain, but also its eyes. Other research using high-speed recordings has shown that, in the fraction of a second just before their beaks strike wood, woodpeckers’ thick nictitans—membranes beneath the lower lid of their eyes, sometimes called the “third eyelid”—close over the eyes. This protects them from debris and keeps them in place. They act like seatbelts, says ophthalmologist Ivan Schwab, author of Evolution's Witness: How Eyes Evolved, and they keep the retina from tearing and the eye from popping right out of the skull.

There’s also a behavioral aspect to the damage control. The researchers found that woodpeckers are pretty good at varying the paths of their pecks. By moving their heads and beaks around as they hammer away, they minimize the number of times in a row that the brain and skull make contact at the same point. Older research also showed that the strike trajectories, as much as they vary, are always almost linear. There’s very little, if any, rotation of the head and almost no movement immediately after impact, minimizing twisting force that could cause injury.

Earlier this year, another group of researchers in China found that, with all of these adaptations, 99.7 percent of the impact energy from striking a tree is absorbed by the body, but a little bit—that last 0.3 percent—does go to the head and the brain. That mechanical energy gets converted into heat, which causes the temperature of a woodpecker’s brain to increase, but the birds seem to have a way dealing with that, too. Woodpeckers usually peck in short bursts with breaks in between, and the researchers think that these pauses give the brain time to cool down before the head banging starts again and brings the temperature back up.

This story was originally published in 2012. It was updated with new information in 2014.

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November 24, 2014 - 6:30am
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