Why Don't Woodpeckers Get Brain Damage?

Woodpecker image via Shutterstock

If you or I bump our heads hard enough on a hunk of wood, it might smart for a while. But to get through an average day, a woodpecker might ram its head into a tree trunk at a speed of 6 or 7 meters per second some 12,000 times without seeming the least bit bothered by it.

Lucky for them. The life of a woodpecker revolves around slamming its face into trees at high speeds. It’s how they feed themselves most of the time, excavating bugs from the wood when fruit or nuts aren’t available. It’s also how many of them make their homes, hollowing out a space in a trunk some 8 inches wide and up to two feet deep to make a nest. This is the niche they’ve come to fill, and over millions of years of doing it, they’ve evolved some intense headgear to prevent brain damage, cracked skulls and retinal detachment.

To figure out what all goes into woodpecker head trauma prevention, a team of Chinese scientists took a look at the birds’ heads, brains and behavior in several different ways last year.

They watched woodpeckers peck at force sensors while recording them with high-speed cameras, scanned their heads with x-rays and electron microscopes, squished a few preserved woodpecker skulls in a hydraulic compressing material testing machine, and even built 3D computer models of the birds’ skulls and smashed them into walls in a simulation.

When all was said and done and both the virtual and actual woodpecker heads had taken a sound beating, the researchers found that there are several factors and anatomical features that all come together to create a shock absorption system for a woodpecker’s head.

Built for the Job

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 tissue that give the bone a tightly woven “mesh” for support and protection. On their scans, the scientists found that this spongy bone is unevenly distributed in woodpeckers, and occurred more around the forehead and the back of the skull, where it could act as a shock absorber.

Woodpeckers' hyoid bones act as an additional support structure. In humans, the horseshoe-shaped hyoid is an attachment site for certain throat and tongue muscles. Woodpeckers’ hyoids do the same thing, but they’re much larger, are differently shaped and wrap all the way around the skull and, in some species, even around the eye socket or into the nasal cavity. This bizarre-looking bone, the researchers think, acts like a safety harness for the skull and brain, absorbing shock and stress as they shake, rattle and roll with each peck.

The woodpecker’s beak helps prevent trauma, too. Their upper beaks are longer than their lower beaks, creating a kind of overbite, but, conversely, the bone structure of the lower beak is stronger than the upper. The researchers think that the unequal build diverts impact and stress away from the brain and lets the upper beak bear it instead.

Beyond the brain, the woodpecker’s anatomy also protects its eyes. Previous research using the same sort of high-speed recording in the Chinese study revealed that, in the fractions of a second just before their beaks strike wood, woodpeckers’ thick nictitans - membranes found beneath the lower lid of many animals’ eyes and sometimes called the “third eyelid” - close over their eyes to protect them from debris and keep them in place. They act sort of like seatbelts, says ophthalmologist Ivan Schwab, author of Evolution's Witness: How Eyes Evolved, and 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 Chinese study found that woodpeckers are pretty good at varying the path 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 torsional force that could cause injury.

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May 23, 2012 - 3:31am
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