Trigger head 'acts like a stiff hammer', not a safety helmet, study finds

Scientists have debunked the popular theory of how a woodpecker can repeatedly beat its beak against a tree at high speed without damaging its own brain.

The researchers analyzed high-speed video of three woodpecker species — the stacked woodpecker, the black woodpecker, and the great spotted woodpecker.

They found their skulls did not act like shock-absorbing helmets as previously thought, but more like rigid metal hammers.

In fact, their calculations showed that any shock absorbers would hinder the woodpecker’s pecking ability.

Woodpeckers quickly drill into trees to extract insects deep in the wood with their long tongues.

An international team of researchers has analyzed high-speed video of three woodpecker species. Here is a sequence of frames from a high-speed video of a woodpecker pecking (Dryocopus pileatus)

Photo of a black woodpecker (Dryocopus martius) filmed in a study in Alpenzoo Innbruck, Austria

HAMMER OR HELMET?

Scientists have long wondered how woodpeckers can repeatedly pound their beaks into tree trunks without damaging their brains.

This led to the idea that their skulls should act like shock-absorbing helmets.

But researchers have debunked this idea, saying their heads act more like stiff hammers.

Although their skulls did not act as shock absorbers, it did not harm their brains, the researchers said.

‘By analyzing high-speed video of three woodpecker species, we found that woodpeckers do not absorb the shock of impact with trees,’ said study author Sam Van Wassenbergh at the University of Antwerp, Belgium.

When a moving head strikes a stationary object, a sudden deceleration of the head (a ‘impact deceleration’) will cause compression at the site of impact of the brain and expansion at the back side, which can damage neurons and cause dysfunction.

Woodpeckers have a spongy bone in their skull, right in front of their beak, which has previously been identified as a shock absorber.

Engineers of shock-absorbing materials and equipment, such as helmets, even use woodpecker morphology as a source of inspiration.

However, the shock absorber theory is ‘controversial’, say Van Wassenbergh and his colleagues, because of ‘the apparent paradox of absorbing the shock that woodpeckers wish to impart to trees’.

‘If the beak absorbs most of the impact on its own, the poor bird will have to hit harder,’ they say in their paper.

‘Consequently, since strong selective pressure may have improved hammering performance through woodpecker evolution, how could this performance-reducing trait develop as well?’

The well-developed spongy bone zone in the frontal region of the skull, which is thought to absorb shock, is highlighted in green.

Van Wassenbergh and colleagues saw the slowing of impact during pecking at three woodpecker species, by filming them and re-watching the footage.

They used the data to build a biomechanical model, which led them to the conclusion that any shock absorption to the skull would be detrimental to them.

Also, although their skulls didn’t act as shock absorbers, angry pecks didn’t harm their brains, the researchers said.

While the deceleration shock with each peck exceeds the known threshold for concussion in monkeys and humans, the woodpecker’s smaller brain can withstand it, they claim.

Van Wassenbergh said woodpeckers could risk brain damage if they peck at the metal with full force.

But their habit of pecking at tree trunks is generally well below the threshold for causing a concussion, even without their skull acting as a protective helmet.

This contrasts with the findings of a 2018 study, which found woodpecker brains exhibit high levels of an Alzheimer’s-causing protein called tau, which is linked to neurodegenerative diseases such as dementia.

High-speed video recording setup at the University of British Columbia for recording woodpeckers (Dryocopus pileatus)

According to Van Wassenbergh, the absence of shock absorbers does not mean their brains are in danger during the ‘seemingly violent impact’.

‘Even the strongest shock of the more than 100 blows analyzed should still be safe for the woodpecker brain because our calculations show a lower brain load than humans suffering from concussions,’ he said.

This finding refutes the old theory of shock absorption, which has been popularized in the media, books, zoos, and more.

‘While filming woodpeckers in zoos, I’ve watched parents explain to their children that woodpeckers don’t get headaches because they have shock absorbers attached to their heads,’ he says.

‘This myth of shock absorption in woodpeckers is now debunked by our findings.’

The new study has been published today in the journal Current Biology.

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