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Monday, July 25, 2022

Hard-headed woodpeckers

Heads of Woodpeckers Function as Stiff Hammers during Pecking

By Science News Staff / Source

The skull of a woodpecker has long been hypothesized to serve as a shock absorber that minimizes the harmful deceleration of its brain upon impact into trees. 

However, this hypothesis is paradoxical since any absorption or dissipation of the head’s kinetic energy by the skull would likely impair the bird’s hammering performance and is therefore unlikely to have evolved by natural selection. 

New research shows that a woodpecker’s skull is used as a stiff hammer to enhance pecking performance, and not as a shock-absorbing system to protect the brain.

Van Wassenbergh et al. found that woodpecker heads behave very stiffly during pecking impacts. Image credit: Van Wassenbergh et al., doi: 10.1016/j.cub.2022.05.052.

The forceful impact of the beak and the associated abrupt deceleration of woodpeckers’ heads when hammering into trees for feeding and nesting, or when drumming for interspecific communication has long intrigued scientists who wonder how these birds protect their brain against injury.

When a moving head strikes a stationary object, the sudden deceleration of the head will cause compressions at the impact site of the brain and expansions at the back side, which can both damage neurons and cause dysfunction.

A straightforward way to decrease these harmful decelerations (i.e., shocks) of the brain is to absorb or dissipate the head’s kinetic energy during impact in a compliant material located between the brain and the impact site, as, for example, in airbags or bike helmets.

This diminishing of shock impulses is fundamentally different from other types of protective adaptations such as rigid body armors that withstand high local forces.

Since the lifestyle of woodpeckers inevitably subjects these birds to high decelerations of the head, multiple studies have sought adaptations related to shock absorption within the cranial musculoskeletal system of woodpeckers.

The spongy bone in a woodpecker’s skull, which is particularly well developed at the frontal region of the skull, has been identified as a prime candidate for shock absorption.

Impact energy could also be absorbed through contraction of the protractor muscles of the quadrate and lower beak if the lower beak is pushed relative to the skull when impacting a tree.

Despite the lack of evidence for biologically significant shock absorption during pecking in woodpeckers, engineers of shock-absorbing materials and tools, such as helmets, have used the morphology of woodpeckers as a source of inspiration.

However, not only do these hypotheses on shock absorption by the skull musculoskeletal system remain untested in a natural situation, but they are also controversial.

The reason for this controversy is the apparent paradox of absorbing the shock the woodpecker wants to impart on the tree. In other words, if the beak absorbed much of its own impact the unfortunate bird would have to pound even harder.

“By analyzing high-speed videos of three species of woodpeckers, we found that woodpeckers do not absorb the shock of the impact with the tree,” said lead author Dr. Sam Van Wassenbergh, a researcher at the Universiteit Antwerpen.

For the study, Van Wassenbergh and colleagues captured high-speed videos of six individuals from three woodpecker species (Dryocopus martiusDryocopus pileatus, and Dendrocopos major) kept in aviaries as they hammered into wood.

The researchers quantified the impact decelerations during pecking in the three species.

They then used the data to build biomechanical models, which led them to the conclusion that any shock absorbance of the skull would be disadvantageous for the birds.

But if their skulls don’t act as shock absorbers, does the furious pecking put their brains at risk? It turns out that it doesn’t.

While the deceleration shock with each peck exceeds the known threshold for a concussion in monkeys and humans, the woodpeckers’ smaller brains can withstand it.

“Woodpeckers could make a mistake, for instance if they were to peck on metal at full power,” Dr. Van Wassenbergh said.

“But their usual pecking on tree trunks is generally well below the threshold to cause a concussion, even without their skulls acting as protective helmets.”

“The absence of shock absorption does not mean their brains are in danger during the seemingly violent impacts.”

“Even the strongest shocks from the over 100 pecks that were analyzed should still be safe for the woodpeckers’ brains as our calculations showed brain loadings that are lower than that of humans suffering a concussion.”

The findings refute the long-held theory of shock absorption, which has been popularized in the media, books, zoos, and more.

“From an evolutionary point of view, the findings may explain why there aren’t woodpeckers with much larger heads and neck muscles,” Dr. Van Wassenbergh said.

“While a larger woodpecker could deliver more powerful pecks, concussions likely would cause them major problems.”

The findings were published in the journal Current Biology.


Sam Van Wassenbergh et al. Woodpeckers minimize cranial absorption of shocks. Current Biology, published online July 14, 2022; doi: 10.1016/j.cub.2022.05.052