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.
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 martius, Dryocopus 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