The forces endured by a woodpecker’s brain as it hammers away at a dead tree branch must be considerable, or so we always thought. Conventional wisdom has it that the woodpecker’s skull must act as some sort of shock-absorbing helmet and cushion the blows. When a brain experiences sudden deceleration it undergoes compression at the impact site and expansion at the opposite side, both of which can damage neurons and result in injury. And, since woodpeckers don’t seem to suffer brain injuries, the logical inference is that some dissipation of the impact force must occur between the beak and the brain.
New research published today in the journal Current Biology refutes this long-held notion and finds instead that woodpeckers’ heads act more like stiff hammers than protective helmets. The authors state that hypotheses on shock absorption by the woodpecker’s cranial musculoskeletal system remain untested in natural situations, and would, in fact, run counter to natural selection pressures to increase pecking performance.
The researchers used high-speed video footage of six woodpecker individuals from three species to test whether the deceleration of the brain is significantly reduced compared with the deceleration of the beak upon impact. They analyzed 109 videos of the birds as they hammered into wood, tracking the positions of various markers on the skulls and beaks. The experts found no significant difference between the deceleration of the beak and that of the skull on impact.
“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 Sam Van Wassenbergh of Universiteit Antwerpen, Belgium.
The researchers then used the data collected to model the functional implications of varying the degree of cranial shock. Their biokinetic models included springs between the beak and the brain that could absorb varying amounts of shock and measure the implications of this for hammering performance.
Their simulations confirmed that absorbing part of the kinetic energy of the head into a compressing electric structure during impact significantly reduced the penetration of the beak into the tree. This means that, if a woodpecker’s beak absorbed any of the impact from its pecking action, the bird would actually have to peck harder in order to perform successful pecks. In fact, model birds with cranial shock absorbers would have to expend more energy to peck to a certain depth than those without cranial shock absorbers.
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 that causes concussion in monkeys and humans is around 103 kPa at the site of impact, hammering woodpeckers experience only between 40 and 60 percent of this pressure, depending on species. This means these species would need to hit their selected spot twice as fast as observed or strike at its top speed on wood that is four times as stiff to suffer a concussion, state the authors.
“The absence of shock absorption does not mean their brains are in danger during the seemingly violent impacts,” said Van Wassenbergh. “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.”
Van Wassenbergh says that woodpeckers could make a mistake, for instance if they were to peck on metal at full power. But their usual pecking on tree trunks is generally well below the threshold that causes a concussion, even without their skulls acting as protective helmets. They are known to have smaller brains and less cerebrospinal fluid around the brain, which are factors that could also protect their brains during impacts.
The findings refute the long-held theory of shock absorption, which has been popularized in the media, books, zoos, and more, noted Van Wassenbergh. “While filming the woodpeckers in zoos, I have witnessed parents explaining to their kids that woodpeckers don’t get headaches because they have shock absorber built into their head. This myth of shock absorption in woodpeckers is now busted by our findings.”
Van Wassenbergh and colleagues conclude that the heads of woodpeckers function as stiff hammers during pecking, and that this ensures maximum penetration of the wood at each peck. They emphasize that, from an evolutionary point of view, there may well be limits to the head size, maximal strike speed and hardness of trees selected by woodpeckers.
The findings also have some practical implications, adds Van Wassenbergh, given that engineers have previously used the anatomy of the woodpecker’s cranial skeleton as a source of inspiration for the development of shock-absorbing materials and helmets. The new findings show that’s not such a good idea, given that woodpecker anatomy actually negates shock absorption.