How woodpeckers avoid head injury
Slow-motion footage, X-ray images and computer simulations have shed light on how woodpeckers avoid injuries to their brains as they peck.
Their heads move some 6m/s (20ft/s), at each peck enduring a deceleration more than 1,000 times that of gravity.
But researchers reporting in Plos One say that unequal upper and lower beak lengths and spongy, plate-like bone structure protect the birds' brains.
The findings could help design more effective head protection for humans.
For years, scientists have examined the anatomy of woodpeckers' skulls to find out how they pull off their powerful pecking without causing themselves harm.
The birds have little "sub-dural space" between their brains and their skulls, so the brain does not have room to bump around as it does in humans. Also, their brains are longer top-to-bottom than front-to-back, meaning the force against the skull is spread over a larger brain area.
A highly-developed bone called the hyoid - which in humans is just above the "Adam's apple" - has also been studied: starting at the underside of the birds' beaks, it makes a full loop through their nostrils, under and around the back of their skulls, over the top and meeting again before the forehead.
However, Ming Zhang of the Hong Kong Polytechnic University, a co-author of the new work, said that he and his colleagues wanted to get to the bottom of the problem numerically.
"We thought that most of the previous studies were limited to the qualitative answer to this question," he told BBC News.
"More quantitative studies are necessary to answer this interesting problem, which would aid in applying the bio-mechanism to human protective device design and even to some industry design."
First, the team had a look at woodpeckers in a controlled environment: two slow-motion cameras captured images of the birds striking a force sensor that measured their pecking power.
They found that the birds slightly turn their heads as they peck, which influences how forces are transmitted.
The team also gathered computed tomography and scanning electron microscope analyses of woodpecker skulls, laying out in detail how the parts fit together and where bone density varied.
With those data in hand, they were able to use a computer simulation to calculate the forces throughout the birds' skulls in the process of pecking.
The team's simulations showed that three factors were at work in sparing the birds injury.
Firstly, the hyoid bone's looping structure around the whole skull was found to act as a "safety belt", especially after the initial impact.
The team also found that the upper and lower halves of the birds' beaks were uneven, and as force was transmitted from the tip of the beak into the bone, this asymmetry lowered the load that made it as far as the brain.
Lastly, plate-like bones with a "spongy" structure at different points in the skull helped distribute the incoming force, thereby protecting the brain.
The team stresses that it is the combination of the three, rather than any one feature, that keeps woodpeckers pecking without injury.