Why don't woodpeckers suffer from concussions despite their high-speed pecking on tree trunks? Woodpeckers are well-known "forest doctors". In order to eat insects hidden in tree trunks, woodpeckers can peck at tree trunks up to 20 times per second, even pecking up to 12,000 times a day, with the acceleration of the head during each peck reaching up to 1,200 times the acceleration of gravity. How can woodpeckers ensure that their heads are not injured under such conditions? A research team led by Ivan Schwab, an ornithologist at the University of California, studied this question and won the 2006 Ig Nobel Prize for it. The researchers set up a specific environment to observe woodpeckers, tested the pecking force with sensors, and captured the pecking process with two slow-motion cameras. They used computed tomography and scanning electron microscopy to collect and analyze the data of the woodpecker's skull, and detailedly marked the combination of its skull structure and the location of bone density changes. With the collected data, scientists used computer simulations to demonstrate the forces exerted on the skull during the pecking process.
Through the simulations, the researchers found that multiple factors can reduce the damage of impact forces to woodpeckers. The fundamental reason is that woodpeckers have evolved a series of devices to protect their brains and eyes from impact. Firstly, the tongue bone of woodpeckers starts from the lower side of the beak, forks to the back of the skull on both sides, and then extends upward, meeting again in front of the forehead. This is like a "safety belt", which can effectively prevent the cervical spine from being bent backward. Its jaw is connected to the skull by a strong muscle, which quickly contracts before impact, also serving as a buffer. Secondly, the lower beak of the woodpecker's beak is longer than the upper beak. When the impact force is transmitted from the tip of the beak to the bone, this structure weakens the impact force and diverts it away from the brain. Moreover, the skull of woodpeckers is spongy. Their skulls are not only very thick and strong, but also have many small voids, a bit like a sponge, which can disperse the impact force and protect the brain. There are three layers of membranes between the woodpecker's skull and brain, from the outside to the inside: the dura mater, arachnoid membrane, and pia mater. The space between the arachnoid membrane and the pia mater is called the subarachnoid space. The human subarachnoid space is filled with cerebrospinal fluid. However, the subarachnoid space of woodpeckers is very narrow and almost devoid of cerebrospinal fluid, which weakens the liquid transmission of shock waves and can buffer and dampen external forces. The brain of woodpeckers is relatively small, and the surface area of small objects is relatively large. Moreover, the size of their brain is longer from top to bottom than from front to back, so the pressure applied to it is easier to be dispersed, so they are not as prone to concussions as humans. In addition, the tip of the woodpecker's beak and head always remain in a straight line, and the direction of pecking is very vertical, which avoids the twisting force caused by shaking leading to meningeal tearing and concussion. Therefore, despite its daily pecking, it can withstand strong impact forces for years.
The structure of woodpeckers' eyes is also very ingenious. High-speed cameras show that in the instant before impact, the woodpecker's nictitating membrane quickly closes, avoiding the impact of flying wood chips on the eyes and wrapping the eyes like a safety belt to prevent them from popping out. Their eyes are filled with a viscous polysaccharide in the choroid, which can act as a buffer. There is also a comb-like membrane on the choroid, which may also play a shock-absorbing role, because once it is congested, it can temporarily increase the intraocular pressure and protect the lens and retina.
The researchers emphasized that the protection of woodpeckers' brains from injury is the result of the combined effects of multiple factors, rather than the sole effect of any one of them. This fully shows that the structure determines the function of the system. This scientific discovery will help scientists design more effective protective devices to protect the human brain.