Woodpeckers do not resist the injurious consequences of head impacts; they avoid them. They peck at velocities from 5.6 to 7.5 m/s for 10–15 ms. The estimated peak impact force they sustain is below 1,000 g, where g is the gravitational acceleration (g=9.81 m/s2). By measuring intracranial pressures and cranial accelerations in anaesthetized woodpeckers, it was shown that impact forces above 1,250 g (12.2 kPa) resulted in significant head injury, whereas forces up to 1,000 g (9.8 kPa) did not.
At very high impact forces there is a transient “concussion” state in which the brain is pressed to the skull (brain deformation), there is a short blackout period, and the woodpecker loses its grip. The ability of woodpeckers to peck repeatedly for long periods without sustaining injury requires rapid recovery from these impact consequences.
The three features of woodpecker anatomy and behavior that are responsible for preventing (or minimizing) head injury are a stiff skull, a reduced-mass brain, and a rapid-recoil action of the hyoid apparatus (Fig. 1). The skull is stiff so as to minimize the forces transmitted to the brain (strain reducing). Much of the energy of impact is absorbed by the skull, which deforms elastically. The skull is reinforced with a mesh of crossed bony trabeculae, which is particularly well developed in the posterior part of the skull, behind the brain (Fig. 1). The brain mass of woodpeckers is reduced relative to the body mass, reducing the inertial force during impact. The posterior part of the skull is concave, leaving sufficient space for the small cerebellum to be withdrawn from a direct impact path between the bill tip and the skull. Finally, the woodpecker’s hyoid apparatus works like a built-in crash helmet, with the anterior part of the tongue wrapped around the cranium like a shock absorber. The shock-absorbing action of the hyoid apparatus is also associated with the rapid recoiling of the beak at the end of impact. This action provides a counterforce that rapidly reverses the acceleration of the head and brain, driving them forward just before impact forces the head backward.
The main function of the hyoid is to hold the tongue in the bill, but by acting like a crash helmet it is effectively a shock absorber (Fig. 2). As the tip of the bill strikes the wood with a force up to 1,250 g (12.2 kPa), it stops suddenly, but the head keeps on moving forward. This movement compresses the hyoid apparatus and the air in the sinus, which provides the cushioning mechanism against impact. The anterior end of the flexible hyoid apparatus extends forward just over the bone plate of the upper jaw, providing the first barrier to the brain when the bill comes to a sudden stop. The hyoid apparatus continues as thin bony rods that lie against the inside of the skull, reaching all the way to the back of the skull. These rods are encased within the tongue, which, in turn, fills the upper jaw up to the tip of the bill. It is not just the hyoid itself, but the air surrounding the hyoid and the tongue that provides the “hydraulic effect” required to prevent head injury. At the end of impact (10–15 ms), the elastic spring-like forces of the bill and the recoil action of the hyoid apparatus drive the beak forward again as the head is snapped back into place (Fig. 2).
These three special adaptations of woodpeckers enable them to peck at high speed to locate food in rotten wood without damaging their brains; however, they do not protect against injury from collisions with hard objects, such as telephone wires and plate glass windows, which often prove fatal.
