Species of woodpeckers, such as the golden-fronted woodpecker, drum with their beak to establish their territories and attract mates. The high-speed pecking motion of the golden-fronted woodpecker causes a tremendous amount of stressed force on the animal. To prevent physical and neurological trauma, the frontal portion of the woodpecker’s skull is comprised of plate-like spongy bone called cancellous bone.
When the woodpecker’s beak strikes an object, the high impact force at its tip is relieved by the anatomy of its beak and the spongy hyoid bone. As a result, the stress force from the impact is reduced two to eight times from the beak tip to the point where the beak meets its skull.
Of the forces that do reach the woodpecker’s skull, the unique structure of its cranial bone prevents forces from reaching its brain and cranial cavity. This cranial bone is a mixture of tightly packed, dense compact bone surrounding a deeper bone that is layered in staggered, plate-like structures that create a dense shock absorption system. When forces intersect with this deeper bone, its porous and layered structure scatters frequencies in divergent directions away from the central point of impact. This bone, while flexible, can be fragile on its own. But by being encased in compact bone, the overall system maintains flexibility within, allowing for movement that absorbs shock.
This diagram represents (A) the point of impact and (B) the point opposite impact that receives residual forces experienced at the point of impact. Illustration by Allison Miller.
The left image above represents a view of the golden-fronted woodpecker’s skull and beak from above. The area highlighted in red is the point of impact that receives residual forces from the bird’s pecking activity.
Illustration by Allison Miller.
The photo and illustration at right reflect how the structure of this area is comprised of compact and spongy bone.
Photo and illustration taken from Wang et al. under creative commons licensing.
This strategy was contributed by Allison Miller.
Check out these related strategies that collectively protect the woodpecker’s brain from impact:
“The woodpecker’s beak…is a specialized chisel effective in cutting into a tree; unlike a human-made chisel, the beak is self-sharpening…; the beak, made of elastic material, is relatively large compared to the body. This endoskeletal feature prevents incident mechanical excitations [i.e., the impact] of drumming from directly reaching the brain. [Another shock-absorbing] feature is a hyoid which rigidly supports the tongue. This musculotendinous tissue serves as an attachment site for the muscles around the throat and tongue…[and] encompasses the head…This feature, not seen in other birds, aids the woodpecker in extending its tongue in order to evenly distribute [the impact] from drumming and to reinforce the head—in other words, the hyoid bypasses the vibrations generated from drumming. [Another shock-absorbing] feature, a spongy bone, which is specially located at the contrecoup position from the beak, allows the woodpecker to avoid brain damage (May et al 1976a, 1976b). This bone is relatively dense but spongy compared to other bones…The spongy bone is thought to evenly distribute incident mechanical excitations [the drumming impact] before they reach the brain…Finally, a skull bone with CSF [cerebrospinal fluid] plays…a key role in dissipating mechanical excitations from drumming…[T]he woodpecker has a very narrow space for CSF between the skull bone and brain. This bird therefore has…relatively little CSF, thereby reducing the transmission of the mechanical excitations into the brain through the CSF (May et al 1976a, 1976b, Schwab 2002).” (Yoon and Park 2011:3)