The high-speed drumming motion of the golden-fronted woodpecker causes a tremendous amount of stressed force on the animal, termed “incident mechanical excitations.” The hyoid bone, located in the bird’s cranium, secures and diverts vibrational forces away from the brain.
The hyoid bone is a strong, flexible bone covered in muscle that allows the woodpecker to extend its tongue out of its beak to grab food. It also serves as an attachment site for muscles around the throat, tongue, and head.
The hyoid bone begins in the nostril of the upper beak, where it divides into two parts between the eyes, and then travels over the top of the skull and around the back. At the base of the skull, the separate pieces rejoin and attach to the muscle of the tongue (see illustration below). The woodpecker not only uses the hyoid bone to gather food for meals, but to protect its brain from neurological trauma. This happens in two ways.
First, when the woodpecker pecks, the muscles surrounding the relaxed hyoid bone contract, propelling the tongue forward inside the beak, and even further when collecting food. This tension stabilizes the cranium and spine, acting as a seat belt to prevent excessive movement of the brain.
Second, the hyoid bone design diverts vibrations (and any forceful impact) away from the cranium. Because of its longer length, the upper beak absorbs more of the shock than the lower beak when striking a surface. The forces then travel up the beak, where they encounter the hyoid bone in the nostril before they hit the spongy bone in the skull. The stress forces then travel along the path of the hyoid bone, rather than continuing to the skull, diffusing into the muscles covering the bone, or traveling to the tongue.
This illustration represents the location of the hyoid bone in a (A) relaxed state, and (B) constricted state. When reaching for food, the hyoid tightens around the cranium, reinforcing the skull and spine, and diverting forces away from the brain. Illustration 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)
A mechanical analysis of woodpecker drumming and its application to shock-absorbing systemsBioinspir. Biomim.January 19, 2011
“[The hyoid] originates from the dorsum of the maxilla, passes through the right nostril, divides into two parts between the eyes, and the dividends then arch over the superior portion of the skull and around the occiput by passing on either side of the neck, coming forward through the lower mandible, and uniting into one again below the forehead” (Wang et al. 2011:6)