The subarachnoid cavity of the golden-fronted woodpecker protects its brain from injury through decreased volume of cerebrospinal fluid.

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Species of woodpeckers, such as the golden-fronted woodpecker, peck 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. One mechanism that prevents physical and neurological trauma is the combination of the small volume of the woodpecker’s cranial space and the smooth surface area of the brain.

Similar to mammals, bird skulls contain a space between the brain’s gray matter and the skull’s vascular tissue, called a subarachnoid cavity. The cavity houses cerebrospinal fluid (CSF) that provides cushioning from minor bumps and jostling, as well as protection from diseases. However, CSF will not protect the brain from strong vibrations or blows, like those experienced by woodpeckers or football players. In these instances, CSF will allow excessive movement of the brain, potentially resulting in bruising and concussions.

The woodpecker has proportionally less CSF than other birds, an adaptation to protect its brain during drumming. Less CSF combats the risk of brain injury in two ways. First, the decreased volume makes it more difficult for the brain to be disrupted if the head is struck. Second, with a decreased CSF volume, there is less medium for stress forces to transmit through, therefore reducing the impact on neural tissue.

In addition to cavity volume, the surface area of the woodpecker brain is smooth with fewer pronounced peaks and valleys. Therefore, if the skull were to be struck, the impact would be more evenly transmitted around the brain, rather than hitting one point with a greater amount of force.

This illustration represents a species with a complex brain. The brain has a good number of peaks and valleys. When struck, the site of impact receives a greater amount of force due to its raised ridges. Illustration by Allison Miller.

In a smooth brained species, such as the golden-fronted woodpecker, the force is distributed more evenly, resulting in a decreased residual impact. Illustration by Allison Miller.

This strategy was contributed by Allison Miller.

Check out these related strategies that collectively protect the woodpecker's brain from impact:

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"The woodpecker’s 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)

Journal article
Why Do Woodpeckers Resist Head Impact Injury: A Biomechanical InvestigationPLoS ONEOctober 26, 2011
Lizhen Wang, Jason Tak-Man Cheung, Fang Pu, Deyu Li, Ming Zhang, Yubo Fan
Editor/s: Mark Briffa

Journal article
A mechanical analysis of woodpecker drumming and its application to shock-absorbing systemsBioinspir. Biomim.January 19, 2011
Sang-Hee Yoon, Sungmin Park

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WoodpeckerMelanerpes aurifronsSpecies

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