Last night on CBC TV News after nos glorieux Canadiens beat the big, bad Bruins there was a brief mention of studying woodpeckers to understand their head motion activity asking the question, why don’t woodpeckers get concussions?
I have referred in an earlier post to University of Auckland, Nigel Shaw’s amazing article from Progress in Neurobiology 67 (2002) 281-344 The neurophysiology of concussion. This was the same blog post I referred to during my meeting with Dr Karen Johnston reporting on our summer project with my summer student, now with his neurology doctorate, Stephan Ong Tone, in which she appeared not to have read Shaw’s article concerning concussions. This omission on her part of not even having read Shaw forced me to stop my meeting with her.
I diverge, back to Woody. All the quotations are from Shaw’s article.
“While a wide variety of vertebrates seem to be susceptible
to a concussive blow, a diverse minority are reputed to be virtually
immune. These range from birds such as the nuthatch
and woodpecker to mammals such as the billygoat and ram.
These animals are believed to be able to routinely withstand
accelerative or decelerative forces one hundred times greater
than can be tolerated by humans (Ropper, 1994).”
Basically Shaw contemplates trying to understand what he terms the biodynamic mechanisms which might improve protective devices for protecting minor head injury. I will talk about this later in a separate blog about the H.A.N.S. device that current race drivers wear to protect their heads.
“The woodpecker is the name given to a group of scansorial and insectivorous birds of which there are more than 200 species. Among its distinctive features, the woodpecker possesses a strong, straight chisel-like bill with which it can incessantly pound on a tree trunk with astonishing force. According to May et al. (1979), as the beak tip strikes the wood, the impact velocity may be 6–7 m/s with an impact deceleration of about 1000 × g (10,000 m/s2).”
Basically the woodpecker’s high capacity repetitive head pecking has a lot to do with its regular biological patterns, such as looking for food, creating a nest in a tree trunk plus making food storage holes. Woodpeckers also signal in a similar fashion to other birds singing by beating a rapid tattoo of their bills against a hollow branch called drumming as a territorial signal in addition to courtship ritual, exerting much less force on their heads.
Various authors have speculated on the woodpecker’s small brain size according to Shaw is not convincing since similar size birds can be stunned/concussed flying into office building windows. Mammals with small brains also suffer concussions so that brain size alone does not appear to be relative to reduction of brain concussions. The next speculation is that woodpeckers possibly have elastic connections between their bill and skull will reduce the shock wave of striking the bill against a tree trunk, which in Shaw’s words, ‘nullifies the utility.’
“May et al. (1979) reasoned that no structural aspect of the
skull or head could provide a completely satisfactory account
of how the woodpecker can evade concussion.” May used high speed cinematography to capture head motion of the woodpecker. May’s group noticed that a woodpeckers head precisely attacked a wooden target in, ‘a fundamentally straight line manner.’ This means the deceleration impact as the woodpeckers beak strikes the tree is linear rather than angular, or rotational. Here is May’s strong interpretation describing such linear deceleration as a, “discovery as being consistent with Holbourn’s theory that it was sudden rotational movements of the head possibly generating peripheral shearing type of injury in the brain which was principally responsible for concussion.” Which sounds very similar to our rotating brain, doesn’t it? Shaw reiterates to empathize this pertinent May observation with the following, ” It will be recalled that concussion always ensued following angular acceleration of the head of the squirrel monkey but never after translational (or linear) acceleration.” This specific strike path orientation of the woodpeckers beak is, according to May, ” seems to provide compelling circumstantial evidence regarding the role of a particular type of head movement in the prevention of concussion.” Yet Shaw is still skeptical, insisting that there may be both linear and angular components affecting head components together in the wild, when the woodpecker strikes its beak against a tree trunk. Shaw sought further detailed analysis at this point, “to suggest that there may be yet more factors operating to
shield the woodpecker from concussive trauma.”
Shaw appears to have resolved the explanation of how a woodpecker avoids concussion despite repeated head deceleration impacts in its daily behavior patterns with a detailed anatomical study of the woodpeckers brain anatomy. Shaw describes how according to May, “the woodpecker’s brain was securely and firmly encased within the skull by what they described as dense spongy bone. The subarachnoid space was very constricted and consequently there would be only a minuscule amount of CSF
present. Judging by these findings, it can be concluded that
the woodpecker’s brain is carefully stowed within the cranium
with virtually no freedom to move at all.”
The essence of Shaw’s article describing his convulsive theory applies to the head motion of the woodpecker in terms of brain motion as, “prevention of concussion in the woodpecker
would seem to involve control of brain movement. Irrespective
of which technique is employed to stop the roiling (rotation) motion of the brain which is assumed to initiate a concussive
episode, the net result and the mechanical principles operating
are basically the same. In both instances, the kinetic
energy which is either absorbed or released by the head
at the time of the accelerative or decelerative impact is allowed
to flow harmlessly through the brain before being
diffused elsewhere in the body.” In other words if the brain no longer is allowed to self rotate as the brain appears to spin about an imaginary central axis, the deceleration flows right through the brain into the skull with no Penfield brain pull happening any where inside the head.
To conclude Shaw states, “one of the advantages of the convulsive
theory is that it can explain the significance of why the woodpecker possesses a tightly packaged immobilized brain.”