Considering how a boxer fares after having received several blows to the head, it is rather surprising how woodpeckers can ram their beaks into hard wood all day without any dizzying consequence. Nature must have given these birds some tricks to shed the potentially devastating brain injuries resulting from the high force impact of their beak onto the tree trunk. Otherwise they would never reach their 15-year life span while pecking at trees at 7 m s-1 and rates of 20 Hz – reaching deceleration forces of up to 1200 g.
No wonder, that woodpeckers have long been a favorite subject of scientists. Lessons taught by the woodpecker, could be a driving force in materials design, after all. It has been established that woodpeckers have adapted to their pecking behavior by a specialized beak, skull bone, and hyoid bone that help in absorbing the energy at the time of impact. However, the question of how those adaptations aid the woodpecker is not exactly fully solved, yet.
In Advanced Theory and Simulations, Jae-Young Jung from the University of California, San Diego and his co-workers perform a comparative study of the woodpecker skull based on function and morphology to determine whether anatomical differences in the frontal bone of some woodpecker species offer any mechanical advantage.
Initial investigation showed a frontal overhang in the skull of frequently pecking species like the white woodpecker which is absent in the golden-fronted woodpecker, who preferably feeds from the ground. Tests with 3D-printed skull models hitting on wooden plates at 7 m s-1 impact speed did, indeed, only cause damage the wood, not the skull. Instead the skull model was dropped onto a metal plate at up to 3 m s-1 to measure the acceleration at impact.
The jugal bones and the beak tip were the only structures damaged by the impact in both, overhang and non-overhang skull models. In consequence, the impact energy must dissipate at these structures. Frequency modal analysis of the skull model implies a natural frequency of the woodpecker brain below 4 kHz to avoid synchronized resonance with the skull bone.
Computational analysis using Ricker wavelets implies that the structure of the skull bone is designed to be isolated from the vibration of the beak, which takes the highest stress, matching the failure detected in actual experiments. Modelling stress propagation over time did show minimal stress propagation from the upper beak to the brain case for both skull models.
This powerful combination of experiments and computational modelling suggest that the beam-like bar structure of the woodpeckers jugal bone is the main stress deflector along with the natural frequency of the skull bone, both being crucial to protect the bird’s brain from concussion.
To learn more about this investigation and other theory-assisted research, please visit Advanced Theory and Simulations.