Why do woodpeckers resist head impact injury: a biomechanical investigation

Abstract

Head injury is a leading cause of morbidity and death in both industrialized and developing countries. It is estimated that brain injuries account for 15% of the burden of fatalities and disabilities, and represent the leading cause of death in young adults. Brain injury may be caused by an impact or a sudden change in the linear and/or angular velocity of the head. However, the woodpecker does not experience any head injury at the high speed of 6-7 m/s with a deceleration of 1000 g when it drums a tree trunk. It is still not known how woodpeckers protect their brain from impact injury. In order to investigate this, two synchronous high-speed video systems were used to observe the pecking process, and the force sensor was used to measure the peck force. The mechanical properties and macro/micro morphological structure in woodpecker's head were investigated using a mechanical testing system and micro-CT scanning. Finite element (FE) models of the woodpecker's head were established to study the dynamic intracranial responses. The result showed that macro/micro morphology of cranial bone and beak can be recognized as a major contributor to non-impact-injuries. This biomechanical analysis makes it possible to visualize events during woodpecker pecking and may inspire new approaches to prevention and treatment of human head injury.

Figure 1. The 3D pecking trajectory during woodpecker's pecking.

(a) 3D schematic diagram; (b) Pecking trajectory on the sagittal plane; (c) Pecking trajectory on the transverse plane; (d) Pecking trajectory on the coronal plane.

Figure 2. Micro-morphology of cranial bone.

(a) The micro-CT scanning images of Great Spotted woodpecker's head on the coronal plane; (b) The micro-CT scanning images of Eurasian hoopoe's head on the coronal plane; (c) The SEM image of Great Spotted woodpecker's cranial bone; (d) The SEM image of Eurasian hoopoe's cranial bone.

Figure 3. Anatomical structures of head and hyoid bone.

(a) Great Spotted woodpecker's head; (b) Great Spotted Woodpecker's hyoid bone; (c) Eurasian hoopoe's hyoid bone.

Figure 4. Micro-CT image and the FE models of Great Spotted Woodpecker' head.

(a) Micro-CT image of Great Spotted Woodpecker' head; (b) Beak_Lower>Beak_Upper FE model; (c) Beak_Lower = Beak_Upper FE model; (d) Beak_Lower<Beak_Upper FE model.

Figure 5. The pecking force-time histories at the initial velocity of 1 m/s for both the FE analysis and pecking circle-maximal pecking force in the experiment.

Figure 6. The time histories of effective stress at the selected points on beak, orbit and skull.

(Beak_U-the point on the tip of upper beak; Beak_L-the point on the tip of lower beak; Orbit-the point on the orbit; Skull_A-the point on the anterior part of skull; Skull_P-the point on the posterior part of skull).

Figure 7. The effective stress distribution of woodpecker's head during pecking.

Figure 8. The effective stress distribution of the hyoid bone during pecking.