Functional and evolutionary morphology of woodpeckers

Abstract

Bock, W.J. 1999. Functional and evolutionary morphology of woodpeckers. In: Adams, N.J. & Slotow, R.H. (eds) Proc. 22 Int. Ornithol. Congr., Durban. Ostrich 70 (1): 23–31. Woodpeckers are the first example of adaptive evolution by Natural Selection mentioned by Darwin who commented that their' feet, tail, beak and tongue' are ‘so admirably adapted to catch insects under the bark of trees’. Over the ensuing decades, the adaptiveness and evolution of these diverse woodpecker features has been examined by many workers but with limited success. A major problem was that these evolutionary explanations were not supported by logically prior functional explanations. Structures of the toes, hind limb and tail are associated with climbing. All forces acting on the climbing bird can be determined with the Method of Free-Body Analysis, which together with measurements of hind limb and tail features and of the tree surfaces used by different species, will permit understanding of the evolution of climbing adaptations. Bill shape and M. protractor pterygoidei development correlate with forces on the bill during drilling. Again Free-Body Analysis shows that compressive shocks acting on the bill do not travel directly into the brain case and hence the brain, but result in a compressive stress in the base of the skull. The frontal overhang in specialised drilling woodpeckers provides a bony stop that prevents excessive abduction of the upper jaw during non-impact periods while drilling into trees. Specialisations of the tongue are connected with greater protraction of the tongue to obtain food. These features not only include longer protractor and retractor muscles, but a universal joint between the basihyal and the fused paraglossalia and the enlargement of two pairs of intrinsic muscles inserting on the paraglossalia enabling movement of the tip of the corneous tongue in all directions. With a better understanding of the adaptiveness and evolution of these diverse features of woodpeckers, it is possible to obtain an improved comprehension of their ecological associations, adaptations and evolutionary history.