Testing an inference of function from structure: Snake vertebrae do the twist

The zygapophyses and zygosphene-zygantrum articulations of snake vertebrae are hypothesized to restrict or eliminate vertebral torsion. This hypothesi s is apparently based solely on the inference of function from structure, d espite the limitations of such inferences, as well as contradictory observa tions and measurements. In this study, I observed and measured axial torsio n in gopher snakes, Pituophis melanoleucus. To examine the structural basis of axial torsion, I measured the vertebral articulation angles along the b ody and the insertion angles of five epaxial muscles. To examine torsion in a natural behavior, I digitized video images and measured the degree of ap parent axial torsion during terrestrial lateral undulation. Finally, I meas ured the mechanical capacity of the vertebral joints for actual torsion ove r intervals of 10 vertebrae in fresh, skinned segments of the trunk. Verteb ral articulation angles vary up to 30 degrees and are associated with varia tion in torsional capacity along the trunk. The freely crawling P. melanole ucus twisted up to 2.19 degrees per vertebra, which produced substantial ov erall torsion when added over several vertebrae. The vertebral joints are m echanically capable of torsion up to 2.89 degrees per joint. Therefore, des pite the mechanical restriction imposed by the complex articulations, verte bral torsion occurs in snakes and appears to be functionally important in s everal natural behaviors. Even in cases in which mechanical function appear s to be narrowly constrained by morphology, specific functions should not b e inferred solely from structural analyses. J. Morphol. 241:217-225, 1999. (C) 1999 Wiley-Liss, Inc.