Fast running restricts evolutionary change of the vertebral column in mammals

Galis, F., Carrier, D.R., van Alphen, J., van der Mije, S.D., Van Dooren, T.J.M., Metz, H.J.A.J., & ten Broek, C.M.A. (2014). Fast running restricts evolutionary change of the vertebral column in mammals. IIASA Interim Report. IIASA, Laxenburg, Austria: IR-14-029

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Abstract

The mammalian vertebral column is highly variable, reflecting adaptations to a wide range of lifestyles, from burrowing in moles to flying in bats. Yet, in many taxa the number of trunk vertebrae is surprisingly constant. We argue that the latter constancy result from strong selection against initial changes of thse numbers in fast-running or agile mammals, while such selection is weak in slower-running, sturdier mammals. The rationale is that changes of the number of trunk vertebrae require homeotic transformations from trunk into sacral vertebrae, or vice versa, and mutations outwards such transformations generally produce transitional lumbosacral vertebrae that are incompletely fused to the sacrum. We hypothesize that such incomplete homeotic transformations impair flaxibility of the lumbosacral joint and, thereby threaten survival in species that depend on axial mobility for speed and agility. Such transformations will only marginally affect performance in slow sturdy species, so that sufficient individuals with transitional vertebrae survive to allow eventual evolutionary changes of trunk vertebral numbers. We present data on fast and slow carnivores and artiodactyls and on slow afrotherians and monotremes that strongly support this hypothesis. The conclusion is that the selective constraints on the number of trunk vertebrae stem from a combination of developmental and biomechanical constraints.

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