title: Fast running restricts evolutionary change of the vertebral column in mammals.
abstract: The mammalian vertebral column is highly variable, reflecting adaptations to a wide range of lifestyles, from burrowing in moles to flying in bats. However, in many taxa, the number of trunk vertebrae is surprisingly constant. We argue that this constancy results from strong selection against initial changes of these numbers in fast running and agile mammals, whereas 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 toward such transformations generally produce transitional lumbosacral vertebrae that are incompletely fused to the sacrum. We hypothesize that such incomplete homeotic transformations impair flexibility 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 count of trunk vertebrae stem from a combination of developmental and biomechanical constraints.
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands; frietson.galis@naturalis.nl.
authors.forename: Frietson
authors.initials: F
authors.lastname: Galis
authors.affiliation: Department of Biology, University of Utah, Salt Lake City, UT 84112;
authors.forename: David R
authors.initials: DR
authors.lastname: Carrier
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands;
authors.forename: Joris
authors.initials: J
authors.lastname: van Alphen
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands;
authors.forename: Steven D
authors.initials: SD
authors.lastname: van der Mije
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands;Centre National de la Recherche Scientifique/Université Pierre et Marie Curie/Université Paris-Est Créteil Val de Marne/Université Paris Diderot/Institut de Recherche pour le Développement/Institut National de la Recherche Agronomique-Unité Mixte de Recherche 7618, Institute of Ecology and Environmental Sciences Paris, Université Pierre et Marie Curie, 75005 Paris, France;
authors.forename: Tom J M
authors.initials: TJ
authors.lastname: Van Dooren
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands;International Institute for Applied Systems Analysis, Laxenburg, Austria; and.
authors.forename: Johan A J
authors.initials: JA
authors.lastname: Metz
authors.affiliation: Naturalis Biodiversity Center, 2333 CR, Leiden, The Netherlands;Group of Evolutionary Ecology, University of Antwerp, 2020 Antwerp, Belgium.
authors.forename: Clara M A
authors.initials: CM
authors.lastname: ten Broek
mesh_terms: D000818:Animals; D000829:Animals, Domestic; D005075:Biological Evolution; D001696:Biomechanical Phenomena; D049628:Body Size; D007178:Inbreeding; D008159:Lumbar Vertebrae / Q000033:anatomy & histology / Q000502:physiology; D008322:Mammals / Q000033:anatomy & histology* / Q000502:physiology*; D012420:Running / Q000502:physiology*; D012447:Sacrum / Q000033:anatomy & histology / Q000502:physiology; D013045:Species Specificity; D013131:Spine / Q000033:anatomy & histology* / Q000502:physiology*
keywords: Eco-Evo-Devo; body plans; evolutionary conservation; locomotion; musculoskeletal system; stabilizing selection