Patterns of morphological variation of the vertebral column in dolphins

Cetaceans swim by the alternate action of their epiaxial and hypaxial muscles and their propulsive movements are confined to the vertical plane. Changes in the shape and mechanical properties of vertebrae strongly affect their function during oscillatory swimming. The first objective of this study was to provide a quantitative characterization of vertebral morphology in representatives of the Delphinidae and Pontoporiidae families. A novel morphometric approach was applied, using nine vertebral measurements and three indices. The second objective was to assess the relationship between morphology and both habitat and size through regression analyses. The phylogenetic structure of the distribution of characters was also explored by estimating phylogenetic signal. No relationship was found between morphology and habitat or size, but vertebral measurements and indices showed a significant phylogenetic signal. Morphological profiles indicated that coastal and oceanic delphinid species had a conservative regionalization of the vertebral column. All delphinid species showed discoidal centra morphology, while Pontoporia blainvillei presented a spool-shaped morphology. Differences in vertebral morphology and inferred muscular architecture between P. blainvillei and delphinids could indicate distinct dynamics of vertebral movement during swimming. However, other complex and specific functional relationships and life-history traits may also be influencing vertebral morphology. The detailed study of the complex evolutionary history of lineages could bring to light other clarifying dimensions for understanding morphological evolution in odontocetes.