Morphology of the vertebral centra in dolphins from the southwestern South Atlantic: A 3D morphometric approach and functional implications

Abstract

In dolphins, centrum shape is one of the features that allows determination of stable and flexible regions in the vertebral column. The Commerson's (Cephalorhynchus commersonii; n = 37), Peale's (Lagenorhynchus australis; n = 24), dusky (Lagenorhynchus obscurus; n = 29), and hourglass dolphins (Lagenorhynchus cruciger; n = 10) are closely related species inhabiting the Southern Hemisphere that have diverse prey and habitat preferences. We applied 3D geometric morphometrics to describe differences in centrum shape along the vertebral columns of these species, and hypothesize how these differences may affect swimming. On each column, we chose a maximum of eight vertebrae and digitized 18 landmarks on each centrum with a Microscribe G2X. We explored shape differences amongst regions employing principal components analyses and computing Mahalanobis distances. We describe differences in centrum shape in relation to functional regions and among species; and analyze shape changes in relation to particular biomechanical requirements. The species studied here may be partially sympatric in the Southern Hemisphere, but they have important differences in foraging ecology and habitat preferences that could be related to differences in centrum shape along the vertebral column.

Vertebral Morphology in Partially Sympatric Dolphins: A 3D Approach

“…sima). In contrast, Marchesi et al (2020a) found morphological differences related to potential differences in thorax flexibility in the four species studied here. This finding was also supported here, as the thoracic region showed heterogeneity in vertebral morphology, allowing to identify two potentially stable areas (Th and ThTa).…”

“…This projection was positioned so that its center would coincide with the anatomical landmark on the center of the centrum face. Accordingly, the vertical line of the "asterisk" passed through the vertical axis of the neural spine (see also Marchesi et al, 2020a). After positioning the projection, we placed one landmark in the center of the face and landmarks on the centrum outline at the intersection of the equiangular radial arrangement and the centrum face outline.…”

“…In both species, stability in the mid-body might help reduce recoil movements and minimize drag forces, whereas long neural processes provide a mechanical advantage for attached swimming muscles. In these species, the torso could also be acting as an oscillatory beam, storing potential energy (Pabst, 1996;Marchesi et al, 2017Marchesi et al, , 2020a. This elastic energy could complementarily replace some of the muscle work that is required to accelerate or decelerate the fluke, functioning as an elastic spring (Pabst, 1996).…”

Vertebral morphology in extant porpoises: Radiation and functional implications

“…Caudally, the vertebra with a neural process perpendicular to the body axis after which neural process have caudal orientation was chosen to represent the synclinal point (SP; sensu Slijper, 1936). This vertebra was equivalent to the boundary between mid-and posterior torso employed in previous studies on dolphins (Marchesi et al, 2021;Marchesi, Boy, et al, 2020;. Finally, the central vertebra of the tailstock (TS) was chosen (Figure 2).…”

“…These authors discussed four possible mechanical scenarios that could affect vertebral column regions: areas of potential elastic stability, with short centrum length and low convexity of the faces; areas of potential rigidity, with long centra and low convexity of the faces; areas with high rotation potential but low vertical displacement, with short centrum length and highly convex faces; and areas with high rotation potential and vertical displacement potential, with long centrum length and highly convex faces (see Buchholtz & Schur, 2004;Marchesi, Boy, et al, 2020). Particular vertebral morphologies with adaptive biomechanical implications have been reported for coastal and pelagic cetaceans (see, Buchholtz & Schur, 2004;Gillet et al, 2019;Marchesi, Boy, et al, 2020;Woodward, 2006).…”