ETNIER, S.A.*; PABST, D.A.; MCLELLAN, W.A.; University of North Carolina, Wilmington: Ontogenetic increases in the structural stiffness (“EI”) of bottlenose dolphins (Tursiops truncatus)

Fetal bottlenose dolphins (Tursiops truncatus) are bent laterally with the tail flukes and throat juxtaposed, requiring tremendous flexibility of the axial skeleton and associated soft tissues. Immediately upon birth, neonatal dolphins must swim to the surface to breathe, yet the dorsoventral oscillations used during locomotion may be compromised by the lateral flexibility required en utero. Anecdotally, neonates are uncoordinated swimmers, which may be associated with the inability to stabilize lateral deformations. Thus, the unique fetal position of dolphins, coupled with the immediate locomotor demands at birth, place conflicting mechanical demands on the tailstock. We examined lateral and dorsoventral flexibility of the tailstock in an ontogenetic series of stranded bottlenose dolphins including fetuses, neonates, and young of the year (TL: 86.5-146 cm). We determined flexibility by immobilizing the head and thorax, applying a known load to the tail, and measuring the resultant deformation. We calculated structural stiffness (“EI”) and measured curvature across the entire tailstock, as well as measuring regional curvature at the dorsal fin, anus, and mid-peduncle. Fetuses are more flexible, with respect to both structural stiffness and curvature, than neonates or juveniles. During fetal growth, flexibility decreases rapidly, but this rate of change slows after birth. The ratio of lateral to dorsoventral stiffness varies only slightly in all age classes. Fetuses are most flexible near the anus, while the position of maximum flexibility shifts caudally in more mature animals. The conflict between the functional demands of fetal and neonatal dolphins appears to be minimized by the rapidly changing mechanical properties of the tailstock.