Locomotor behaviour among pinnipeds

Abstract

Pinnipeds (Phocidae, Otariidae, Odobenidae) form a group of semi-aquatic carnivorous mammals whose locomotion, both in marine and terrestrial environments, is the result of a long process of adaptive evolution from a terrestrial quadrupedal ancestor. This review of the scientific literature focuses on the aquatic and terrestrial locomotor behaviours of the three extant pinniped families. In the aquatic environment, Phocidae swim primarily through pelvic and lateral oscillations of the hind limbs, made possible by a more flexible lumbar spine and a stiffer thoracic region. Otariidae propel themselves using their forelimbs, thanks to pectoral flippers functioning as wings, a flexible spine and poorly developed epaxial musculature in the lumbar region. The Odobenidae adopt an intermediate mode, primarily pelvic but with some involvement of the forelimbs at low speeds, combined with a relatively rigid thoracolumbar spine due to their large body mass. Regarding terrestrial movement, Phocidae move by dorsoventral or sinusoidal vertebral undulations, without cranial rotation of the hind limbs, which makes their locomotion energetically costly and confines them to coastal areas. Otariidae retain functional terrestrial locomotion, combining a sideways gait with an asymmetrical gallop, thanks in particular to a robust big toe that provides the main propulsion. The energy expenditure associated with movement, although lower than that of Phocidae, remains higher than that of terrestrial carnivores due to the enlargement of the forelimbs and the absence of tendinous-ligamentous elastic storage. There is very little literature on the terrestrial locomotion of Odobenidae. However, given the significant rigidity of the thoracolumbar segment, it is highly likely that the initial impulse comes from trunk flexion. In conclusion, locomotion in pinnipeds illustrates evolutionary trade-offs between aquatic performance and terrestrial mobility, with different morphofunctional solutions depending on the family. This knowledge is fundamental to the veterinary medicine of these species, both in clinical assessment and in understanding their biological constraints.