Comparative terrestrial locomotion of the Hooker's sea lion (Phocarctos hookeri) and the New Zealand fur seal (Arctocephalus forsteri): evolutionary and ecological implications

Abstract

Quantitative and qualitative analyses of filming studies reveal that fundamental differences exist between the gaits of the New Zealand fur seal (Arctocephalus forsteri) and the Hooker's sea lion (Phocarctos hookeri). Terrestrial locomotion of the latter species is similar to that of terrestrial vertebrates in which the limbs are moved in sequence, alternately and independently. In contrast, the gait of the New Zealand fur seal does not conform to this sequence, the hind limbs being moved in unison. The gaits of both species are defined and illustrated. The limbs of otariids are structurally adapted for a semi-aquatic lifestyle and consequently large oscillations of the centre of gravity are necessary to enable the limbs to be lifted and protracted during terrestrial locomotion. Phocarctos hookeri achieves this by transferring weight in the transverse plane while in A.forsteri it is in the sagittal plane. Hind limb movements are distinctly different; P. hookeri moves each hind limb individually by the combined action of limb protraction and rotation of the pelvis while A. forsteri moves its hind limbs together, predominantly by flexion of the posterior axial skeleton. While terrestrial locomotion in these species is achieved by fundamentally different gaits, post cranial elements of A. forsteri and P. hookeri are barely distinguishable; selection for the behavioural control of terrestrial locomotion has apparently preceded structural modifications. The gaits are considered to be ecological specializations which are adaptations to the mechanical problems imposed by different habitats. Gaits of these species appear typical or representative of members of their inferred subfamilies (Arctocephalinae and Otariinae). The gaits of A. forsteri and P. hookeri are however paradoxical in light of their inferred evolutionary history since the gait of the Hooker's sea lion resembles more closely that of the putative ancestors of otariids (arctoid fissiped carnivores) than does the gait of the supposedly more primitive New Zealand fur seal.