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Pinnipedia, an order of semi-aquatic marine mammals, adapted a body design to locomote both aquatically and terrestrially. The limbs of these amphibious mammals are modified as flippers, which are beneficial for aquatic locomotion, but can limit their locomotion on land. Phocids (true seals) have reduced forelimbs and are unable to bring their hindlimbs beneath them during terrestrial locomotion. Otariids, like the California sea lion (Zalophus californianus), have enlarged forelimbs and can bring their hindlimbs under the body to locomote quadrupedally on land. Due to these differences, phocids are expected to move on land with greater energetic costs compared to otariids. The energetic cost of terrestrial locomotion in pinnipeds has only been examined in the elephant seal. To compare the mechanical costs of transport of terrestrial locomotion between otariids and phocids, one male and two female adult California sea lions were video recorded galloping across a runway. The center of mass, along with six other anatomical points, were digitized to obtain variables such as velocity (𝑉𝑉), amplitude of heave (𝐴𝐴), and the frequency (𝑓𝑓) of oscillations during the gallop cycle of the California sea lions. These variables represent the principal parameters of a biomechanical model that computes the power output of individuals. Power input and cost of transport were compared between four pinniped species: the California sea lion, the harbor seal, the gray seal, and the Northern elephant seal. The model indicates that the quadrupedal gait of otariids has a lower cost of transport than the undulatory gait of phocids.

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