Terrestrial locomotion of the northern elephant seal (Mirounga angustirostris): Limitation of large aquatically adapted seals on land?

Abstract

The aquatic specializations of phocid seals have restricted their ability to locomote on land. The amphibious northern elephant seal, Mirounga angustirostris, is the second largest phocid seal in the world, with males reaching 2700 kg. Although elephant seals are proficient swimmers and deep divers, their extreme size and aquatic specializations limit terrestrial movement. The kinematics of terrestrial locomotion in northern elephant seals were analyzed from video recordings of animals observed on the beach of Añõ Nuevo State Reserve, CA, USA. The seals moved using a series of rhythmic undulations produced by dorsoventral spinal flexion. The traveling spinal wave moved anteriorly along the dorsal margin of the body with the chest, pelvic region and foreflippers serving as the main points of contact with the ground. The hindflippers were not used. The spinal wave and foreflippers were used to lift the chest off the ground as the body was pushed forward from the pelvis as the foreflippers were retracted to pull the body forward. Seals moved over land at 0.41–2.56 m s−1 (0.12–0.71 body lengths s−1). The frequency and amplitude of spinal flexions both displayed a direct increase with increasing speed. The duty factor for the pelvic region decreased with increasing velocity while the duty factor of the foreflipper remained constant. Kinematic data for elephant seals and other phocids were used in a biomechanical model to calculate the mechanical energy expended during terrestrial locomotion. The elephant seals were found to expend more energy when traveling over land for their size than smaller phocids. The unique method of terrestrial movement also exhibited greater energy expenditure on land than values for large quadrupeds. The trade-off for the northern elephant seal is that its massive size and morphology have well adapted it to an aquatic existence but limited its locomotor performance (i.e. speed, endurance) on land.