Locomotion, Terrestrial

Abstract

This chapter discusses the terrestriality of the obligate marine mammals. Among Cetacea, Sirenia, and Pinnipedia, only pinnipeds are able to locomote on land. Extant pinnipeds exhibit a general body plan consisting of a streamlined torso with major portions of the appendicular skeleton concealed within the body contour. The knee joint is held in an obligate flexed posture and proximal limb elements are shortened, whereas distal limb elements are elongated. In addition the feet are modified as flippers with retention of five digits and elongation of the thumb, big toe, and little toe. Three patterns of combined aquatic and terrestrial locomotion are recognized: forelimb propulsion and quadrapedal walking (otariids); hindlimb propulsion and quadrupedal walking coupled with sternal lunging (odobenids); and hindlimb propulsion and essentially limb-free undulation (phocids). Otariids have retained the greatest degree of terrestriality and, surprisingly, represent the latest diverging group of crown pinnipeds. Like otariids, their fore- and hindlimbs are capable of quadrupedal support and locomotion, their feet are held in a plantigrade stance, and their hind feet are able to rotate under the body. Further Phocids have a very divergent mode of terrestrial locomotion. Their fore- and hindlimbs are, for the most part, incapable of supporting the body, the feet are typically held free of the substrate, and the hind feet cannot be rotated forward under the body and instead are held in a relatively hyperextended posture and directed backward with palmar surfaces opposed.