Abstract

The pelvic limbs of the California sea lion show peculiar morphological adaptations, which enable rapid turns when swimming, but also allow weight bearing when walking on land.^1-3 Specifically, the California sea lion is able to rotate the lower leg and foot, despite the fact that the pelvic limb is encased by the body wall to the level of the tarsus.

Statements in the literature regarding how sea lions are able to walk on land are contradictory. Rotation of the pelvis, flexion of the vertebral column, or flexion of the hip and tarsus combined with extension of the stifle are said to enable the sea lion to bear weight when walking on land.^4,5 Our hypothesis is that sea lions are not able to rotate their pelvis, but flex the lumbar part of the vertebral column. The range of motion of the pelvic limb is limited by the fact that these are enclosed in the body wall.

The objective of this study is to analyze the range of motion of the iliosacral, hip, stifle, and tarsal joints. The extrinsic and intrinsic muscles and the joints of the pelvic limbs are studied morphologically to determine their actions on the pelvic limb in swimming and walking.

Three California sea lions were obtained from The Marine Mammal Center in Sausalito, California. Prior to dissection, series of computed topographic images (CTs) of the pelvic region in the flexed (walking), intermediate, and extended (swimming) position were obtained. Virtual 3D-reconstructions of the skeleton were analyzed to visualize the actual range of motion. Meticulous dissections of the intrinsic and extrinsic muscles of the pelvic limbs were done to reveal the species-specific morphology of the muscles and joints. In addition, videos of sea lions recorded in four zoos were analyzed frame by frame to understand the range of motion, e.g., extension versus flexion, adduction versus abduction, of the vertebral column, thigh, lower leg, tarsus, and foot in the transition from swimming to walking.

Three-dimensional reconstructions of the skeleton reveal no difference in the angles of the sacrum to the pelvis between the walking and swimming positions. For walking on the pelvic limbs, the hip joint is slightly flexed, the femur is adducted, and the angle of the stifle remains relatively unchanged, while tibia and fibula, as well as the tarsus are rotated medially. In addition, sea lions flex the lumbar part of the vertebral column to move their pelvic limbs forward for walking while maintaining the plantar surface of the foot in a ventral position. The tarsus enables a unique rotation of the foot, which is not seen in terrestrial but in arboreal mammals.

This study demonstrates unique morphological adaptations of the musculoskeletal system of California sea lions to their semi-aquatic lifestyle.

Acknowledgements

The authors gratefully acknowledge the support of Dr. Daniel J. Hillmann and of our colleagues in the Section of Diagnostic Imaging, Department of Veterinary Clinical Sciences, School of Veterinary Medicine, LSU.

The support of Chris Yates, National Oceanic and Atmospheric Administration, of Joe Cordaro, NOAA's Southwest Region Stranding Coordinator, and of Dr. Bill Van Bonn, Dr. Denise Greig and Lauren Rust, The Marine Mammal Center in Sausalito, California, is also gratefully appreciated.

References

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2.  Howell AB. Contributions to the comparative anatomy of the eared and earless seals. Proc US Nat Museum. 1929;73:1–143.

3.  Mori M. The skeleton and musculature of Zalophus. Okajima Folia Anat Jpn. 1958;3/4:203–284.

4.  Beentjes MP. Comparative terrestrial locomotion of the Hooker's sea lion and the New Sealand fur seal. Zool J Linnean Soc. 1990;98:307–325.

5.  English AW. Limb movements and locomotor function in the California sea lion. J Zool (London). 1976;178:361–364.