Date of Award

Fall 2020

Document Type


Degree Name

Master of Science (MS)



Committee Chairperson

Frank Fish, PhD

Committee Member

Jennifer Maresh, PhD

Committee Member

Michael Rosario, PhD


The hindflippers of California sea lions (Zalophus californianus) have previously been overlooked as aquatic control surfaces. Although passively trailed in rectilinear swimming, the hindflippers are abducted into a delta-wing shape during aquatic maneuvers. As the anatomy of sea lion hindflippers had not previously been described, anatomical/morphological examinations were completed via scaled measurements and dissections. It was found that the tendons of Flexor Hallucis Longus and Flexor Digitorum Longus insert into the collagen matrix of the crenellations instead of onto bone and the tendons of Flexor Digitorum Brevis contain foramen through which the tendons of Flexor Digitorum Longus pass. The anatomy indicates an increased demand for the ability to abduct and flex the digits and crenellations. Between the adducted and abducted postures of the hindflipper, there is a mean 22.6% increase in planar surface area. It was also found that the crenellations, which contain no bone, represent 17.2% of the total length of the hindflipper. To examine hindflippers during maneuvering, trained sea lions were video recorded performing porpoising behaviors and 180° banking turns. Anatomical points of reference were digitized from videos to measure velocity and angle of attack. During porpoising, the mean hindflipper angle of attack through the submerged lift-producing phase was 14.6°. During banking turns, the mean angle of attack was 28.3 ± 7.3°. Zalophus owes much of its maneuvering capacity to immense body flexibility, however this study provides evidence that supports the assertion that the hindflippers act as aquatic control surfaces analogous to biological delta-wings to achieve high-performance maneuvers.