Zalophus hindflipper turning metrics and angle of attack

California sea lions (Zalophus californianus) are a highly maneuverable
species of marine mammal. During uninterrupted, rectilinear swimming, sea
lions oscillate their foreflippers to propel themselves forward without
aid from the collapsed hindflippers, which are passively trailed. During
maneuvers such as turning and leaping (porpoising), the hindflippers are
spread into a delta-wing configuration. There is little information
defining the role of otarrid hindflippers as aquatic control surfaces. To
examine Z. californianus hindflippers during maneuvering, trained sea
lions were video recorded underwater through viewing windows performing
porpoising behaviors and banking turns. Porpoising by a trained sea lion
was compared with sea lions executing the maneuver in the wild. Anatomical
points of reference (ankle and hindflipper tip) were digitized from videos
to analyze various performance metrics and define the use of the
hindflippers. During a porpoising bout, the hindflippers were considered
to generate lift when surfacing with a mean angle of attack of 14.6±6.3
deg. However, while performing banked 180 deg turns, the mean angle of
attack of the hindflippers was 28.3±7.3 deg, and greater by another 8-12
deg for the maximum 20% of cases. The delta-wing morphology of the
hindflippers may be advantageous at high angles of attack to prevent
stalling during high-performance maneuvers. Lift generated by the
delta-shaped hindflippers, in concert with their position far from the
center of gravity, would make these appendages effective aquatic control
surfaces for executing rapid turning maneuvers.

加州海狮（Zalophus californianus）是一种高度灵活的海洋哺乳动物。在持续直线游泳时，海狮通过摆动前鳍推进自身前进，无需折叠后鳍的辅助——后者仅被动拖曳。在转弯、豚跃（porpoising）等机动动作中，后鳍会展开成三角翼形态。关于海狮科动物后鳍作为水生控制面的作用，目前相关研究信息较少。为探究加州海狮在机动过程中的后鳍功能，研究人员通过观察窗对受训海狮在水下执行豚跃和倾斜转弯动作的过程进行了视频记录。研究将受训海狮的豚跃动作与野生海狮的同类动作进行了对比。研究人员从视频中提取了解剖学参考点（脚踝及后鳍尖端）的数字化数据，以分析各项性能指标并明确后鳍的功能。在豚跃过程中，后鳍在浮出水面时被认为会产生升力，其平均攻角为14.6±6.3度。然而，在执行180度倾斜转弯时，后鳍的平均攻角为28.3±7.3度，且在20%的极端案例中，攻角还会再增加8-12度。后鳍的三角翼形态在高攻角条件下可能具有优势，可防止其在高性能机动过程中发生失速。三角形态后鳍产生的升力，结合其远离重心的位置，使这一附肢成为执行快速转弯机动的高效水生控制面。