A novel, bounding gait in swimming turtles: implications for aquatic locomotor diversity

Turtles are an iconic lineage in studies of animal locomotion, typifying the use of slow, alternating footfalls during walking. Alternating movements of contralateral limbs are also typical during swimming gaits for most freshwater turtles. Here, we report a novel gait in turtles, in which the pleurodire Emydura subglobosa swims using a bounding gait that coordinates bilateral protraction of both forelimbs with bilateral retraction of both hindlimbs. Use of this bounding gait is correlated with increased limb excursion and decreased stride frequency, but not increased velocity when compared to standard swimming strokes. Bounding by E. subglobosa provides a second example of a non-mammalian lineage that can use bounding gaits, and may give insight into the evolution of aquatic flapping. Parallels in limb muscle fascicle properties between bounding turtles and crocodylids suggest a possible musculoskeletal mechanism underlying the use of bounding gaits in particular lineages.

龟类是动物运动学研究中的标志性类群，其行走行为以缓慢的交替式步行动作为典型特征。多数淡水龟的游泳步态同样以对侧肢体交替运动为典型模式。本研究首次报道了龟类的一种新型步态：侧颈龟亚目（Pleurodire）的圆澳龟（Emydura subglobosa）在游泳时采用奔跃步态（bounding gait），即双侧前肢同步前伸，并与双侧后肢同步后伸形成协调配合。相较于常规游泳划水动作，采用该奔跃步态时，龟类的肢体位移幅度提升、步频降低，但运动速度并未随之提升。圆澳龟的奔跃行为为可使用奔跃步态的非哺乳类类群提供了第二个研究案例，或可为水生拍水运动的演化机制提供新的研究视角。奔跃龟类与鳄科（Crocodylidae）动物在肢体肌束特性上的相似性，提示了特定类群采用奔跃步态背后可能存在的肌肉骨骼调控机制。