Summers A P, Darouian K F, Richmond A M, Brainerd E L
Department of Biology, University of Massachusetts, Amherst 01003, USA.
J Exp Zool. 1998 Jul 1;281(4):280-7. doi: 10.1002/(sici)1097-010x(19980701)281:4<280::aid-jez4>3.0.co;2-k.
Studies of aquatic prey capture in vertebrates have demonstrated remarkable convergence in kinematics between diverse vertebrate taxa. When feeding in water, most vertebrates employ large-amplitude hyoid depression to expand the oral cavity and suck in water along with the prey. In contrast, vertebrates feeding on land exhibit little or no hyoid depression. In this study we compared the kinematics of terrestrial and aquatic prey capture within one species of turtle, Terrapene carolina, in order to determine whether an individual species can modulate the magnitude of hyoid depression between air and water. High-speed video (250 frames per second) showed that hyoid depression was over three times greater in aquatic than in terrestrial feedings, indicating that T. carolina is able to modulate hyoid depression magnitude depending on the medium in which feeding occurs. In addition, we observed medium-dependent modulation of hyoid depression in another turtle, Heosemys grandis, and large-amplitude hyoid depression during aquatic feeding in Kinosternon leucostomum, Platysternon megacephalum, and juvenile Chelydra serpentina. In all of these turtles, hyoid depression produced oral cavity expansion during aquatic feeding, but the earthworm prey were never sucked toward the predators. Prey were captured by neck extension (ram feeding), and we conclude that the function of hyoid depression during aquatic feeding in cryptodire turtles is to prevent the forward motion of the predator from pushing the prey away (compensatory suction). Aquatic feeding is probably the primitive condition for all extant turtles, and thus terrestrial feeding in T. carolina and other turtles is a secondarily derived characteristic. We conclude from this historical pattern that it is not appropriate to use extant turtles in attempts to reconstruct the terrestrial feeding mechanisms of primitive amniotes.
对脊椎动物水生猎物捕获的研究表明，不同脊椎动物类群在运动学上存在显著趋同。在水中觅食时，大多数脊椎动物会大幅度压低舌骨以扩张口腔，并连同猎物一起吸入水。相比之下，在陆地上觅食的脊椎动物很少或根本不会压低舌骨。在本研究中，我们比较了一种龟类——卡罗莱纳箱龟（Terrapene carolina）在陆地和水生环境中捕获猎物的运动学，以确定单个物种是否能够调节在空气和水中舌骨压低的幅度。高速视频（每秒250帧）显示，水生觅食时舌骨压低幅度比陆生觅食时大三倍多，这表明卡罗莱纳箱龟能够根据觅食所处的介质调节舌骨压低幅度。此外，我们在另一种龟——巨鳖（Heosemys grandis）中观察到了舌骨压低的介质依赖性调节，并且在白吻动胸龟（Kinosternon leucostomum）、大头平胸龟（Platysternon megacephalum）和幼年蛇鳄龟（Chelydra serpentina）的水生觅食过程中也观察到了大幅度的舌骨压低。在所有这些龟类中，水生觅食时舌骨压低会导致口腔扩张，但蚯蚓猎物从未被吸向捕食者。猎物是通过伸展颈部捕获的（猛冲式捕食），我们得出结论，隐颈龟类水生觅食时舌骨压低的功能是防止捕食者向前运动将猎物推开（补偿性吸食）。水生觅食可能是所有现存龟类的原始状态，因此卡罗莱纳箱龟和其他龟类的陆生觅食是次生衍生特征。基于这种历史模式，我们得出结论，试图利用现存龟类来重建原始羊膜动物的陆生觅食机制是不合适的。
