Department of Evolution, Ecology, and Organismal Biology, University of California, Riverside, CA, USA.
UTEP Biodiversity Collections, The University of Texas at El Paso, El Paso, TX, USA.
J Anat. 2021 Dec;239(6):1503-1515. doi: 10.1111/joa.13511. Epub 2021 Jul 15.
The remarkable ability of geckos to adhere to smooth surfaces is often thought of in terms of external structures, including the branching setae that make contact with the surface producing van der Waals forces. Some geckos also exhibit unique movements of the distal segments of the limbs during locomotion and static clinging, including active digital hyperextension and considerable pedal rotation. During static clinging, geckos can exhibit considerable adduction/abduction of the pes while the crus and thigh remain firmly adpressed to the substratum. This decoupling of pedal adduction/abduction from ankle flexion/extension and pedal long-axis rotation is a significant departure from pedal displacements of a typical lizard lacking adhesive ability. The structure of the ankle is likely key to this decoupling, although no detailed comparison of this complex joint between pad-bearing geckos and other lizards is available. Here we compare the configuration of the mesotarsal joint of nongekkotan lizards (Iguana and Pristidactylus) with that of the Tokay gecko (Gekko gecko) using prepared skeletons, scanning electron microscopy, and micro-computed tomographic (µCT) scans. We focus on the structure of the astragalocalcaneum and the fourth distal tarsal. The mesotarsal joint exhibits a suite of modifications that are likely associated with the secondarily symmetrical pes of pad-bearing geckos. For example, the lateral process of the astragalocalcaneum is much more extensive in G. gecko compared with other lizards. The mesotarsal joint exhibits several other differences permitting dissociation of long-axis rotation of the pes from flexion-extension movement, including a reduced ventral peg on the fourth distal tarsal, an articulatory pattern dominated by a well-defined, expansive distomesial notch of the astragalocalcaneum, and an associated broad proximodorsal articulatory facet of the fourth distal tarsal. Pad-bearing geckos are capable of effectively deploying their intricate adhesive system across a broad array of body angles because of this highly modified ankle. Future research should determine whether the differences encountered in G. gecko (and their extent) apply to the Gekkota as a whole and should examine how the elements of the ankle move dynamically during locomotion across a range of taxa.
壁虎能够在光滑表面上附着的非凡能力通常被认为与外部结构有关，包括与表面接触产生范德华力的分支刚毛。一些壁虎在运动和静态附着时还表现出肢体远端节段的独特运动，包括主动的数字超伸展和相当大的足旋转。在静态附着时，壁虎可以在跗跖骨和小腿仍然牢固地压在基质上时表现出相当大的内收/外展。这种足的内收/外展与踝关节弯曲/伸展和足长轴旋转的解耦是典型缺乏粘附能力的蜥蜴足位移的显著偏离。踝关节的结构可能是这种解耦的关键，尽管没有关于具有粘性能力的垫状壁虎和其他蜥蜴之间这种复杂关节的详细比较。在这里，我们使用准备好的骨骼、扫描电子显微镜和微计算机断层扫描（µCT）扫描比较了非壁虎蜥蜴（鬣蜥和大蜥蜴）和壁虎（壁虎）的跗跖关节的结构。我们专注于距骨-跟骨和第四远跗骨的结构。跗跖关节表现出一系列的变化，这些变化可能与垫状壁虎的二次对称的跗跖骨有关。例如，距骨-跟骨的外侧突在壁虎中比在其他蜥蜴中更为广泛。跗跖关节还表现出其他一些差异，允许足的长轴旋转与弯曲-伸展运动分离，包括第四远跗骨上较小的腹侧钉、由距骨-跟骨的明显、扩展的远中面切迹主导的关节模式，以及第四远跗骨的相关宽近背侧关节面。由于这种高度改良的踝关节，垫状壁虎能够有效地在广泛的身体角度上部署其复杂的粘附系统。未来的研究应该确定在壁虎中遇到的差异（及其程度）是否适用于整个壁虎科，并应该检查在跨越一系列分类群的运动过程中，踝关节的各个元素如何动态移动。
