MacLaren Jamie A, Nauwelaerts Sandra
Department of Biology, Universiteit Antwerpen, Building D, Campus Drie Eiken, Universiteitsplein, Wilrijk, Antwerp, 2610, Belgium.
Centre for Research and Conservation, Koninklijke Maatschappij voor Dierkunde (KMDA), Koningin Astridplein 26, Antwerp, 2018, Belgium.
J Morphol. 2017 Nov;278(11):1517-1535. doi: 10.1002/jmor.20728. Epub 2017 Jul 13.
The distal forelimb (autopodium) of quadrupedal mammals is a key morphological unit involved in locomotion, body support, and interaction with the substrate. The manus of the tapir (Perissodactyla: Tapirus) is unique within modern perissodactyls, as it retains the plesiomorphic tetradactyl (four-toed) condition also exhibited by basal equids and rhinoceroses. Tapirs are known to exhibit anatomical mesaxonic symmetry in the manus, although interspecific differences and biomechanical mesaxony have yet to be rigorously tested. Here, we investigate variation in the manus morphology of four modern tapir species (Tapirus indicus, Tapirus bairdii, Tapirus pinchaque, and Tapirus terrestris) using a geometric morphometric approach. Autopodial bones were laser scanned to capture surface shape and morphology was quantified using 3D-landmark analysis. Landmarks were aligned using Generalised Procrustes Analysis, with discriminant function and partial least square analyses performed on aligned coordinate data to identify features that significantly separate tapir species. Overall, our results support the previously held hypothesis that T. indicus is morphologically separate from neotropical tapirs; however, previous conclusions regarding function from morphological differences are shown to require reassessment. We find evidence indicating that T. bairdii exhibits reduced reliance on the lateral fifth digit compared to other tapirs. Morphometric assessment of the metacarpophalangeal joint and the morphology of the distal facets of the lunate lend evidence toward high loading on the lateral digits of both the large T. indicus (large body mass) and the small, long limbed T. pinchaque (ground impact). Our results support other recent studies on T. pinchaque, suggesting subtle but important adaptations to a compliant but inclined habitat. In conclusion, we demonstrate further evidence that the modern tapir forelimb is a variable locomotor unit with a range of interspecific features tailored to habitual and biomechanical needs of each species.
四足哺乳动物的前肢远端（手足）是参与运动、身体支撑以及与地面相互作用的关键形态学单元。貘（奇蹄目：貘属）的 manus 在现代奇蹄目中独一无二，因为它保留了原始的四指（四趾）状态，这种状态在基础马科动物和犀牛中也有体现。尽管种间差异和生物力学中轴性尚未经过严格测试，但已知貘的 manus 呈现解剖学上的中轴对称。在这里，我们使用几何形态测量方法研究了四种现代貘属物种（印度貘、拜氏貘、山貘和南美貘）的 manus 形态变异。对手足骨骼进行激光扫描以获取表面形状，并使用三维地标分析对形态进行量化。使用广义普罗克拉斯提斯分析对齐地标，并对对齐的坐标数据进行判别函数和偏最小二乘分析，以识别能显著区分貘属物种的特征。总体而言，我们的结果支持了之前的假设，即印度貘在形态上与新热带貘不同；然而，先前从形态差异得出的关于功能的结论需要重新评估。我们发现有证据表明，与其他貘相比，拜氏貘对外侧第五指的依赖程度降低。掌指关节的形态测量评估以及月骨远端小关节面的形态为大型的印度貘（体重较大）和小型、四肢修长的山貘（地面冲击）的外侧指承受高负荷提供了证据。我们的结果支持了最近关于山貘的其他研究，表明其对顺应性但倾斜的栖息地有微妙但重要的适应性。总之，我们进一步证明，现代貘的前肢是一个可变的运动单元，具有一系列根据每个物种的习性和生物力学需求定制的种间特征。
