Comparative Zoology, Institute of Biology, Humboldt University of Berlin, Philippstrasse 12/13, 10115 Berlin, Germany.
Museum für Naturkunde, Leibniz Institute for Evolution and Biodiversity Science, Invalidenstraße 43, 10115 Berlin, Germany.
Proc Biol Sci. 2023 Nov 8;290(2010):20231592. doi: 10.1098/rspb.2023.1592. Epub 2023 Nov 1.
Besides manatees, the suspensory extant 'tree sloths' are the only mammals that deviate from a cervical count (CC) of seven vertebrae. They do so in opposite directions in the two living genera (increased versus decreased CC). Aberrant CCs seemingly reflect neck mobility in both genera, suggesting adaptive significance for their head position during suspensory locomotion and especially increased ability for neck torsion in three-toed sloths. We test two hypotheses in a comparative evolutionary framework by assessing three-dimensional intervertebral range of motion (ROM) based on exhaustive automated detection of bone collisions and joint disarticulation while accounting for interacting rotations of roll, yaw and pitch. First, we hypothesize that the increase of CC also increases overall neck mobility compared with mammals with a regular CC, and vice versa. Second, we hypothesize that the anatomy of the intervertebral articulations determines mobility of the neck. The assessment revealed that CC plays only a secondary role in defining ROM since summed torsion (roll) capacity was primarily determined by vertebral anatomy. Our results thus suggest limited neck rotational adaptive significance of the CC aberration in sloths. Further, the study demonstrates the suitability of our automated approach for the comparative assessment of osteological ROM in vertebral series.
除海牛外，现存唯一的悬停“树懒”是颈椎数（CC）偏离 7 个的哺乳动物。它们在两个现存属中向相反的方向偏离（增加与减少 CC）。异常的 CC 似乎反映了这两个属的颈部活动性，表明它们在悬停运动中头部位置的适应性意义，尤其是三趾树懒颈部扭转能力的增强。我们通过评估基于骨骼碰撞自动检测的三维椎间活动范围（ROM），并考虑滚转、偏航和俯仰的相互旋转，在比较进化框架中检验了两个假设。首先，我们假设与具有常规 CC 的哺乳动物相比，CC 的增加也会增加整体颈部活动性，反之亦然。其次，我们假设椎间关节的解剖结构决定了颈部的活动性。评估结果表明，CC 在定义 ROM 方面仅起次要作用，因为总扭转（滚转）能力主要由椎体解剖结构决定。因此，我们的研究结果表明，在树懒中，CC 异常对颈部旋转的适应性意义有限。此外，该研究表明我们的自动方法适用于对脊椎系列中骨骼 ROM 的比较评估。
