Centre for Cell Engineering, University of Glasgow, Joseph Black Building, University Avenue, Glasgow G12 8QQ, UK.
J R Soc Interface. 2013 Jan 16;10(80):20120838. doi: 10.1098/rsif.2012.0838. Print 2013 Mar 6.
To live and clamber about in an arboreal habitat, tree frogs have evolved adhesive pads on their toes. In addition, they often have long and slender legs to facilitate not only long jumps, but also to bridge gaps between leaves when climbing. Both adhesive pads and long limbs are used in conjunction, as we will show in this study. Previous research has shown that tree frogs change from a crouched posture (where the limbs are close to the body) to a sprawled posture with extended limbs when clinging on to steeper inclines such as vertical or overhanging slopes. We investigated this change in posture in White's tree frogs (Litoria caerulea) by challenging the frogs to cling onto a tiltable platform. The platform consisted of an array of 24 three-dimensional force transducers, which allowed us to measure the ground reaction forces of the frogs during a tilt. Starting from a crouched resting position, the normal forces on the forelimbs changed sign and became increasingly negative with increasing slope angle of the platform. At about 106° ± 12°, tilt of the platform the frogs reacted by extending one or two of their limbs outwards. At a steeper angle (131° ± 11°), the frogs spread out all their limbs sideways, with the hindlimbs stretched out to their maximum reach. Although the extension was strongest in the lateral direction, limbs were significantly extended in the fore-aft direction as well. With the extension of the limbs, the lateral forces increased relative to the normal forces. The large contribution of the in-plane forces helped to keep the angle between the force vector and the platform small. The Kendall theory for the peeling of adhesive tape predicts that smaller peel angles lead to higher attachment forces. We compare our data with the predictions of the Kendall model and discuss possible implications of the sliding of the pads on the surface. The forces were indeed much larger for smaller angles and thus can be explained by peeling theory.
为了在树木栖息地中生活和攀爬，树蛙的脚趾上已经进化出了粘性垫。此外，它们的腿通常又长又细，不仅便于长距离跳跃，还便于在攀爬时跨越树叶之间的间隙。正如我们将在本研究中展示的那样，粘性垫和长肢是协同作用的。以前的研究表明，树蛙在攀爬更陡峭的斜坡（如垂直或悬垂的斜坡）时，会从蹲伏姿势（四肢靠近身体）转变为伸展四肢的伸展姿势。我们通过让树蛙在可倾斜的平台上攀爬来研究这种姿势的变化。该平台由一组 24 个三维力传感器组成，使我们能够在倾斜时测量树蛙的地面反作用力。从蹲伏的休息位置开始，前肢的法向力的符号发生变化，并且随着平台坡度角的增加而变得越来越负。当平台倾斜约 106°±12°时，树蛙会向外伸展一到两条肢体。在更陡峭的角度（131°±11°）下，树蛙会将所有的肢体向侧面伸展，后腿伸展到最大伸展范围。尽管在侧向方向上的伸展最强，但肢体在前后方向上也明显伸展。随着肢体的伸展，侧向力相对于法向力增加。平面力的大贡献有助于保持力矢量与平台之间的角度较小。Kendall 理论预测，较小的剥离角度会导致更高的附着力。我们将我们的数据与 Kendall 模型的预测进行了比较，并讨论了垫在表面上滑动的可能影响。较小的角度确实会产生更大的力，因此可以用剥离理论来解释。
