Sticking like sticky tape: tree frogs use friction forces to enhance attachment on overhanging surfaces

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...

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Bibliographic Details
Published in:Journal of the Royal Society interface 2013-03, Vol.10 (80), p.20120838-20120838
Main Authors: Endlein, Thomas, Ji, Aihong, Samuel, Diana, Yao, Ning, Wang, Zhongyuan, Barnes, W. Jon P., Federle, Walter, Kappl, Michael, Dai, Zhendong
Format: Article
Language:English
Subjects:
Adhesion
Animals
Anura - anatomy & histology
Anura - physiology
Climbing
Friction
Friction - physiology
Hindlimb - anatomy & histology
Hindlimb - physiology
Locomotion - physiology
Models, Biological
Peeling
Ranidae
Sliding
Tree Frog
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Summary: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.
ISSN:1742-5689
1742-5662
DOI:10.1098/rsif.2012.0838