title: The use of clamping grips and friction pads by tree frogs for climbing curved surfaces.
abstract: Most studies on the adhesive mechanisms of climbing animals have addressed attachment against flat surfaces, yet many animals can climb highly curved surfaces, like twigs and small branches. Here we investigated whether tree frogs use a clamping grip by recording the ground reaction forces on a cylindrical object with either a smooth or anti-adhesive, rough surface. Furthermore, we measured the contact area of fore and hindlimbs against differently sized transparent cylinders and the forces of individual pads and subarticular tubercles in restrained animals. Our study revealed that frogs use friction and normal forces of roughly a similar magnitude for holding on to cylindrical objects. When challenged with climbing a non-adhesive surface, the compressive forces between opposite legs nearly doubled, indicating a stronger clamping grip. In contrast to climbing flat surfaces, frogs increased the contact area on all limbs by engaging not just adhesive pads but also subarticular tubercles on curved surfaces. Our force measurements showed that tubercles can withstand larger shear stresses than pads. SEM images of tubercles revealed a similar structure to that of toe pads including the presence of nanopillars, though channels surrounding epithelial cells were less pronounced. The tubercles' smaller size, proximal location on the toes and shallow cells make them probably less prone to buckling and thus ideal for gripping curved surfaces.
authors.affiliation: Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, 70569 Stuttgart, Germany endlein@is.mpg.de.
authors.forename: Thomas
authors.identifier: 0000-0002-6358-5523
authors.initials: T
authors.lastname: Endlein
authors.affiliation: Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People's Republic of China.
authors.forename: Aihong
authors.initials: A
authors.lastname: Ji
authors.affiliation: Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People's Republic of China.
authors.forename: Shanshan
authors.initials: S
authors.lastname: Yuan
authors.affiliation: Centre for Cell Engineering, University of Glasgow, Joseph Black Building, University Avenue, Glasgow G12 8QQ, UK.
authors.forename: Iain
authors.initials: I
authors.lastname: Hill
authors.affiliation: Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People's Republic of China.
authors.forename: Huan
authors.initials: H
authors.lastname: Wang
authors.affiliation: Centre for Cell Engineering, University of Glasgow, Joseph Black Building, University Avenue, Glasgow G12 8QQ, UK.
authors.forename: W Jon P
authors.initials: WJ
authors.lastname: Barnes
authors.affiliation: Institute of Bio-inspired Structure and Surface Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street, Nanjing 210016, People's Republic of China.
authors.forename: Zhendong
authors.initials: Z
authors.lastname: Dai
authors.affiliation: Max Planck Institute for Intelligent Systems, Heisenbergstrasse 3, 70569 Stuttgart, Germany.
authors.forename: Metin
authors.initials: M
authors.lastname: Sitti
mesh_terms: D000268:Adhesiveness; D000825:Animal Structures / Q000033:anatomy & histology*; D000818:Animals; D001001:Anura / Q000033:anatomy & histology*; D017276:Friction; D013499:Surface Properties
keywords: adhesive pad; climbing; friction grip; subarticular tubercle; tree frog