title: Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing.
abstract: Organismal functions are size-dependent whenever body surfaces supply body volumes. Larger organisms can develop strongly folded internal surfaces for enhanced diffusion, but in many cases areas cannot be folded so that their enlargement is constrained by anatomy, presenting a problem for larger animals. Here, we study the allometry of adhesive pad area in 225 climbing animal species, covering more than seven orders of magnitude in weight. Across all taxa, adhesive pad area showed extreme positive allometry and scaled with weight, implying a 200-fold increase of relative pad area from mites to geckos. However, allometric scaling coefficients for pad area systematically decreased with taxonomic level and were close to isometry when evolutionary history was accounted for, indicating that the substantial anatomical changes required to achieve this increase in relative pad area are limited by phylogenetic constraints. Using a comparative phylogenetic approach, we found that the departure from isometry is almost exclusively caused by large differences in size-corrected pad area between arthropods and vertebrates. To mitigate the expected decrease of weight-specific adhesion within closely related taxa where pad area scaled close to isometry, data for several taxa suggest that the pads' adhesive strength increased for larger animals. The combination of adjustments in relative pad area for distantly related taxa and changes in adhesive strength for closely related groups helps explain how climbing with adhesive pads has evolved in animals varying over seven orders of magnitude in body weight. Our results illustrate the size limits of adhesion-based climbing, with profound implications for large-scale bio-inspired adhesives.
authors.affiliation: Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom; dl416@cam.ac.uk.
authors.forename: David
authors.initials: D
authors.lastname: Labonte
authors.affiliation: School of Science and Engineering, University of the Sunshine Coast, Sippy Downs, QLD 4556, Australia;
authors.forename: Christofer J
authors.initials: CJ
authors.lastname: Clemente
authors.affiliation: Department of Life Sciences, Anglia Ruskin University, Cambridge CB1 1PT, United Kingdom;
authors.forename: Alex
authors.initials: A
authors.lastname: Dittrich
authors.affiliation: Biology Department, University of Massachusetts Amherst, Amherst, MA 01003;
authors.forename: Chi-Yun
authors.initials: CY
authors.lastname: Kuo
authors.affiliation: Department of Polymer Science and Engineering, University of Massachusetts Amherst, Amherst, MA 01003.
authors.forename: Alfred J
authors.initials: AJ
authors.lastname: Crosby
authors.affiliation: Biology Department, University of Massachusetts Amherst, Amherst, MA 01003;
authors.forename: Duncan J
authors.initials: DJ
authors.lastname: Irschick
authors.affiliation: Department of Zoology, University of Cambridge, Cambridge CB2 3EJ, United Kingdom;
authors.forename: Walter
authors.initials: W
authors.lastname: Federle
mesh_terms: D000268:Adhesiveness; D000818:Animals; D009068:Movement
keywords: adhesion; bio-inspired adhesives; evolution; scaling