Extreme positive allometry of animal adhesive pads and the size limits of adhesion-based climbing.

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Clemente, Christofer J 
Dittrich, Alex 
Kuo, Chi-Yun 
Crosby, Alfred J 

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.

adhesion, bio-inspired adhesives, evolution, scaling, Adhesiveness, Animals, Movement
Journal Title
Proc Natl Acad Sci U S A
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Proceedings of the National Academy of Sciences
Biotechnology and Biological Sciences Research Council (BB/I008667/1)
Biotechnology and Biological Sciences Research Council (BB/E004156/1)
Human Frontier Science Program (HFSP) (RGP0034/2012)
We are sincerely grateful to all our colleagues who readily shared published and unpublished data with us: Aaron M. Bauer, Jon Barnes, Niall Crawford, Thomas Endlein, Hanns Hagen Goetzke, Thomas E. Macrini, Anthony P. Russell & Joanna M. Smith. We also thank Casey Gilman, Dylan Briggs, Irina Showalter, Dan King and Mike Imburgia for their assistance with the collection of gecko toepad data. This study was supported by research grants from the UK Biotechnology and Biological Sciences Research Council (BB/I008667/1) to WF, the Human Frontier Science Programme (RGP0034/2012) to DI, AJC and WF, the Denman Baynes Senior Research Fellowship to DL and a Discovery Early Career Research Fellowship (DE120101503) to CJC.