Dataset Information

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.

SUBMITTER: Labonte D

PROVIDER: S-EPMC4747726 | biostudies-literature | 2016 Feb

REPOSITORIES: biostudies-literature

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

Labonte David D Clemente Christofer J CJ Dittrich Alex A Kuo Chi-Yun CY Crosby Alfred J AJ Irschick Duncan J DJ Federle Walter W

Proceedings of the National Academy of Sciences of the United States of America 20160119 5

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 positi ...[more]

PMID: 26787862

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Project description:BackgroundMany arachnids possess adhesive pads on their feet that help them climb smooth surfaces and capture prey. Spider and gecko adhesives have converged on a branched, hairy structure, which theoretically allows them to adhere solely by dry (solid-solid) intermolecular interactions. Indeed, the consensus in the literature is that spiders and their smooth-padded relatives, the solifugids, adhere without the aid of a secretion.Methodology and principal findingsWe investigated the adhesive contact zone of living spiders, solifugids and mites using interference reflection microscopy, which allows the detection of thin liquid films. Like insects, all the arachnids we studied left behind hydrophobic fluid footprints on glass (mean refractive index: 1.48-1.50; contact angle: 3.7-11.2°). Fluid was not always secreted continuously, suggesting that pads can function in both wet and dry modes. We measured the attachment forces of single adhesive setae from tarantulas (Grammostola rosea) by attaching them to a bending beam with a known spring constant and filming the resulting deflection. Individual spider setae showed a lower static friction at rest (26%±2.8 SE of the peak friction) than single gecko setae (Thecadactylus rapicauda; 96%±1.7 SE). This may be explained by the fact that spider setae continued to release fluid after isolation from the animal, lubricating the contact zone.SignificanceThis finding implies that tarsal secretions occur within all major groups of terrestrial arthropods with adhesive pads. The presence of liquid in an adhesive contact zone has important consequences for attachment performance, improving adhesion to rough surfaces and introducing rate-dependent effects. Our results leave geckos and anoles as the only known representatives of truly dry adhesive pads in nature. Engineers seeking biological inspiration for synthetic adhesives should consider whether model species with fluid secretions are appropriate to their design goals.

Dataset Information

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

Publications

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

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