Journal of Comparative Physiology A

, Volume 192, Issue 11, pp 1179–1191 | Cite as

Whole animal measurements of shear and adhesive forces in adult tree frogs: insights into underlying mechanisms of adhesion obtained from studying the effects of size and scale

  • W. Jon. P. Barnes
  • Christine Oines
  • Joanna M. Smith
Original Paper

Abstract

This allometric study of adhesion in 15 Trinidadian tree frog species investigates how relationships between length, area and mass limit the ability of adult frog species of different sizes to adhere to inclined and overhanging surfaces. Our experiments show that hylid frogs possess an area-based wet adhesive system in which larger species are lighter than expected from isometry and adhere better than expected from their toe pad area. However, in spite of these adaptations, larger species adhere less well than smaller species. In addition to these adhesive forces, tree frogs also generate significant shear forces that scale with mass, suggesting that they are frictional forces. Toe pads detach by peeling and frogs have strategies to prevent peeling from taking place while they are adhering to surfaces, including orienting themselves head-up on slopes. The scaling of tree frog adhesion is also used to distinguish between different models for adhesion, including classic formulae for capillarity and Stefan adhesion. These classic equations grossly overestimate the adhesive forces that tree frogs produce. More promising are peeling models, designed to predict the pull-off forces of adhesive tape. However, more work is required before we can qualitatively and quantitatively describe the adhesive mechanism of tree frogs.

Abbreviations

b

Width of tape

E

Modulus of elasticity

Fadhesion

Adhesive force

FC

Capillarity force

FP

Peeling force

FSA

Stefan adhesion force

Ft and Fp

Tensile and pressure components of capillarity forces

FPOJKR

Pull-off force according to JKR theory

g

Acceleration due to gravity

h

Distance of separation (of components adhering by wet adhesion)

m

Mass

N

Normal force

R

Radius of curvature

r

Radius (except in a statistical context when r is the correlation coefficient)

SVL

Snout-vent length of frogs

T

Tangential force

v

Velocity

w

Half-width of backing

γ

Surface tension

η

Viscosity

θF

Angle of fall

θS

Angle of slip

θ1 and θ2

Contact angles between fluid and adjoining surfaces

μ

Coefficient of friction

Notes

Acknowledgements

We are extremely grateful to the University of the West Indies in Trinidad for laboratory facilities. We also wish to thank numerous members of Glasgow University expeditions to Trinidad who helped with both frog capture and in carrying out the experiments, especially Tristan Hatton-Ellis, Gary Mason, Nan Swannie, Dan Thornham and Georgina Wood. WJPB is indebted to Eduard Arzt and Walter Federle for useful discussions. We acknowledge funding from the Carnegie Trust for the Universities of Scotland, Glasgow University’s John Robertson Bequest and Continental Tyres (WJPB) and a postgraduate studentship from the Natural Environment Research Council (JMS).

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Copyright information

© Springer-Verlag 2006

Authors and Affiliations

  • W. Jon. P. Barnes
    • 1
  • Christine Oines
    • 1
  • Joanna M. Smith
    • 1
  1. 1.Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, Graham Kerr BuildingUniversity of GlasgowGlasgowScotland, UK

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