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Structural correlates of increased adhesive efficiency with adult size in the toe pads of hylid tree frogs

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Abstract

Tree frogs are able to climb smooth, vertical substrates using specialised toe pads which adhere via an area-based wet adhesive mechanism. Although the link between pads and arboreality in frogs is well-established, few studies have investigated the influence of morphology on adhesion. Trinidadian tree frogs from the genus Hyla are geometrically similar. There is a tendency towards comparatively reduced mass in larger species, but toe pad area increases as expected with isometry. As adhesion is area-dependent, forces are affected directly by the increase in mass relative to pad area, and there is a decrease in the ability of larger species to adhere to smooth rotation platforms. However, there is an increase in force per unit area that suggests larger species have more efficient toe pads. Toe pad structure is very similar though there are variations in the details of a number of features. Crucially, although differences in morphology appeared small they had demonstrable effects on adhesive efficiency of the pads. Epithelial cell area correlates positively with frog length and adhesive efficiency, related features of cell density and intercellular channel length correlate negatively. These findings are discussed in relation to the different forces involved in the tree frogs’ wet adhesive system.

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Abbreviations

F :

force normal to the surface

F c :

capillarity

F sa :

stefan adhesion

g :

the earth’s gravitational acceleration

h :

meniscal height

IRM:

interference reflection microscopy

m :

mass

r :

pad radius

SEM:

scanning electron microscopy

SVL:

snout–vent length

v :

speed of separation

α detachment :

angle at which frogs fall from rotation platform

γ :

surface tension

η :

viscosity

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Acknowledgments

JMS thanks Margaret Mullin of Glasgow University SEM Unit and Mathis Riehle for advice. In Trinidad our thanks to the late Professor Peter Bacon, Karen Duhn, Mary Alkins-Koo, Mike Oatham and Grace Sirju-Charran of the University of the West Indies. Thanks also to Nadra Nathai-Gayan and David Boodoo of the Wildlife Division for permits and friendship. At Simla Research Station, thanks to the Asa Wright Nature Centre and Ronnie Hernandez. For frogging above and beyond the call of duty: Glasgow University Expedition members—Andy Barnes, Beccy Johnson, Gary Mason, Jim McVeigh, Carrie-Ann McCulloch, Christine Oines, Dan Thornham; and Leeds University’s Darren Croft, Beth Arrowsmith, Jon Bielby, Kirsten Skinner and Liz White. Funded with assistance from: NERC Grant No.—GT0499TS99 (JMS), Wellcome Trust Showcase Award (WJPB, JRD and Dr Mathis Riehle), The Carnegie Trust for Scotland (JRD and WJPB), The Glasgow Natural History Society’s Blodwen Lloyd Binns Trust (JMS), Continental Tires (WJPB) and the Percy Sladen Trust (WJPB). All experiments comply with the current laws of the country in which they were performed.

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Authors and Affiliations

  1. Division of Environmental and Evolutionary Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK

    Joanna M. Smith, W. Jon. P. Barnes, J. Roger Downie & Graeme D. Ruxton

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  1. Joanna M. Smith
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  2. W. Jon. P. Barnes
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  3. J. Roger Downie
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  4. Graeme D. Ruxton
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Correspondence to Joanna M. Smith.

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Smith, J.M., Barnes, W.J.P., Downie, J.R. et al. Structural correlates of increased adhesive efficiency with adult size in the toe pads of hylid tree frogs. J Comp Physiol A 192, 1193–1204 (2006). https://doi.org/10.1007/s00359-006-0151-4

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  • DOI: https://doi.org/10.1007/s00359-006-0151-4

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