|MadSci Network: Zoology|
Hmm. Your project topic is fascinating, and either not many folks have worked on the specifics of tree frog toe pad adhesives, or they're not widely distributing their findings. I found a bunch of way-too-general information on the Web (e.g. tree frogs have adhesive toe pads which help them 'cling' to trees), a couple of which I've listed below, just in case they contain other information of use to you in the background parts of your write-up. However, I did find some highly technical stuff which I must admit was over my head on first read. I'll provide the citations so you can look for the references themselves, and I'll quote a chunk so you can see if this is the type of information you're missing. I found two references that may be useful; I'll start with the older and less technical: From: Smyth, H. Rucker. 1962. Amphibians and their ways. Macmillan Company, New York. Pp. 174-175. "The pads of treefrogs are round, but they are not, as many poeople believe, suction disks. They are far more complex than that. The undersurface of a toe pad is made up of many wedge-shaped cells, each separate from the others. These cells penetrate the cracks and irregularities of the surface to be climbed. In addition, glands in the toe pad excrete a sticky substance that increases their efficiency. It would seem as if some frogs have stickier toes than others. For instance, when one holds a peeper one is not aware of any stickiness on the toe pads at all. How different are the toes of the Eastern Grey Treefrog! If this amphibian wishes to cling to your finger, it is quite difficult to disengage his toes. Incidentally, it should be mentioned that after the animal is removed one's fingers do not feel sticky." From: Duellman, W.E. and L. Trueb. 1986. Biology of Amphibians. The Johns Hopkins University Press, Baltimore, MD. p. 372. (get ready, this is a mouthful! the part in [brackets] was added by me) "Also related to locomotion is the grapsing ability of the toes of many kinds of arboreal frogs that have expanded adhesive toepads. Light and electron microscopical studies by Ernst (1973a, 1973b) and D. Green (1979) have shown that the epidermal cells in the toepads are structurally different from other epidermis on the body. The toepads of arboreal frogs of the families Hylidae, Hyperoliidae, and Rhacophoridae are nearly hemispherical structures on the ventral surfaces of the distal segments of the fingers and toes. The pad is bordered, except proximally, by a circumferal groove (transverse groove or circummarginal groove of some authors). The epidermal cells of the toepad are columnar, usually hexagonal in shape, and clearly separated from one another at their apices [there's a photograph in the book of this-- looks like a closeup of a Koosh ball, but the Koosh strands would be six-sided, not round]. The outermost surfaces of these cells are flat but covered with small, round hemidesmosome plaques. Epidermal cells elsewhere on the digits are squamous [means smooth], except in an area of transition where the circumferal groove is absent; in this area the cells are cuboidal. In some frogs, cuboidal epidermis also is present on the subarticular tubercles. Interspersed among the columnar cells are mucous pores; these are numerous and bordered by unmodified cells....The mucous glands imbedded in the dermis are large, convoluted, and surrounded by a thin myoepithelium of smooth muscle. The toepads are offset from the plane of the digit by an intercalary element between the distal and penultimate phalanges; this allows the entire surface of the toepad to be in contact with the substrate. Experiments by S. Emerson and Diehl (1980) and D. Green (1981) provided evidence that surface tension capillarity enhanced by mucous secretions is the principal means by which anurans adhere to smooth surfaces. Adhesion by toepads is supplemented by adhesion of the skin of the belly, also by surface tension. On rough surfaces, the structure of the epidermis allows interlocking of the toepad with the surface." ------ All right! So. You're becoming a frog toepad expert, so that may have made perfect sense. In case it didn't, the basic idea is that tree frogs have amazingly cool toes, whose pads are structured differently from the rest of the skin of the frog. These structural differences pave the way for tree frogs to hang on tight to climbing surfaces. The physical structure of the toepads appears to increase the total surface area with which the frog can 'attach' itself to surfaces. Perhaps even more important are the mucous pores in the toepads. Muscles control the mucous pores, and I would guess these muscles control the secretion of 'sticky goo' so it's only released when tree frogs are climbing or 'hanging out' on slippery or vertical substrates. The 1980 and 1981 references listed above serve to back up the argument that this sticky goo is the main way tree frogs manage to keep from sliding off whatever they're climbing or sitting on. I hadn't really thought about the role of frog bellies in the whole process, but that makes good sense too. ------- Since I firmly believe in students spending time in real live libraries, looking up stuff themselves, I'm just going to point you in the right direction for the references listed in the previous section. If you get terribly terribly stuck, or you can convince me that your local library and librarian absolutely cannot help you find these articles, then you can write me again for more clues. I don't want to be mean, but I happen to enjoy hunting things down in libraries, and like to encourage others to do the same. "Green, D.M. 1981. Adhesion and the toe-pads of tree frogs" can be found in the journal called "Copeia" which is dedicated to reptile and amphibian research. My apologies if this is old news to you, but some other readers might not know about it. "Green, D. 1979. Treefrog toepads: comparative surface morphology using scanning electron microscopy" can be found in the Canadian Journal of Zoology. Ernst's 1973 articles, "The digital pads of the treefrog Hyla cinerea I. The epidermis." and "... II. The mucous glands." can be found in the journal called "Tissue and Cell". Emerson and Diehl's 1980 article "Toe pad morphology and adhesive mechanisms in frogs" can be found in the Biological Journal of the Linnean Society. Have fun, and keep asking hard questions! Ruth ___________________ Not of the scientific level you need, but worth a look: http://allaboutfrog s.org/weird/general/feet.html h ttp://www.csu.edu.au/faculty/commerce/account/frogs/frog.htm#legs
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