Re: How do the suction pads on the toes of treefrogs work?

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