One of our key research topics is the evolution and the function of warning colors (aposematism) in toxic prey. We study Neotropical harlequin frogs, Atelopus spumarius and related forms from the Amazon basin and adjacent areas.
Interesting in these frogs is:
(1) They are all toxic and possess the highly potent tetrodotoxin (TTX), a substance known from many animals including puffer fish (for more information on TTX see our recent Quick Guide in Current Biology). The degree of intra- and interspecific variation is subject to ongoing research.
(2) These harlequin frogs highly vary in dorsal color conspicuousness: some are vividly colored while others are virtually non-aposematic.
(3) Some have colorful ventral sides and some even hand/foot soles. These are red and we consider them to represent flash marks to warn potential predators. Actually, we have been able to show that red hand/foot soles can almost only be seen by birds. In contrast, dorsal colors are seen by a variety of potential predators including crabs. See an interesting video on a crab predation attempt here; the frog in the end is apparently too toxic for the crab.
Recent fieldwork by Daniela C. Rössler and Max N. Lorentz, doctorate candidates of the lab, has been very successful and they are currently analyzing relationships between color conspicuousness and toxicty aspects. For more videos from the field, see here.
Harlequin frogs are not the only amphibians with red hand/foot soles. This character is also notable in toads of the toxic genus Melanophryniscus; see here for the extraordinary Melanophryniscus admirabilis, a Critically Endangered species from southern Brazil.Lorentz, M.N., A.N. Stokes, D.C. Rössler & S. Lötters (2016): Tetrodotoxin in animals. — Current Biology, 26: R870-R872.