A detailed chemical examination reveals frogs’ sticky-tongue hunting technique

Frogs capture their prey with the sticky mucus covering their tongues, but this mucus isn’t inherently adhesive. Frog mucus is thought to be pressure-sensitive, with tongue retraction strain triggering adhesion. Mucus stuck to prey has been observed to form fibrils, leading to the hypothesis that tongue retraction reorders glycosylated mucin proteins. Understanding the chemical mechanism of this process can inform the design of biomimetic materials, and so Fowler et al. used new techniques to examine the surface chemistry of frog tongues.

“We are able to take a chemical image of the frog tongue print and then look at the outer 4 to 5 nanometers of the surface,” said co-author Joe E. Baio, whose group was involved in developing the near-edge X-ray absorption fine structure (NEXAFS) microscopy techniques used in this study. “Our techniques are really surface-sensitive, and that’s where we think most of the action takes place.”

NEXAFS and sum frequency generation of vibrational spectra enabled analysis of the interfacial chemistry of “mucus prints.” These prints were gathered by tempting horned frogs into launching their tongues at crickets the authors cunningly placed behind glass microscope slides.

They found that the secondary structures of frog tongue mucins are randomly arranged, but upon tongue retraction, molecules align into well-ordered fibrils. Hydrophobic groups orientated toward the slide surface and hydrophilic groups orientated toward the mucus bulk. “The physical input [tongue retraction] changes the chemistry of the mucus and that’s how it turns the glue on,” said Baio.

Next, the team wants to find the specific mucin sequence in order to replicate the inherent reversibility in artificial pressure-sensitive adhesives.

Source: “The surface chemistry of the frog sticky-tongue mechanism,” by J. Elliot Fowler, Thomas Kleinteich, Johannes Franz, Cherno Jaye, Daniel A. Fischer, Stanislav N. Gorb, Tobias Weidner, and Joe E. Baio, Biointerphases (2018). The article can be accessed at https://doi.org/10.1116/1.5052651 .