Independent origins of frog glue and its role in predator evasion

Skin-secreted adhesives, or glues, are highly effective defense adaptations that have evolved recurrently in a small number of amphibians. From an ecological standpoint, this rapidly solidifying material—essentially, a sticky slime—encumbers the predator long enough for its would-be prey to escape.

But what makes some skin secretions stickier than others, and why has it arisen multiple times throughout the history of amphibian evolution?

Adhesives in nature: Ancient tools for survival

There’s no denying that materials that help us stick things to other things—that is, glue—are omnipresent. On any given day, you may find yourself reaching for a stack of sticky notes or a universally reliable roll of sticky tape. But what about glue in other, less dexterous animals? What is it used for, and how does it work?

Before we go any further, let’s clarify what exactly is meant by “animal glue.” In the context of glue-producing organisms, the materials I’m referring to are called “biological adhesives.” These are naturally secreted materials that occur in a wide range of species, with many serving vital functions necessary for that particular organism’s survival.

The potential uses of these glues are as diverse as the animals producing them, and include substrate attachment (typical of sessile marine organisms such as mussels, barnacles and tubeworms), locomotion (used by starfish to move across the ocean floor) and prey capture (most easily recognizable in the form of spider silk).

Indeed, the sheer utility and adaptive value of glue is reflected in its broad taxonomic distribution, spanning several anciently diverged lineages.

There’s one thing you may have noticed: All the animals I’ve described so far are invertebrates. What about species that look and feel a little closer to home—say, a fellow tetrapod?

If you ever find yourself in the company of herpetologists, simply utter the words “sticky” and “frog” and you will inevitably hear about that time a wild specimen oozed vast amounts of slime all over the offender’s hands.

This slime rapidly took on the properties of superglue: Hands were stuck together, the frog was stuck to the hands—all in all, a very sticky situation. This may just be anecdotal evidence, but search the literature and you will find scant mention of this relatively obscure phenomenon. However, that doesn’t make the stories any less true.

How frogs use glue as a defensive strategy

Skin-secreted chemicals constitute the most widespread antipredator adaptation among amphibians. In a small number of amphibians, this mechanism occurs in the form of glue. When stressed, the amphibian discharges a viscous fluid from its back that quickly solidifies into a sticky mass (i.e., glue).

This glue functions as an effective defense weapon, incapacitating the attacking predator—often a snake—by clogging its mouth and making the act of swallowing impossible. The energetic cost of overcoming this stickiness eventually becomes too high, forcing the predator to give up and release the amphibian.

While toxic skin secretions (i.e., poisons) have long persisted as the focus of most biochemical investigations, research on glue remains scarce and superficial. One possible reason for this disparity may be the fact that glue is a rare feature in frogs, having emerged only sporadically in species that are—on an evolutionary timescale—distantly related.

Although frog glue has been discovered throughout the world, its absence in most species (and especially close relatives) is conspicuous. For instance, a glue-producing frog in Madagascar might not share the island with glue-producing amphibians from other lineages.

Instead, similar sticky secretions may be found in frogs with distributions restricted to Australia or South America, for example.

This brings us to the crux of my study published in Nature Communications : How, exactly, did glue as a defense adaptation evolve in only some frogs, but not in others?

And, as an aside: Does frog glue have anything in common with that sticky tape you have at home?

The bonding process: From interdisciplinary to intermolecular

To answer these questions, I investigated the glue produced by a species endemic to Madagascar: the tomato frog, Dyscophus guineti. Together with several collaborating research institutions, I integrated functional, molecular and evolutionary analyses to uncover what, exactly, makes frog glue stick.

To this end, I identified two proteins that demonstrably interact within the glue milieu to sustain its adhesive and cohesive strength. One is a large glycoprotein (cleverly assigned the acronym PRIT, courtesy of my supervisor) with a presumed glue-specific role, and contains duplicate copies of an evolutionarily conserved domain that is also present in many extracellular metazoan proteins.

The second, much smaller protein is a glycan-binding member of an ubiquitous protein family known as galectin. These findings are consistent with previous reports on the importance of both glycoproteins and glycan-binding proteins in other animal glues, although their interactions and probable mechanism of action were unresolved until recently.

Structural models predicted that while the conserved domains within PRIT are well defined, their intervening regions are structurally heterogenous. This is in contrast to most (nonadhesive) proteins, which have rigid and defined structures, whereas the structural dynamism of PRIT renders it highly flexible.

In practical terms, this means that frog glue can conformationally adapt to any surface it comes into contact with—for example, the oral epithelia of a snake. The transition from a viscous but fluid slime into a tough, fast-acting adhesive occurs once pressure is applied, such as the force exerted by a predator’s bite.

Reverting to our earlier question: What do the humble sticky tape and frog glue have in common? They’re both pressure sensitive, meaning that compressive force is required to fully “activate” their sticking power.

A recipe for recurrent evolution: Reuse, recycle, re-evolve

With the glue proteins identified, I could finally begin to examine the genetic and structural changes that lead to its evolution in distantly related lineages. As noted above, neither protein is inherently unique to D. guineti, or even frogs in general. In fact, the protein domains found in frog glue are present in all animals, including humans.

The specific architecture of the gene encoding PRIT, however, involves a deviation that evolved in an early amphibian ancestor. In other words, glue genes evolved before the glue itself.

Intriguingly, a second glue-producing species (the Mozambique rain frog, Breviceps mossambicus) also encodes a PRIT gene. Dyscophus and Breviceps diverged about 100 million years ago and belong to distinct radiations of frogs (Microhylidae and Afrobatrachia, respectively).

Other members of these lineages produce nonadhesive toxins that are known to have originated early in frog evolution, thus leaving little doubt that: (1) Dyscophus and Breviceps both descended from a poisonous ancestor; and (2) their skin secretions evolved into glues independently.

Alongside structural changes, shifting gene expression was identified as a decisive factor in the recurrent evolution of glue: PRITs and galectins exhibit the same pattern of elevated expression in both glue-producing species, from which we can surmise that regulatory changes also contributed to the parallel evolution of frog glue.

Unlike other glue-producing animals, each of which evolved a unique method of adhesion, highly diverged frog lineages have repeatedly recruited the same pre-existing genes, simply by boosting their expression.

Frog glue is therefore a culmination of evolutionary processes that came before it, with recurrent structural and regulatory changes acting on an ancient and near-universal template.

From forest floors to operating tables: The future of biomimetics

My recent work represents the first detailed analysis of a vertebrate defense glue, thus advancing our understanding of these unusual adaptations while simultaneously opening the door for the development of new, rapid-acting adhesive technologies.

The effectiveness of animal slimes as surgical sealants has already been demonstrated using slug defensive glue; for instance, now we know how it functions in a vertebrate, using a model with the potential to encompass biological glues from phylogenetically diverse sources.

Watch this space: Soon, frog glue derivatives may become as crucial and commonplace in surgical practices as the sticky tape is in our households today.

This story is part of Science X Dialog, where researchers can report findings from their published research articles. Visit this page for information about Science X Dialog and how to participate.

From the molecular to the most majestic of biological marvels, Shabnam Zaman has always been driven to understand the “why” behind the whimsical. That’s how she ended up as a Ph.D. researcher at the Amphibian Evolution Lab (Vrije Universiteit Brussel, Belgium) with a mission to investigate a strange but little-known phenomenon: frog glue. Together with her trusty companion, Bob the Tomato Frog, they are on a quest to unravel the enduring mysteries of what makes these creatures so incredibly sticky.