Sticky Defense: How Frogs Use Adhesive Secretions for Protection

The Frog Prince. Author provided (no credit)

In fact, many species of frogs produce a poison in their skin, the effects of which can range from mild nausea to death, so this display of affection is generally discouraged. But what if the frog had found a more unusual way to defend itself, by leaving our heroine's lips sealed with the kiss?

[Article issu de The Conversation, écrit par Shabnam Zaman, PhD Researcher in Biology, Vrije Universiteit Brussel]

Biological adhesives: a story as old as time

While humans mainly use synthetic materials to glue things together, our princely frog – like other glue-producing organisms – produces what is called a “biological adhesive”. These naturally secreted substances are widespread among animals and are often essential for their survival.

For example, mussels produce a type of glue that permanently attaches them to underwater surfaces, while other ocean-dwelling animals, such as starfish, use a different, much more temporary type of glue to help them move around.

On land, the most famous example is spider glue, which it uses to create silk to capture prey. If we focus on terrestrial vertebrates, the stickiest specimens are the feet of geckos and those of some tree frogs. These are examples of “dry” and “wet” adhesion, meaning that geckos adhere to surfaces without producing anything resembling glue, while the pads of tree frogs are covered in a thin layer of slime or mucus.

Despite their obvious differences, both types of adhesion have been described as “self-cleaning” and could help us develop synthetic materials that share this characteristic. Indeed, the way different biological glues are formulated and then secreted often involves marvelous feats of natural engineering.

In other cases, however, the secretion of glue has little to do with complex geometric operations, but rather with the act of pouring out large quantities of slime in one go.

Sticky secretions: an unlikely defense mechanism

From traditional medicine to shamanic rituals to folktales and myths, frogs and toads hold great cultural significance around the world. The poison glands of frogs are particularly important, as they can be used to make weapons, medicines, or even hallucinogens.

Until now, studies of amphibian skin defenses have focused on molecules that function as toxins. However, in addition to being poison providers, a small number of species (including the world's largest amphibian, the Chinese giant salamander) have evolved a more unusual survival strategy: glue.

Meet the tomato frog.

When stressed, the animal's skin releases a thick liquid that becomes extremely sticky within seconds. From the frog's perspective, this stress usually takes the form of an attack by a predator (or princess). The speed at which the viscous secretion—sticky slime, really—turns into glue makes it nearly impossible for a predator to ingest the frog, likely due to the discomfort of having its mouth and face coated in glue.

Although this tactic may seem crude and inelegant, it is an effective defense mechanism, as it gives the frog time to escape.

Tracing evolutionary mechanisms

Although glue is a rare trait among frogs, it has evolved repeatedly in species across different continents. My recently published research explores the origins of this remarkable survival strategy and explains why it is present in some frogs and not others.

To answer these questions, we first had to identify the ingredients responsible for frog glue's stickiness. To do this, we used technologies ranging from low-tech Lego bricks to high-tech microscopes capable of magnifying to the nanoscale (one billionth of a meter).

Surprisingly, the basic ingredients needed to make this glue exist in almost all animals, including humans, but only amphibians have evolved the toolset needed to turn them into glue. And even among amphibians, only a few species, living as far apart as Madagascar, Brazil, and Australia, have developed this ability.

We found that the Mozambique rain frog, which is separated from the tomato frog by about 100 million years of evolution, uses the same basic ingredients and toolkit to create its own adhesive secretion.

From biology to biomimicry: a very sticky surgical solution

Frog glue is fascinating, it's fast and flexible, which means it has huge potential for practical applications. That's where biomimetics comes in, a field that strives to mimic biological processes that nature has taken millions of years to develop.

Thanks to our research, we now know, for the first time, how a four-legged animal produces glue. Imagine if medical adhesives inspired by frog glue could be used as surgical glues: not only is it strong and non-toxic, but it is able to adapt and adhere to virtually any surface.