During a hike through a sweaty Peruvian jungle in 2016, biologist Kim Laurents of the Free University of Brussels in Belgium caught a green tree frog. As the large amphibian squirmed, the slime on Laurents' hands turned to glue.
“My fingers just stuck together,” he says, “and then I suddenly realized my hands and my flashlight and everything was stuck together.”
Similar sticky secretions are found in salamanders as well as other frogs. The animals use this natural, powerful adhesive to thwart both predators and curious scientists. The mechanism behind this fast glue has been a mystery until now, Dr. Loranz said, including how it evolved in unrelated amphibians. In a study published last month in Nature Communications, he and his team have provided the answer: a noodle-like protein found throughout the amphibian family tree has been adapted by many species to make their own fast glue.
Many amphibians produce a layer of mucus on their skin that keeps them moist and helps them breathe, and some species, including toads and some of the more unusual and deadly species like poison dart frogs, secrete powerful venom to ward off predators. Unfortunately, such venom doesn't always work quickly, with unfortunate consequences for the amphibian that's attacked, says Dr. Laurents.
But amphibian glue “works almost immediately,” Dr Loranz said.
The stress of biting “triggers the release of a viscous liquid that quickly solidifies,” added Shabnam Zaman, a doctoral student at the Vrije Universiteit Brussel and an author of the study.
That's bad news for amphibian lovers.
“When an amphibian gets caught in the jaws of a predator like a snake, the snake is in trouble,” Dr. Loranz says. “At some point, the snake will decide it can't do anything more, give up, and start trying to chase the frog away.”
But how frogs produce such glue is a question that hasn't received much research attention, Zaman says. To understand how the glue works, the team studied tomato frogs, plump red frogs that live in Madagascar and often produce a sticky white glue when prodded.
The team carefully handled some captive frogs, using their mucus to stick toy blocks together and measure how much force was needed to separate them. They also tested the fluid, including scrutinizing it under a high-powered microscope. Other organisms, such as mussels, use biochemical glues they produce to stick to rocks. But the proteins that make the amphibian glue are unusual, Zaman says.
Most proteins have a rigid, well-defined structure that helps them perform a specific function. But the protein the team found in the frog glue was distinctly soft. “It acts like boiled spaghetti,” says Dr. Loranz. “You apply its shape to a surface and it adapts to that surface.”
The protein in question is also full of sticky sugars, and when you add a second protein that actively binds the first, you get a viscous, pressure-sensitive slime. “It's basically glue,” says Dr Loranz.
Zaman said the study is a first step that could “help identify many more adhesive proteins” in other amphibians, as well as in the prey-trapping mucus of velvet worms and the defensive adhesive of slugs. “This study sets a precedent that will allow us to identify components more easily,” he said.
The study also offers clues as to how glue evolved in other amphibians, says Dr. Loranz. When scientists looked at Mozambique rain frogs — small, plump frogs known for making glue — they found that their sticky proteins are related to those of tomato frogs. The two species, separated by more than 100 million years of different evolutionary years, have converged on similar adaptations. And they're not alone: The phenomenon has only been formally reported in five families of frogs, but anecdotal reports suggest it's more widespread.
Similar proteins are present, in much smaller numbers, in most other amphibians that don't make glue. “We found that the genes that make these proteins are present in almost every amphibian we looked at,” said Dr. Loranz, including frogs, salamanders and caecilians, a type of worm-like amphibian. Although the genes are expressed differently in different animals, most amphibians have the biological tools to make their own glue, even if the majority don't use it.
“This kind of thing can happen if you have a good, widespread template,” Dr Loranz said. “This glue is a great example of how evolutionary innovations evolve.”
