You never know when a frog playing an electronic game will lead to an experiment on the physics of saliva....Alexis C. Noel, a Ph.D. student in mechanical engineering at Georgia Tech, and her supervisor, David L. Hu, were watching a viral YouTube video in which a frog is attacking the screen of a smartphone running an ant-smashing game. It appears to be winning. They started wondering how — in reality — frog tongues stick to insects so quickly when they shoot out to grab them, and decided it was a phenomenon worth studying. David Hu is an associate professor of mechanical engineering and of biology, as well as an adjunct associate professor of physics, at Georgia Tech.
By Alexis Noel and David Hu
Note: This article was first published in The Conversation on Jan. 31, 2017. It is republished here under the Creative Commons License.
How does one get stuck studying frog tongues? Our study into the sticky, slimy world of frogs all began with a humorous video of a real African bullfrog lunging at fake insects in a mobile game. This frog was clearly an expert at gaming; the speed and accuracy of its tongue could rival the thumbs of texting teenagers.
The versatile frog tongue can grab wet, hairy and slippery surfaces with equal ease. It does a lot better than our engineered adhesives – not even household tapes can firmly stick to wet or dusty surfaces. What makes this tongue even more impressive is its speed: Over 4,000 species of frog and toad snag prey faster than a human can blink. What makes the frog tongue so uniquely sticky? Our group aimed to find out.
Baseline Frog Tongue Knowledge
Early modern scientific attention to frog tongues came in 1849, when biologist Augustus Waller published the first documented frog tongue study on nerves and papillae – the surface microstructures found on the tongue. Waller was fascinated with the soft, sticky nature of the frog tongue and what he called:
“the peculiar advantages possessed by the tongue of the living frog…the extreme elasticity and transparency of this organ induced me to submit it to the microscope.”
Fast-forward 165 years, when biomechanics researchers Kleinteich and Gorb were the first to measure tongue forces in the horned frogCeratophrys cranwelli. They found in 2014 that frog adhesion forces can reach up to 1.4 times the body weight. That means the sticky frog tongue is strong enough to lift nearly twice its own weight. They postulated that the tongue acts like sticky tape or a pressure-sensitive adhesive – a permanently tacky surface that adheres to substrates under light pressure.
To begin our own study on sticky frog tongues, we filmed various frogs and toads eating insects using high-speed videography. We found that the frog’s tongue is able to capture an insect in under 0.07 seconds, five times faster than a human eye blink. In addition, insect acceleration toward the frog’s mouth during capture can reach 12 times the acceleration of gravity. For comparison, astronauts normally experience around three times the acceleration of gravity during a rocket launch.
On To The Materials Testing
Thoroughly intrigued, we wanted to understand how the sticky tongue holds onto prey so well at high accelerations. We first had to gather some frog tongues. Here at Georgia Tech, we tracked down an on-campus biology dissection class, who used northern leopard frogs on a regular basis.
The plan was this: Poke the tongue tissue to determine softness, and spin the frog saliva between two plates to determine viscosity. Softness and viscosity are common metrics for comparing solid and fluid materials, respectively. Softness describes tongue deformation when a stretching force is applied, and viscosity describes saliva’s resistance to movement.
Determining the softness of frog tongue tissue was no easy task. We had to create our own indentation tools since the tongue softness was beyond the capabilities of the traditional materials-testing equipment on campus. We decided to use an indentation machine, which pokes biological materials and measures forces. The force-displacement relationship can then describe softness based on the indentation head shape, such as a cylinder or sphere.
However, typical heads for indentation machines can cost US$500 or more. Not wanting to spend the money or wait on shipping, we decided to make our own spherical and flat-head indenters from stainless steel earrings. After our tests, we found frog tongues are about as soft as brain tissue and 10 times softer than the human tongue. Yes, we tested brain and human tongue tissue (post mortem) in the lab for comparison.
For testing saliva properties, we ran into a problem: The machine that would spin frog saliva required about one-fifth of a teaspoon of fluid to run the test. Sounds small, but not in the context of collecting frog spit. Amphibians are unique in that they secrete saliva through glands located on their tongue. So, one night we spent a few hours scraping 15 dead frog tongues to get a saliva sample large enough for the testing equipment.
How do you get saliva off a frog tongue? Easy. First, you pull the tongue out of the mouth. Second, you rub the tongue on a plastic sheet until a (tiny) saliva globule is formed. Globules form due to the long-chain mucus proteins that exist in the frog saliva, much like human saliva; these proteins tangle like pasta when swirled. Then you quickly grab the globule using tweezers and place it in an airtight container to reduce evaporation.
After testing, we were surprised to find that the saliva is a two-phase viscoelastic fluid. The two phases are dependent on how quickly the saliva is sheared, when resting between parallel plates. At low shear rates, the saliva is very thick and viscous; at high shear rates, the frog saliva becomes thin and liquidy. This is similar to paint, which is easily spread by a brush, yet remains firmly adhered on the wall. Its these two phases that give the saliva its reversibility in prey capture, for adhering and releasing an insect.
To Catch A Cricket
How does soft tissue and a two-phase saliva help the frog tongue stick to an insect? Let’s walk through a prey-capture scenario, which begins with a frog tongue zooming out of the mouth and slamming into an insect.
During this impact phase, the tongue deforms and wraps around the insect, increasing contact area. The saliva becomes liquidy, penetrating the insect cracks. As the frog pulls its tongue back into the mouth, the tissue stretches like a spring, reducing forces on the insect (similar to how a bungee cord reduces forces on your ankle). The saliva returns to its thick, viscous state, maintaining high grip on the insect. Once the insect is inside the mouth, the eyeballs push the insect down the throat, causing the saliva to once again become thin and liquidy.
It’s possible that untangling the adhesion secrets of frog tongues could have future applications for things like high-speed adhesive mechanisms for conveyor belts, or fast grabbing mechanisms in soft robotics.
Most importantly, this work provides valuable insight into the biology and function of amphibians – 40 percent of which are in catastrophic decline or already extinct. Working with conservation organization The Amphibian Foundation, we had access to live and preserved species of frog. The results of our research provide us with a greater understanding of this imperiled group. The knowledge gathered on unique functions of frog and toad species can inform conservation decisions for managing populations in dynamic and declining ecosystems.
While it’s not easy being green, a frog may find comfort in the fact that its tongue is one amazing adhesive.
Mark Mandica of The Amphibian Foundation collaborated on the research published in Journal of the Royal Society Interface and coauthored this article.
Alexis Noel is a Ph.D. student in Biomechanics at Georgia Institute of Technology.
David Hu is an associate professor of mechanical engineering and of biology and an adjunct associate professor of physics at Georgia Institute of Technology.
For More Information Contact
A. Maureen Rouhi, Ph.D.
Director of Communications
College of Sciences
Over the past year, scientists have made great strides in the development of brain-machine interfaces (BMIs), wired external devices that are controlled solely by brain activity [see “Roadmapping the Adoption of Brain-Machine Interfaces”]. Last October, Nathan Copeland, a man who had been paralyzed from the chest down for more than 10 years, made headlines when he fist-bumped President Obama with a BMI-controlled robotic arm using only his thoughts. As BMI-related technologies and neuroprosthetics become more sophisticated, researchers are learning that these tools can make some fascinating changes to the brain, engaging its natural plasticity in sometimes unanticipated ways. Understanding those changes to underlying plasticity, some say, could offer clues to how to rewire and rehabilitate the damaged brain—perhaps even without the need of external hardware. Prosthetics, even without the addition of a BMI component, can alter the brain’s connections, says Lewis Wheaton, director of the Cognitive Motor Control Lab at the Georgia Institute of Technology says.
Negotiating uneven ground can be challenging for people who use lower-limb prostheses to walk, so researchers spend time searching for solutions that will allow greater stability in these situations. Manufacturers of prosthetic feet have contributed to a solution by adding multiaxial features that better reproduce the behavior of human ankles, which can stiffen as the terrain warrants. However, School of Biological Sciences Senior Lecturer W. Lee Childers found that there was a lack of evidence evaluating the prosthetic ankle stiffness as it relates to the user’s dynamic balance and gait over uneven terrain. Thus, his continuing research focuses on defining the effect of multiaxial stiffness on gait stability among people with unilateral transtibial amputations....“The main focus of this work was to justify that it is a good thing for prosthetic feet to have multiaxial function,” Childers says, because if it can prevent falls among its users, its value is demonstrated to the payers.
According to a study published by Georgia Tech researchers in the journal Scientific Reports, healthy people who take measures to avoid getting sick cannot fully eradicate the spread of disease without an infected individual taking preemptive steps first. Instead, the sick individual in question needs to take steps to avoid infecting anyone else, and the main motivator for taking those steps seems to be empathy -- the ability to understand the feelings of others. Ceyhun Eksin and Joshua S. Weitz collaborated on the study with a researcher from King Abdullah University of Science and Technology. Eksin is a postdoctorate fellow in Weitz's lab in the School of Biological Sciences.
Bees perform a crucial function for nature by pollinating crops. Now, researchers at Georgia Tech have explored how they keep themselves clean while dealing with that messy pollen.... David L. Hu, an associate professor who has a joint appointment in the School of Biological Sciences and the School of Mechanical Engineering, co-authored the study. Hu says this is the first quantitative analysis of how honeybees clean themselves and carry pollen using the 3 million hairs on their bodies. The study appeared in the journal Bioinspiration and Biomimetics.
The story of warring bacterial armies started as a Georgia Tech research published in February. Now it's a nationally distributed podcast produced by the National Science Foundation (NSF), and you can thank the researchers' unique mix of biology and math for inspiring NSF to tell the story widely in this format.
"The Discovery Files" recently highlighted the work of Brian Hammer, Will Ratcliff, Samuel Brown, and Peter Yunker in a 90-second radio feature titled "A Gut Reaction." The podcast is based on a paper published on Feb. 6, 2017, in the journal Nature Communications.
The researchers used math and physics equations to find patterns and consistency in how two competing armies of cholera bacteria attack each other. The work could someday help scientists develop targeted therapies using engineered microbes that could kill infectious, harmful bacteria while sparing helpful ones.
NSF, which helped fund the research, creates a weekly audio report on the latest scientific research. "The Discovery Files" airs on radio stations throughout the U.S.
You can listen to "A Gut Reaction" here.
Hammer and Brown are associate professors in the School of Biological Sciences. Ratcliff and Yunker are assistant professors, respectively, in the School of Biological Sciences and the School of Physics.
For More Information Contact
Renay San Miguel
Communications Officer/Science Writer
College of Sciences
Empathy is a crucial human ability. It’s the basis of the golden rule: do unto others as you’d have them do unto you. New research from Georgia Tech finds that empathy can help prevent the spread of disease during an outbreak. This segment on Georgia Public Broadcasting's "On Second Thought" program featured Ceyhun Eksin, a postdoctoral fellow in the lab of School of Biological Sciences professor Joshua S. Weitz. Eksin and Weitz collaborated with a researcher from King Abdullah University of Science and Technology.
Honeybees have almost three million hairs on their tiny bodies. Each hair is strategically placed to carry pollen and also to brush it off. Researchers at Georgia Tech used high-speed footage of tethered bees covered in pollen to see how these hairs work. David Hu, an assistant professor in the School of Biological Sciences, was a co-author of the study. The Georgia Tech Urban Honey Bee Project assisted in the research.
David Hu, an assistant professor in the School of Biological Sciences, has applied his expertise in the mechanics of interfaces between fluids such as air and water to the biomechanics of animal locomotion, publishing papers ranging from why frog tongues are so sticky to how fire ants form waterproof rafts....Hu, a 1997 graduate of Montgomery Blair High School Magnet in Montgomery County, Maryland, talks to the Magnet Foundation Newsletter about his experiences in the Magnet, how he became a professor, and his research today.