Candler Hobbs/Georgia Tech
Frogs’ have a remarkable power to tongue-grab prey as big as mice or as oddly shaped as tarantulas. How do they do this? The ability stems from a combo of peculiar saliva and a super-squishy tongue, new data show.
The first detailed analysis of the stickiness of frog saliva finds that the fluid can shift rather abruptly from gooey to runny, notes Alexis Noel. She is a mechanical engineer at Georgia Tech in Atlanta. Those quick changes come in handy during the various phases of a single tongue strike. And it all works because the tongue itself is so soft.
Internet videos of frogs feasting sparked Noel’s curiosity about their ability “to eat furry things, hairy things, slimy things,” she says. And, she adds, they do so with speed and power. A frog tongue strikes five times more quickly than a human can blink.
But a frog’s tongue is so soft that none of the standard tools on Noel’s campus could measure it without special modifications. The engineer eventually figured out that this tissue is as soft as a brain. And both the brain and tongue are softer than a marshmallow.
Noel and her colleagues shared their findings February 1 in the Journal of the Royal Society Interface.
Homing in on how the tongue nabs that prey
When a frog’s tongue shoots out at, say, a fly, the soft frog tissue splats on impact. It then spreads and curls around the prey. This action “massively increases the contact area” of frog tissue that can stick to the fly. That enhances its grip, Noel explains. Then frog saliva intensifies the effect.
The human mouth has salivary glands spread around inside. These glands drip saliva onto the tongue. Frog tongues, however, don’t depend on dripping glands. Instead, their saliva comes from glands inside the tongue itself. To see how sticky frog saliva might be, Noel spent several hours per sample scraping some 15 frog tongues to put together one fifth of a teaspoon of spit. That was what she needed for a single test.
Noel and her colleagues found that this saliva is what’s called a shear-thinning liquid. It grows thinner and easier to stir or smear around when force is applied. Smacking into a fly jolts saliva from its sticky phase into a more “liquidy” phase. That lets the fluid flow into all the small cracks on the insect’s body. As the tongue returns to the mouth, the spit thickens again. That makes the glue-like spit grip more strongly.
During that tongue jerk, acceleration can surge to 12 times the pull of Earth’s gravity. Still, in spite of the spit’s stickiness, the insect could be flung loose at this point, Noel calculated. But the soft stretchiness of the tongue lets it act like a natural bungee cord that retracts without too much of a jolt. And that prevents the loss of the prey, she concludes.
Once the fly is in the mouth, the tongue’s grip needs to loosen so that the fly can slide down the gullet. “Frogs actually use their eyeballs while swallowing,” Noel says. The eyeballs sink low into the head. From the outside they look as if they change from bulges to low bumps. Inside the head, they push the food toward the throat. The eyes’ impact jars the saliva into a runnier phase, easing its grip on the prey.
Frogs aren’t the only hunters that tongue-snatch their prey. Chameleon tongues also can be very sticky, notes Pascal Damman. He studies the physics of soft matter (including chameleon tongues) at the University of Mons in Belgium. The new findings remind him a bit of how chameleons catch prey using gooey mucus and a stretchy tongue, he says.
(for more about Power Words, click here)
acceleration A change in the speed or direction of some object.
bungee cord An elastic cord — usually with a woven fabric covering — that has a large hook at either end. When stretched over or around objects and then hooked securely in place, this cord will keep objects from falling, slipping or coming loose.
chameleon A type of lizard known for its ability to change the color of its skin.
colleague Someone who works with another; a co-worker or team member.
engineer A person who uses science to solve problems. As a verb, to engineer means to design a device, material or process that will solve some problem or unmet need.
force Some outside influence that can change the motion of a body, hold bodies close to one another, or produce motion or stress in a stationary body.
gland A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body (or in a body cavity) or for elimination from the body.
gravity The force that attracts anything with mass, or bulk, toward any other thing with mass. The more mass that something has, the greater its gravity.
gullet The tube that carries food from the mouth to the stomach.
insect A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.
internet An electronic communications network. It allows computers anywhere in the world to link into other networks to find information, download files and share data (including pictures).
journal (in science) A publication in which scientists share their research findings with the public. Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, there, is to prevent the publication of mistakes, fraud or sloppy work.
mechanical Having to do with the devices that move, including tools, engines and other machines (even, potentially, living machines); or something caused by the physical movement of another thing.
mucus A slimy substance produced in the lungs, nose, digestive system and other parts of the body to protect against infection. Mucus is made mainly of water, although it also includes salt and proteins (such as mucins). Some animals use mucus for other purposes, such as to move across the ground or to defend themselves against predators.
physics The scientific study of the nature and properties of matter and energy. A scientist who works in that field is known as a physicist.
prey (n.) Animal species eaten by others. (v.) To attack and eat another species.
secrete (noun: secretion) The natural release of some liquid substance — such as hormones, an oil or saliva — often by an organ of the body.
tarantula A hairy spider, some of which grow large enough to catch small lizards, frogs and birds.
tissue Any of the distinct types of material, comprised of cells, which make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms.
viscous The property of being thick, sticky and hard to pour. Molasses and maple syrup are two examples of viscous liquids.
Journal: A.C. Noel et al. Frogs use a viscoelastic tongue and non-Newtonian saliva to catch prey. Journal of the Royal Society Interface. Published online February 1, 2017. doi: 10.1098/rsif.2016.0764.