The tongue is a wonderfully versatile muscle. It helps you speak, taste food and swallow. Animals’ tongues have many important jobs too. For instance, while people may use their tongue to lick a lollipop, hummingbirds and some bats use theirs to slurp up a flower’s sweet, sticky nectar. And those who do it best can get a big assist from tongues that are basically hairy, new data show.
One such animal is the Pallas’ long-tongued bat, or Glossophaga soricina (Gla-SOFF-uh-guh Sor-ih-SEE-nuh). Its tongue is long — longer than its entire head! That allows it to reach deeply into tube-like flowers. But that tongue is extraordinary in another way, too. Its tip is covered with long, hair-like structures, observes Alice Nasto. She works at the Massachusetts Institute of Technology in Cambridge. As a mechanical engineer, she designs, develops, builds and tests mechanical devices.
Nasto has studied hairy structures before. In 2016, she worked with a team to study how hairy surfaces trap air bubbles when they’re dipped into liquids. This time, she wanted to learn more about their ability to trap liquids. The tongues of some bats are idea natural examples, she notes.
Previously, researchers who studied these bats’ had described their tongues as “nectar mops,” observes Nasto. But that’s only partly right, she says. Those stringy structures on their tongues don’t absorb nectar as a cloth mop absorbs water. Instead, they increase the tongue’s surface area. That ups the area available for the nectar to stick to. But those hairs pop up only as needed. Most of the time they lay fairly flat. It’s when the bat extends its tongue to slurp nectar that these “hairs” fill with blood and stand up.
But were the super-slurping tongues on these bats as effective as they could be? Nasto and her colleagues wanted to analyze them to find out. And to do that, they needed to turn to math.
Modeling the hairy tongue
The researchers started by building a model of the hairy tongue. They used lasers to sculpt a mold of the shape. The surface needed to be covered with stiff, stubby structures. So the laser had to cut hundreds of tubular holes into the mold. Then the researchers poured in a liquid, rubber-like silicone. This filled the holes and flowed over the top to form a thin sheet. Once the material gelled into a solid, the researchers peeled off the sheet. It was now covered in little stubs.
Next, Nasto’s team dipped the stubby surface into a basin filled with a thick oil. They did this slowly, to make sure no air got trapped between the silicone stubs. As they pulled the fake tongue material out of the oil, they measured how quickly the fluid drained off of it. For a bat, slower drainage means more nectar will stay on long enough to reach its mouth (and tummy).
The team made four surfaces with different stub sizes. The largest stubs were about 4.2 millimeters across (about 1/6 of an inch). The smallest were just 0.2 millimeters across. That span is about eight thousandths of an inch, or about as thick as two sheets of copy paper.
The researchers tested those surfaces with several oils, each having a different viscosity (Vis-KOSS-ih-tee). A fluid’s viscosity is a measure of its resistance to flow. Molasses is very viscous, so it flows slowly. Water is not viscous, so it flows relatively quickly. Some oils the team tested were as viscous as honey. Others were as quick-flowing as motor oil.
Many combinations of surfaces and oils were put to the test. Then the researchers compared how stub size and oil viscosity affected how quickly the fluid drained off of the model “tongue.” Afterward, they used math to describe those relationships with numbers.
The math behind the nectar-lapping ability of a hairy tongue is complicated, Nasto notes. When tongue hairs are closer together, liquid doesn’t drip off of them very quickly. That means more nectar per slurp — but only up to a point. When the structures get too close, there’s less room in between the hairs for nectar to fit.
So, math showed there is an ideal size and spacing for tiny structures on a tongue. And that ideal combination also depends on the thickness of the fluid it would be lapping up.
Nasto’s team used its mathematical model to estimate the best size and spacing for a bat tongue to lap up the most nectar. And the hairy nectar slurper on the Pallas’ long-tongued bat is nearly perfect, they found. In fact, the team estimates, each slurp with its scoops about 10 times as much nectar as if it would if the tongue were smooth.
The researchers describe their findings in the February Physical Review Fluids.
The team’s study “provides nice insight into how liquid gets loaded onto a hairy tongue,” says Elizabeth Brainerd. She works at Brown University in Providence, R.I. As someone who studies biomechanics, she looks into how living things move and function. Brainerd was not part of this research team, but she has studied the tongues of these bats. And their hairy structures don’t seem to be oddly shaped taste buds, she notes. That suggests they instead serve some physical role, such as boosting nectar-lapping.
This bat can dip its tongue into a flower about eight times per second, Brainerd notes. And each dip gleans nearly the maximum amount of nectar possible. That’s good evidence, she adds, that evolution has fine-tuned the size and shape of this animal’s tongue to do the best job it can.
bat A type of winged mammal comprising more than 1,100 separate species — or one in every four known species of mammal.
biomechanics The study of how living things move, especially of the forces exerted by muscles and gravity on the skeletal structure. Someone who works in this field is a biomechanist.
colleague Someone who works with another; a co-worker or team member.
develop (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.
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.
evolution (v. to evolve) A process by which species undergo changes over time, usually through genetic variation and natural selection. These changes usually result in a new type of organism better suited for its environment than the earlier type. The newer type is not necessarily more “advanced,” just better adapted to the particular conditions in which it developed.
honey Foraging bees visit flowers in search of nectar, a sugary liquid. Back at the hive, honeybees will add some enzymes to the nectar, then deposit the amber colored liquid into the hive’s combs..
insight The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.
laser A device that generates an intense beam of coherent light of a single color. Lasers are used in drilling and cutting, alignment and guidance, in data storage and in surgery.
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.
mechanical engineering A research field in which people use physics to study motion and the properties of materials to design, build and/or test devices.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes.
motor A device that converts electricity into mechanical motion. (in biology) A term referring to movement.
nectar A sugary fluid secreted by plants, especially by flowers. It encourages pollination by insects and other animals. It is collected by bees to make into honey.
physical (adj.) A term for things that exist in the real world, as opposed to in memories or the imagination. It can also refer to properties of materials that are due to their size and non-chemical interactions (such as when one block slams with force into another).
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion.
resistance (in physics) Something that keeps a physical material (such as a block of wood, flow of water or air) from moving freely, usually because it provides friction to impede its motion.
silicone Heat-resistant substances that can be used in many different ways, including the rubber-like materials that provide a waterproof seal around windows and in aquariums. Some silicones serve as grease-like lubricants in cars and trucks. Most silicones, a type of molecule known as a polymer, are built around long chains of silicon and oxygen atoms.
surface area The area of some material’s surface. In general, smaller materials and ones with rougher or more convoluted surfaces have a greater exterior surface area — per unit mass — than larger items or ones with smoother exteriors. That becomes important when chemical, biological or physical processes occur on a surface.
taste buds A collection of 50 to 100 or so taste receptors. They’re found on the tongues of land animals. When certain chemicals in food or other materials trigger a response in these receptors, the brain detects one or more flavors — sweet, sour, salty, bitter or umami.
technology The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.
viscosity (adj. viscous) The measure of a fluid’s resistance to stress. Viscosity corresponds to the idea of how “thick” a liquid is. Honey is very viscous, for instance, while water has relatively low viscosity.