Three simple rules guide fire ants in building towers | Science News for Students

Three simple rules guide fire ants in building towers

The same set of rules build floating rafts
Sep 11, 2017 — 7:00 am EST
fire ants, tower

Fire ants build towers using three simple rules, no leader needed.

Candler Hobbs and David L. Hu

When faced with rushing floodwaters, fire ants are known to build two types of structures. A quickly formed raft lets the insects float to safety. Then, once they find a branch or tree to hold on to, the ants might form a tower up to 30 ants high, with eggs, brood (including larvae) and queen tucked safely inside. Neither structure requires a set of plans or a foreman ant to lead the construction, though. Instead, a new study shows, both structures form by the same set of three simple rules.

Those rules initially came from a study of fire ant rafts. First rule: If you have an ant or ants on top of you, don’t move. Rule two: If you’re standing on top of ants, keep moving a short distance in any direction. Lastly: If you find a space next to ants that aren’t moving, enter it and link up.

When fire ants are in water, these rules compel them to build rafts, notes Sulisay Phonekeo. “The same rules dictate them to build towers when they are around a stem [or] branch,” his team’s new data show. Phonekeo is a mechanical engineer at the Georgia Institute of Technology in Atlanta. He led the new study, published July 12 in Royal Society Open Science.

Tiny builders

Phonekeo and his Georgia Tech colleagues collected the fire ants that they studied from roadsides near Atlanta. While covered in protective gear, the researchers dug up ant mounds and placed them in buckets. They lined those buckets with talc powder so the insects couldn’t climb out. Being quick was a necessity because “once you start digging, they’ll … go on attack mode,” Phonekeo points out. The researchers then slowly flooded the bucket until the ants floated out of the dirt and formed a raft that could be easily scooped out.

Back in the lab, the researchers placed some of these ants in a clear petri dish with a central support. Then they watched — and filmed — as the insects formed a tower. The support had to be covered with the non-stick chemical (Teflon), which the ants could grab onto but not climb without help. Over about 25 minutes, the ants would form a tower stretching up to 30 millimeters (1.2 inches) high. The ants themselves are only 2 to 6 millimeters (0.07 to 0.24 inch) long.

Story continues below video.

Whether it’s a tall tower or a floating raft, fire ants follow the same set of rules.
Candler Hobbs and David L. Hu

The structures they erected looked like the Eiffel Tower or the end of a trombone, with a wide base and narrow top. And the towers weren’t static, like the rafts of ants are. Instead, videos showed that the towers of ants were constantly sinking and being rebuilt.

Peering into the transparent petri dish from below revealed that the ants had built tunnels in the base of their tower. They used those tunnels to exit the base before climbing back up the outside.

“The ants clear a path through the ants underneath much like clearing soil,” Phonekeo says. Ants may be using the tunnels to remove debris from inside the towers. And the constant sinking and rebuilding may give the ants a chance to rest without the weight of any compatriots on their backs, he adds.

To find out what was happening inside the tower, the researchers fed half their ants a liquid laced with radioactive iodide. It emits X-rays. They then filmed the insects using a camera that recorded those X-rays. In the film, radioactive ants appeared as dark dots. The researchers could see that some of those dots didn’t move, but others did.

The team then turned to the three rules that fire ants follow when building a raft and now realized that they also applied to towers.

But there was also a fourth rule: A tower’s stability depends on the ants that have attached themselves to the rod. The top row of ants on the rod isn’t stable unless those ants form a complete ring. So to get a taller tower, there needs to be a full ring of ants gripping to the rod and to each other.

That such simple rules could form two completely different structures is inspiring to Phonekeo. “It makes me wonder about the possibilities of living structures that these ants can build if we can design the right environment for them.”

Power Words

(more about Power Words)

brood     A group of related animals that emerges in a specific region in the same year. Depending on the animal type, the collective group is sometimes also known as a year class.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

colleague     Someone who works with another; a co-worker or team member.

debris     Scattered fragments, typically of trash or of something that has been destroyed. Space debris, for instance, includes the wreckage of defunct satellites and spacecraft.

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.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

fire ant     A tropical American ant that has a painful and sometimes poisonous sting.

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.

larvae    (sing: larva) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)

link     A connection between two people or things.

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.

petri dish     A shallow, circular dish used to grow bacteria or other microorganisms.

radioactive     An adjective that describes unstable elements, such as certain forms (isotopes) of uranium and plutonium. Such elements are said to be unstable because their nucleus sheds energy that is carried away by photons and/or and often one or more subatomic particles. This emission of energy is by a process known as radioactive decay.

static     (in physics) An electronic signal that contains no information but does contain random fluctuations in intensity or wavelength.

technology     The application of scientific knowledge for practical purposes, especially in industry — or the devices, processes and systems that result from those efforts.

X-ray     A type of radiation analogous to gamma rays, but having somewhat lower energy.


Journal:​ S. Phonekeo et al. Fire ants perpetually rebuild sinking towers. Royal Society Open Science. Published online July 12, 2017. doi: 10.1098/rsos.170475.