A Biologist and Systems Engineer Explains How Fire Ants Organize Themselves to Survive Floods

Craig Tovey

Drop a clump of 100,000 fire ants in a pond of water — or flood a huge area of Texas that’s infest­ed with fire ants and dri­ve them out of their nests in large groups. In min­utes the clump will flat­ten and spread into a cir­cu­lar pan­cake that can float for weeks with­out drown­ing the ants. Drop the same clump of ants near a plant on sol­id ground. They’ll climb atop each oth­er to a form a sol­id mass around the plant stem in the shape of the Eif­fel Tow­er — some­times as high as 30 ants tall. The ant tow­er serves as a tem­po­rary encamp­ment that repels raindrops.

How and why do the ants make these sym­met­ri­cal but very dif­fer­ent shapes? They depend on touch and smell — not sight — to per­ceive the world, so they can sense only what’s very close to them. Con­trary to pop­u­lar belief, the queen doesn’t issue orders to the colony; she spends her life lay­ing eggs. Each ant con­trols itself, based on infor­ma­tion gath­ered from its imme­di­ate vicinity.

As both a sys­tems engi­neer and biol­o­gist, I’m fas­ci­nat­ed by the ant colony’s effec­tive­ness in diverse tasks, such as for­ag­ing for food, float­ing on water, fight­ing oth­er ants and build­ing tow­ers and under­ground nests — all accom­plished by thou­sands of pur­blind crea­tures whose brains have less than one ten-thou­sandth as many neu­rons as a human’s.

In ear­li­er research, my col­league David Hu and I inves­ti­gat­ed how these tiny crea­tures weave their bod­ies into water-repel­lent life­sav­ing rafts that float for weeks on flood waters. (This didn’t hap­pen after Kat­ri­na flood­ed New Orleans in 2005 because the storm surge and lev­ee col­laps­es hap­pened so fast the ants couldn’t escape their nests, and drowned. Harvey’s floods result­ed from rain over a much longer peri­od of time.)

Now we want­ed to under­stand how the same ants coor­di­nate to assem­ble into a com­plete­ly dif­fer­ent struc­ture on land — a tow­er made of as many as hun­dreds of thou­sands of liv­ing fire ants.

David Hu describes how fire ants form their escape rafts. (Video: YouTube / The Com­pa­ny of Biol­o­gists)
How sup­port­ive are fire ants?

Half the ants here in Geor­gia are fire ants, Solenop­sis invic­ta. To col­lect our lab sub­jects, we slow­ly pour water into an under­ground nest, forc­ing the ants to the sur­face. Then we cap­ture them, take them to the lab, and keep them in bins. After some painful bites we learned to line the bins with baby pow­der to pre­vent their escape.

To trig­ger their tow­er build­ing, we put a clump of ants in a petri dish and sim­u­lat­ed a plant stem with a small ver­ti­cal pole in the cen­ter. The first thing we noticed about their tow­er was that it was always nar­row at the top and wide at the bot­tom, like the bell of a trum­pet. A pile of dead ants is con­i­cal. Why the bell shape?

Our first guess, that more ants were need­ed toward the bot­tom to sup­port more weight, proved accu­rate. To be pre­cise, we hypoth­e­sized that each ant is will­ing to sup­port the weight of a cer­tain num­ber of oth­er ants, but no more.

From this hypoth­e­sis we derived a math­e­mat­i­cal for­mu­la that pre­dict­ed the width of the tow­er as a func­tion of height. After mea­sur­ing tow­ers made of dif­fer­ent num­bers of ants, we con­firmed our mod­el: ants were will­ing to sup­port the weight of three of their brethren — but not more. So the num­ber of ants need­ed in a lay­er had to be the same as in the next lay­er up (to sup­port the weight of all the ants above the next lay­er), plus one-third the num­ber in the next lay­er (to sup­port the next layer).

Lat­er, we learned that archi­tect Gus­tave Eif­fel used the same prin­ci­ple of equal load-bear­ing for his famous tower.

Ring around the pole

Next we asked how fire ants build the tow­er. Of course they’re not doing the math that would tell them how many ants need to go where to cre­ate this dis­tinc­tive shape. And why does it take them 10 to 20 min­utes rather than the mere one or two min­utes need­ed to build a raft? This took us sev­en tri­al hypothe­ses over two frus­trat­ing years to answer.

Ants build a tow­er in real time. (Video: Sulisay Phonekeo / Geor­gia Tech / YouTube)

Although we think of a tow­er as made of hor­i­zon­tal lay­ers, the ants don’t build the tow­er by com­plet­ing the bot­tom lay­er and adding one com­plete lay­er at a time. They can’t know” in advance how wide the bot­tom lay­er must be. There isn’t any way for them to count how many ants there are, much less to mea­sure a layer’s width or cal­cu­late the nec­es­sary width.

Instead, ants scur­ry­ing about on the sur­face get attached and there­by thick­en the tow­er at all lay­ers. The top lay­er is always formed atop what had just pre­vi­ous­ly been the top lay­er. Being the nar­row­est, it con­sists of a ring of ants around the pole, each grip­ping its two hor­i­zon­tal­ly adja­cent ants.

Our key obser­va­tion was that if a ring does not com­plete­ly encir­cle the pole, it doesn’t sup­port oth­er ants that are try­ing to build anoth­er ring on top of them. After mea­sur­ing ant grip and adhe­sion strengths, we ana­lyzed the physics of the ring and deter­mined that a com­plete ring is 20 to 100 times more sta­ble than an incom­plete one. It looked like ring for­ma­tion might be the bot­tle­neck for tow­er growth.

This hypoth­e­sis gave us a testable pre­dic­tion. A larg­er-diam­e­ter pole has more ring places to be filled, so its tow­er should grow more slow­ly. To get a quan­ti­ta­tive pre­dic­tion, we math­e­mat­i­cal­ly mod­eled the ant move­ments as being in ran­dom direc­tions for a dis­tance of about a cen­time­ter — the same as in our mod­el of ant move­ment for ant raft formation.

Then we filmed close-ups of ants mov­ing into places on the ring. Based on over 100 data points, we got strong con­fir­ma­tion of our mod­el of ring-fill­ing. When we ran tow­er-build­ing exper­i­ments with a range of pole diam­e­ters, sure enough, the tow­ers grew more slow­ly around larg­er-diam­e­ter poles, at rates that matched our pre­dic­tions fair­ly well.

Sink­ing in slow motion

There was one big sur­prise to come. We thought that once the tow­er was com­plete, that was all there was. But in one of our exper­i­men­tal tri­als, we acci­den­tal­ly left the video cam­era run­ning for an extra hour after the tow­er had been built.

Then-Ph.D.-student Nathan Mlot was too good a sci­en­tist to just dis­card obser­va­tion­al data. But he didn’t want to waste an hour watch­ing noth­ing hap­pen. So he watched the video at 10x nor­mal speed — and what he saw was amazing.

Time-lapse video of an ant tow­er. (Video: Sulisay Phonekeo / Geor­gia Tech / Con­ver­sa­tionE­DU)

At 10x speed, the sur­face ants move so quick­ly they are a blur through which the tow­er under­neath is vis­i­ble, and the tow­er is slow­ly sink­ing. It hap­pens much too slow­ly to dis­cern at nor­mal speed. We observed the bot­tom tow­er lay­er from below through the trans­par­ent petri dish. The ants there form tun­nels and grad­u­al­ly exit the tow­er. They then scur­ry about the tow­er sur­face until even­tu­al­ly they join a new top ring. We couldn’t see the ants deep inside the tow­er. Is the entire tow­er or just its sur­face sink­ing? We sus­pect­ed the for­mer, as ants in clumps and rafts grip togeth­er as one mass.

We enlist­ed Daria Mon­aenko­va, who had just invent­ed a nov­el 3D X‑ray tech­nique. We doped some of the ants with radioac­tive iodine and tracked them. Every tracked ant in the tow­er sank.

X‑ray pho­tog­ra­phy reveals ants (black dots) walk up the sides of the tow­er, only to sink when they reach the col­umn. (Video: Sulisay Phonekeo / Geor­gia Tech / Con­ver­sa­tionE­DU)

Per­haps the most remark­able impli­ca­tion of this research is that the ants don’t have to know” whether they are all behav­ing the same way. Appar­ent­ly they fol­low the same sim­ple rules of move­ment: If ants are mov­ing above you, remain in place. If not, move ran­dom­ly, and stop only if you reach an unoc­cu­pied space adja­cent to at least one sta­tion­ary ant.

Once the tow­er is built, the ants cir­cu­late through it while pre­serv­ing its shape. We were sur­prised; we thought the ants would stop build­ing their tow­er once its height was max­i­mal. Pre­vi­ous­ly, when we stud­ied the ant raft, we were sur­prised in the oppo­site way. We thought the ants would cir­cu­late through the raft so as to take turns being under­wa­ter on the bot­tom. Instead, ants on the bot­tom can stay in place for weeks.

Every liv­ing organ­ism I’ve stud­ied has turned out to be more com­pli­cat­ed than it seemed at first. Under­stand­ing how sim­ple rules can lead to elab­o­rate and var­ied struc­tures increas­es our respect for the pow­er of evo­lu­tion, and gives us ideas for how to design mul­ti-func­tion­al self-assem­bling robot teams.

(“How do fire ants form giant rafts to sur­vive floods?was orig­i­nal­ly pub­lished on The Con­ver­sa­tion and is repost­ed on Rur­al Amer­i­ca In These Times thanks to a Cre­ative Com­mons license.)

Craig Tovey is Pro­fes­sor of Indus­tri­al & Sys­tems Engi­neer­ing and Co-Direc­tor of the Cen­ter for Bio­log­i­cal­ly Inspired Design at the Geor­gia Insti­tute of Technology.
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