Nest tomography.
Guy Theraulaz
Scientists u CT scans to create three-dimensional images
of ant nests.THE REMARKABLE SELF-ORGANIZATION OF ANTS
By: Emily Singer | April 9, 2014
Living bridge.
Alex Wild
To speed their foraging excursions, army ants build bridges with their own bodies, allowing others to race across a gap.
Give a colony of garden ants a week and a pile of dirt, and they’ll transform it into an underground edifice about the height of a skyscraper in an ant-scaled city. Without a blueprint or a leader, thousands of incts moving specks of dirt create a complex, spongelike structure with parallel levels connected b
y a network of tunnels. Some ant species even build living structures out of their bodies: Army ants and fire ants in Central and South America asmble themlves into bridges that smooth their path on foraging expeditions, and certain types of fire ants cluster into makeshift rafts to escape floods.
How do incts with tiny brains engineer such impressive structures?
Scientists have been studying the social behavior of ants and other incts for decades,arching for chemical cues and other signals that the incts u to coordinate behavior.Much of this work has focud on understanding how ants decide where to forage or build their homes. But new rearch combining obrvations of ant behavior with modern imaging techniques and computational modeling is beginning to reveal the crets of ant construction.It turns out that ants perform the complex tasks by obeying a few simple rules.
“People are finally starting to crack the problem of producing the structures, which are either made out of soil or the ants themlves,”said Stephen Pratt , a biologist at Arizona State University. The organization of inct societies
is a marquee example of a complex
decentralized system that aris from the interactions of many individuals, he said.
Cracking the problems could lead to improvements in swarm robotics, large numbers of simple robots working together, as well as lf-healing materials and other systems capable of organizing and fixing themlves. More broadly, identifying the rules that ants obey could
Army Ant Bivouac Alex Wild Both army ants and fire ants build bivouacs,temporary nests made of the incts themlves, where they can protect and rai their young.
help scientists understand how biologically complex systems emerge — for example, how groups of cells give ri to organs.
“Self-organizing mechanisms are prent everywhere in nature, from the development of an embryo to the organization of large animal populations,” said Simon Garnier , a biologist at the New Jery Institute of Technology.
Guy Theraulaz , a behavioral biologist at the Rearch Center on Animal Cognition in Toulou, France, and collaborators have been studying inct nests for the last 20 years,building more complex and realistic models as their data improved. They have discovered that three basic guidelines governing when and where ants pick up and drop off their building materials are sufficient to create sophisticated, multilayered structures.
“It all results from local interactions between the individuals,” said Garnier, a former student of Theraulaz’s who now studies living ant bridges. “The final structure emerges without central coordination.”
Theraulaz’s team painstakingly analyzed videos of ants crawling across petri dishes as they attempted to build a shelter, noting each time that an ant picked up or dropped off a grain of sand. The rearchers discovered three main rules: The ants picked up grains at a constant rate, approximately 2 grains per minute; they preferred to drop them near other grains, forming a pillar; and they tended to choo grains previously handled by other ants, probably becau of marking by a chemical pheromone.
The rearchers ud the three rules to build a computer model that mimicked the nest-building behavior. In the model, virtual ants moved randomly around a three dimensional space, picking up pieces of virtual sand soaked in a virtual pheromone. The model ants created pillars that looked just like tho made by their biological counterparts. The rearchers could alter the pillars’ layout by changing how quickly the pheromone evaporates, which could explain why different environmental conditions, such as heat and humidity, influence the structure of ant nests. (They published a preliminary version of the model in a conference report in 2011 but haven’t yet published the more r
efined version,which better mimics real ants.)
“The real novelty here is our newly acquired ability to obrve in detail the formation and the transformations of the structures,” Theraulaz said. “We finally have access to preci data on how living things get together to form complex yet fully functional and reactive structures.”After a weeklong simulation, the virtual ants created something that looked like a real nest;layers stacked together with connections between them. The connections themlves were not explicitly written into the rules, Theraulaz said.
“For the longest time, people never would have believed this is possible,” said Chris Adami, a physicist and computational biologist at Michigan State University, who was not involved in the study. “When looking at complex animal behavior, people assumed they must be smart animals.”
Living Architecture
For David Hu and collaborators at the Georgia Institute of
Technology, rearching ant architecture is both a
livelihood and a workplace headache. Hu’s team studies
living architecture in which “ants are the bricks and the
brick layers,” Hu said. But the fire ants in Hu’s lab are also
adroit escape artists. They build towers to escape their
enclosures and creep under locked doors. Hu is terrified of
three-day weekends, which give the ants more time to break free and build bivouacs — nests made of hundreds of thousands of ants — under his colleagues’ desks. When everyone returns to work, he receives panicked calls from infested offices.
“We have ants escaping from our lab all the time,” Hu said. “The bivouacs are sophisticated, with tunnels and windows that can open and clo in respon to humidity and temperature.”
In his rearch, Hu is focud on first understanding a simpler structure — ant rafts. The incts can escape floods in their habitat by asmbling into rafts made up of up to 100,000 members. The surprisingly buoyant structures, which can be as large as a dinner plate, can float for weeks, enabling the colony to survive and find a new home.
Hu and collaborators had previously shown that after a spoonful of ants is dropped into water, the blob of incts transforms into a pancakelike raft through a simple process: each ant walks randomly on the surface of the blob until it hits the water’s edge. “An individual ant can’t know how big the raft is, where it is in the raft and what other ants are doing,” Hu said.“The only communication goes on at the edge of the structure — that’s where the structure grows.” Hu’s team ud the simple rules to build a virtual ant raft that had the same dynamics as one made by real ants.
David Hu To build a raft, ants that have been dropped into water walk around randomly on the surface of the ant blob until they reach an edge, transforming the blob into a pancakelike structure. The result is a remarkably buoyant raft.
Wanting to understand exactly what gives the ant rafts their remarkable strength and buoyancy, Hu’s team peeked inside the structure. They froze rafts of ants and then created images of them using computed tomography (also known as CT scans).
The findings, which will be published in an upcoming paper in the Journal of Experimental Biology, reveal that ants weave themlves into something like three-dimensional Gore-Tex, a fabric that is both breathable and waterproof. The ants form air pockets by pushing away from whichever ants the
y are connected to, creating highly buoyant rafts that are 75 percent air. The weave of the ant fabric is held together by multiple connections among individual ants, which orient themlves perpendicular to one another. “What’s happening at the big scale is the result of lots of interactions at the small scale,” Hu said. The result is a water-repellant lattice that enables even the ants at the bottom of the structure to survive.
As an engineer, Hu views ant conglomerates like any other material, studying their properties much as one might study plastic, steel or honey. Ants, however, have the unusual ability to act as either a liquid or a solid, and Hu hopes further rearch into this ability will help
Field rearch.
Devanand Saraswati
To study army ant bridges, Chris Reid, right, a postdoctoral
rearcher in Simon Garnier’s lab, and Matthew Lutz, a
graduate student, construct platforms in the columns’ path
and photograph the ants as they create ineers design lf-healing structures such as bridges capable of nsing and mending cracks.
To find his ant architects, Garnier sometimes spends days with his collaborators wandering the rainforest on an island in the Panama Canal. But once in clo range, the target is easy to spot: Huge swaths of army ants in arch of food for their voracious young
sometimes cover the length and almost half the
width of a football field. Ants from this nomadic species, named for their characteristic marching columns, blanket their surroundings. To expedite their relentless foraging, the ants rapidly build bridges over gaps in their path or across trees, using their own bodies as building blocks to create a smooth and expedient path for their kin. Scientists have long studied the curious creatures, exploring the evolutionary advantages of their foraging and bridge-building tactics, but Garnier and collaborators are among the first to study exactly how the structures form. They build obstacles in the path of the marching column and record the ants as they build a bridge.
Like fire ant rafts, bridges are built bad on simple rules and posss surprising strength and flexibility. As soon as an ant ns a gap in the road, it starts to build a bridge, which can reach a s
pan of tens of centimeters and involve hundreds of ants. Once the structure is formed, the ants will maintain their position as long as they feel traffic overhead, dismantling the bridge as soon as the traffic lightens. “The exact timing of their decision to join or leave the structure maximizes stability as a function of traffic on the trail,” Garnier said. “The rules of behavior in forming and dismantling the bridge are optimally designed to handle the traffic.”Garnier’s team is now studying how individual ants cling to one another to create the structure and how ants at the fastening points can hold the weight of 100 comrades. “I think this is a new, very exciting approach,” said Bert H?lldobler , an evolutionary biologist at Arizona State University who has been studying ants for more than 40 years .
One of the most exciting findings to emerge from studies of living architecture “is how dynamic and rich this process is,” said Scott Turner , a biologist at the State University of New York College of Environmental Science and Forestry in Syracu. Garnier’s work shows that ants build and disasmble bridges according to changing needs. Preliminary work from Hu’s group, which also studies bridges, shows that the structure’s properties, such as strength and integrity, evolve with changing conditions.
Ant Traffic
Garnier said he was inspired to examine ant behavior after
studying human pedestrian traffic. “It’s a fascinating question
to understand how individuals that are less cognitively able
than we are can collectively achieve results that are
sometimes better than what we can do with our big brains,” he
said. Ants on a foraging mission are typically carrying loads
two to three times their size and running at a human
equivalent of 60 miles per hour. The incts avoid traffic jams
by spontaneously forming three lanes of traffic, a center lane
of homeward bound ants flanked by two lanes of incts
heading out on the hunt. It’s unlikely that swarms of speeding
humans could organize themlves so effortlessly, Garnier
said. “If you removed traffic lanes in New Jery, it would be
a nightmare,” he said.Chris Reid
Army ant bridges are remarkably strong and adaptive; the incts begin to build them as soon as they n a gap in their path and disasmble them once traffic has cleared.
Although H?lldobler is excited about all three projects, he cautions that just becau a model mimics real ant behavior doesn’t mean it reflects what’s actually happening. He cites the ca of a model of dert ants that re-created their complex foraging expeditions without the need for a chemical trail marker, created at a time when scientists had found no evidence for one. But H?lldobler’s team later discovered that the incts do indeed u chemical markers,limiting the ufulness of the model.
Also currently missing is an evolutionary approach to understanding the ant behavior. “If we can understand how rules emerge from other rules and how they change with the environment, that would be extraordinarily fruitful,” said Adami, who is planning to work with Garnier on this question.
Meanwhile, engineers are already dreaming up uful applications. They hope to u ant constructio
n principles to design modular robots that can lf-organize . Adami imagines a swarm of robots nt to Mars to build a structure from Martian soil ahead of the arrival of humans. The beauty of a decentralized system is that a project can succeed even if individual parts fail.
Dynamic ant architecture might also
provide insight into how to make buildings more adaptive, changing its properties bad on how many people are inside, for example. To make a living building, “you need to continually monitor the environment and what effect the swarm has on the environment,” Turner said.Ants might even shed light on the complex organization of the organ we u to study them —the brain. The behavior of an ant community rembles the organization of neurons into a functioning brain, H?lldobler said. “Each neuron is relatively dumb, but if you take billions of neurons, they interact in a way that we have only scratched the surface of understanding.”
