Fire ants can build miniature look-alikes of the Eiffel Tower from their own bodies, and the insects perpetually rebuild the structures to save them from collapsing, a new study finds.
The insects crawl up and down these structures in a phenomenon that resembles a slow-motion water fountain in reverse, the researchers said.
Fire ants (Solenopsis invicta) evolved in the Pantanal wetlands of Brazil. In 2011, Craig Tovey, a biologist at the Georgia Institute of Technology in Atlanta, and his colleagues discovered the way in which colonies of these insects can shape themselves into raftsthat can stay afloat for months.
Fire ants can use sticky pads at the ends of their feet to link to each other and form a pancake-shaped raft. The 2011 study found that each ant's exoskeleton can trap air bubbles and become slightly water-repellent. Weaving a colony together leads to a more powerful waterproofing effect that keeps the raft dry while afloat in the water. [Video: Watch Fire Ants Build Rafts]
If the ant rafts find optimal spots to settle down, they can form bell-shaped towers that act as temporary shelters in the aftermaths of floods. These structures may each consist of hundreds of thousands of ants and reach more than 30 ants high; until now, it was a mystery how ants could build such tall structures from their own bodies without getting crushed, the researchers said in the new study.
Tovey and his colleagues accidentally found the secret to the tall structures while they were experimenting with fire ant colonies gathered from roadsides near Atlanta. The researchers made the discovery when "we accidentally left the video camera running for an extra hour after the ants had finished building their tower," Tovey told Live Science.
Study co-author Nathan Mlot, also a biologist at the Georgia Institute of Technology, "was too good a scientist to discard data," Tovey said. "But he didn't want to waste an hour watching nothing happen. So he played the video at several times regular speed."
To induce the ants to build towers, the researchers placed them in clear boxes that had plastic rods sticking up from their floors. These rods served as supports on which the ants could build structures made of themselves. In subsequent experiments, the towers the ants built ranged from 0.28 to 1.18 inches (7 to 30 millimeters) high and were built within 17 to 33 minutes. The researchers noted that such towers likely took on a bell shape because in that form, each component bears an equal load.
At high speed, the researchers could see that the towers are constantly sinking, as ants within the depths of the structures tunnel away from the piles of insects around them. However, the structures are constantly rebuilt, as ants scurry up the sides of the towers.
"I was most surprised that the ant tower perpetually sinks and gets rebuilt," Tovey said. "I thought the ants stopped building once the tower was complete. The shape stays the same — who would guess that the ants circulate through an unchanging structure?"
To confirm their findings, the researchers mixed a mildly radioactive iodine-based dye into the drinking water of some of the insects and then put the colony in an X-ray machine to monitor the motions of the ants. "In real time, the surface ants block the view," Tovey said. "Moreover, the sinking is too slow to detect."
By placing transparent sheets of plastic on top of ants, the scientists found that each insect, which on average weighs about 1 milligram, can support up to about 750 times its weight and live to tell the tale. However, the experiments also suggested that in towers, each ant seems to feel most comfortable supporting up to three ants on its back — any more, and they simply give up and walk away, Tovey said.
The researchers noted these structures were built without a leader or coordinated effort. Instead, each ant just wandered aimlessly, following a certain set of rules that could help it construct towers. Computational models the researchers developed could accurately predict tower shapes and growth rates, the study said.
"To build their tall, solid, Eiffel Tower-shaped structure, the ants seem to be following the same simple behavioral rules that they follow to build a pancake-shaped floating raft on water," Tovey said. "It is remarkable that the two large-scale shapes formed by the group of ants are dramatically different and achieve different functions, yet emerge from the same small-scale individual behaviors."
The researchers now want to analyze "the bridges the fire ants make out of their bodies to traverse gaps in terrain," Tovey said. "They are amazing. The ones in front hold each other, dangle downwards and outwards to the other side, and grip firmly at each end. The rest of the ants walk across the bridge. Then, the ants who compose the bridge deconstruct it starting from the first side, so at the end, all of the ants have reached the other side."
Such research could help inspire the creation of swarms of robots that could build complex structures from their bodies, Tovey said.
"Robotics researchers have had some success getting a fleet of robots to form a two-dimensional pattern like a rectangle, but they have not figured out how to get robots to form a stable three-dimensional structure," Tovey said. "This research may show how to do that.
"For example, suppose we send several hundred little robots through a small opening into a collapsed building to search for survivors, or to explore unknown terrain on Mars," Tovey said. "Sometimes, the robots will have to work together to cross crevices or climb over steep obstacles. At other times, they should spread out. This research may help us understand how to design their individual controllers so they can cooperatively accomplish different tasks in different situations."
Still, it may prove difficult creating robots that can do everything ants can do, he said. "Repeatedly drop an ant from 6 feet [1.83 meters], and it won't get injured. Drop a robot from 6 feet a hundred times and good luck," Tovey said.
The scientists detailed their findings online July 12 in the journal Royal Society Open Science.
Original article on Live Science.