Crazy Ants Cooperate To Carry Food

A new study shows that informed individual ants help guide larger groups

Ehud Fonio and Ofer Feinerman

A little more than a week ago, I found ants in my kitchen. A few were helping one another heave a small piece of dog food up my wall toward the window and I spent an embarrassing amount of time watching their efforts, which I documented at The Last Word on Nothing.

Coincidentally, a group of scientists at the Weizmann Institute of Science in Rehovot, Israel have been studying similar behavior in ants for several years, although in a different species, and they may have figured out how the insects cooperate to carry food and other heavy loads. Their work published today in Nature Communications. (Thanks to Friend of Our Modern Plagues Nsikan Akpan for the tip.)

The inspiration for the study struck four years ago when Ehud Fonio, an ethologist at Weizmann and one of the lead authors, moved to a new apartment, where he discovered longhorn crazy ants (Paratrechina longicornis) joining forces to abscond with pieces of cat food. Fonio filmed the crazy ants, which get their name from their erratic zigzagging movements. Those films raised questions about how the ants were carrying the food, and soon his team at Weizmann, led by physicist Ofer Feinerman at the Ant Collective Behavior Group, began filming the same species in other locations, baiting them with Cheerios that had been intentionally scented with cat food to make the pieces more attractive. (The ants aren’t as interested in plain Cheerios, but the cereal slides easily and its lighter color makes it easier to see the ants.)

If you’ve ever helped friends move a couch, you know it takes cooperation and coordination for a group to carry a large object. According to the new paper, the behavior isn’t very common outside of humans and ants. In order to understand how the longhorn crazy ants achieved this feat, the researchers took hours of footage and used it to build models to determine the rules the insects followed to carry the Cheerios to their nests.

“What is especially impressive is that [the study] is based entirely on field data. They captured the behavior of the ants in their natural context and managed to come up with very clean data,” says Simon Garnier, who studies collective behaviors of ants and other organisms at the New Jersey Institute of Technology, and who wasn’t involved with the new research. “It is an excellent example of how tracking technologies, in this case computer vision, are making it possible to collect good data on animal behavior in the wild.”

Conformists Versus Leaders

Ants communicate, in part, through chemicals called pheromones, which can help them follow one another to a piece of food. In order to bring it back to their nest, they can either spend time chomping it into smaller pieces to make it easier for individual ants to carry, or work together to carry it whole.

The latter strategy may be faster, but it’s tricky to coordinate the movements of so many individuals. To understand how the ants coped with this, the Weizmann researchers tracked each individual with numbers using a computer to see which were leading the movement, including pulling, pushing, and turning.

If all the ants pulled and pushed in conflicting directions, the cereal wouldn’t move. But if they all move in generally the same direction, as well as in a straight line to the nest, they could get the food home fastest. “So everyone has to be a conformist. One ant grabs the object and she feels it is going to the left and doesn’t want to fight the other ants, and so she also pulls to the left,” Feinerman says.

What the researchers found was that groups of ants moving the cereal together sometimes lost track of the most direct route home, possibly because the food blocked their antennae, making it difficult for them to navigate. Individual ants moving freely had a better sense of the nest’s location, and came in to correct the others’ path, slowly leading the entire group back on course. The effect only lasted a short while before that leader lost the trail and fell back in with the general direction of the group. Then, another leader would come along and correct the course, repeating the process.

It’s a slick method, but it doesn’t always work. If there are too many conformists the process breaks down. When the researchers introduced an unusually large object, there were so many ants involved that the insects couldn’t figure out how to stay on their course—for example, they couldn’t navigate around obstacles. In effect, says Feinerman, the larger groups were collectively too conservative, which prevented them from completing the task.

“Usually when people talk of ants they have this very romantic view that one ant is really stupid, but if you take many ants you get something very smart—an emergence of intelligence or something like that,” says Feinerman. “If you look at this system it looks like the intelligence of knowing where to go and how to pass obstacles does not come from the big group. The big group just gets stuck at the big obstacle and they never pass it. The problem-solving abilities come from the single ant who knows which way to go to pass the obstacle.”

The Power of the Group

According to Alex Wild, the curator of entomology at the University of Texas at Austin as well as an ant expert, different species of ants forage for food using very different tactics. One is to fight off competitors, which is how fire ants do it. The longhorn crazy ants are a good example of another approach called discovery, where they find food and whisk it away before any other critters notice.

“The cooperative transport mechanism described in this paper is not just a curiosity, but an integral part of the mechanism that allows P. longicornis to persist in habitats populated by more aggressive species,” says Wild, who wasn’t part of the research team.

So how about the ants in my kitchen? Wiled identified these as pavement ants, or Tetramorium caespitum. Were they cooperating under the same rules as the longhorn crazy ants as they carried away my dog’s food?

“We’re looking at more general things, notions about cooperation,” Feinerman says. “Some of these things apply to other species of ants, and some don’t.” The next ants on Feinerman’s list are Pheidole, a different genus from my pavement ants. (I guess I’ll have to wait to learn more about the push-pull tactics going on in my kitchen.)

For Feinerman, how a specific species cooperates isn’t exactly the point.

“For me the ultimate goal is more philosophical. I want to understand what is the power of the group and how biological groups make the most of themselves. In biology, almost everything lives in a group, from the cells in our body to ants to colonies of bacteria,” he says. “We can learn from them what you can achieve with a group and what you can’t maybe achieve with a group.”

Additional reading:

Gelblum et al, “Ant groups optimally amplify the effect of transiently informed individuals,” Nature Communications, July 28, 2015

Correction: Due to a transcription error, Ofer Feinerman was originally quoted as saying “…a convergence of intelligence or something like that…” rather than “…an emergence of intelligence or something like that…” The text has been corrected to reflect the change. This reporter regrets the error and promises to turn up the volume during her next interview transcription.