Physics, Flocks, and Fire Ants

By: Hannah Pell

Many of us have been spending a lot of time on our own lately. It can be difficult to feel like we’re accomplishing all that much individually — especially when social media is always there to remind you of how productive your friends and colleagues have been during quarantine. I long for the days of in-person gatherings; that feeling of being a face in the crowd, one of many.

I’ve found a bit of optimism for my solitary blues in learning about the science of collective behavior. Collective behavior arises as a property of groups but is not necessarily shared by the individual participants themselves. In other words, strange things can happen when groups of things — from particles to bacteria to fish and to people — come together.

Let’s take a deeper look at a few particularly wild examples.

Fire ants: a liquid and a solid 

Though small, ants are mighty; individual ants can withstand more than 5,000 times their own weight by some estimates. And, if the fastest ant in the world were the size of a person, it could run 200 meters per second. (I certainly wouldn’t want something that strong or speedy chasing after me)!

So what happens when ants join forces and work together? Some pretty astounding physics, as it turns out.

Image Credit: The Agricultural Research Service

Researchers at the Georgia Institute of Technology have studied the unique collective behavior that arises when fire ants band together. When a large number of fire ants find themselves floating in water, they actually link together to form a raft. Rather than trying to swim on their own, they act as a cohesive unit, even flexing to withstand applied pressure. Although ants are individually hydrophobic (water repellent), this property is amplified drastically when they form a raft. (Massive fire ant rafts are often found in the aftermath of hurricanes and have caused significant damage in some cases. As if you needed another thing to be afraid of).

Fire ants could be described as viscoelastic, demonstrating properties of both solids and liquids. On the macro-level, a ball of fire ants will respond to outside pressure elastically like a spring (however, if you applied the same pressure to one fire ant, things might not go as well). At the same time, we can imagine the links between two fire ants like the weak bonds that characterize liquids and their ability to flow. If a few ants fall away from the raft, the mass “self-heals,” repairing the damage done to the overall structure.

“Ants are like no living thing on Earth,” said Dr. David Hu, professor of fluid mechanics at the Georgia Institute of Technology. “They can both be a solid and a liquid, and bounce back and forth between those two states at the same time.”

In addition to their strength, speed, and raft-crafting abilities, ants are also excellent at managing their own traffic, which leads us to another form of collective behavior: mass movement.

Flocking physics 

Have you ever wondered why birds don’t crash into each other when flying in a crowded flock? Physics offers a few hints.

Flocking (like raft-forming) is an example of active matter: systems whose constituent elements consume energy. The emergence of collective behavior is a property of dynamic systems that are intrinsically out of thermal equilibrium; there is a tendency toward imbalance. Active matter is still a relatively new classification of soft matter; one of the most well-known mathematical models describing active matter — the Vicsek model — was published in 1995. Additional quantitative models to describe flocking have since been proposed as well.

One important assumption in flocking models is that biological objects exhibit a tendency to behave like their neighbors. There is no single leader of a flock of birds, so the direction and motion of the whole becomes a product of individual birds flying according to how the birds immediately around them are moving. The overall flock shape in the sky arises from a bunch of individual decisions made by the birds, analogous to a phase transition in a physical system. Thus a flock of birds is another example of collective behavior.

The physics of togetherness
Emergent phenomena in ecological communities is a fascinating and interdisciplinary area of research, blending statistical mechanics, engineering, and biology. Physics is all about bodies in motion, and understanding these collective behaviors can lead to a variety of useful applications, such as bio-inspired robotics or maybe even the secret to the perfect wok-tossed fried rice.

If there’s anything we can learn from birds or fire ants in our time of social distancing, it’s that some pretty astounding things can happen whenever we can come back together again.

Starlings — a notoriously invasive species — fly in large groups called murmurations. 

Image Credit: Tanya Hart – Imported from 500px (archived version) by the Archive Team. CC BY-SA 3.0.

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