The things ants do

Ants seem to roam this earth for the sole purpose of demonstrating what emergence is all about. Ant colonies show a remarkable and robust ability to solve problems.

Some species are able to build bridges out of themselves to cross gaps in the road:

Or they form rafts — to survive floods, or travel over water:

They build stunning cities:

Scientists excavating the concrete cast of an ant city

In witnessing a colony perform stunts like these, one might wonder which one of these critters is giving the orders.

Who’s in charge?

The answer is: none of them.

A single ant always seems to know exactly what to do. It never runs off to the ‘Executive Officer Ant’ for instructions. The key lies in the fact that each ant is following a tiny set of rules. Trivial rules, dealing only with its local environment. An ant is just minding its own damn business, really.

When a legion of these simple creatures live together, though, magic happens. The group starts displaying astonishing feats of organisation. It acquires problem solving capabilities a NASA rocket scientist would be proud of.

Systems consisting of simple components sometimes exhibit properties which are absent in any of the components themselves. This is called emergence, and it’s the underlying phenomenon at work in ant colonies. The resulting collective behaviour emerging from such a system has a more dramatic name: Swarm Intelligence.

How do ants gather food?

To illustrate the principle, let’s look at the way ants form those characteristic single file lines when gathering food:

Leafcutter ants gathering food

One might assume that one ant found the food source earlier, went to tell the others to come help collect it. And then promoted one of them to executive officer in charge of coordinating the enterprise.

In reality, a few simple facts explain the entire event:

Ants deposit pheromones when looking for food.

Ants tend to follow the strongest pheromone trail they can find when they don’t carry food

Ants explore randomly if there is no food or trail to follow

Pheromones decay over time

Short paths take less time to travel along

Let’s break the process down in steps.

1: random exploration

Ants go out to find food, randomly exploring their surroundings. They leave pheromones along the way, like Hansel & Gretel left bread crumbs.

2: Gary finds food

Gary finds food and picks up a portion of it.

3: Gary goes home

Gary follows own pheromone trail back to the nest. He releases more pheromones on the way. This reinforces his trail.

4: Cindy finds Gary’s trail

Cindy runs into Gary’s strong trail.

5: Cindy finds the food as well

Cindy follows Gary’s trail and finds the food source as well. She picks up a portion. Gary arrived at the nest and leaves his food there.

6: Cindy reinforces Gary’s trail

Gary sets back out along the strongest trail he can find, the one he started himself. Cindy picks the strongest available trail and follows it which will lead her to the nest. Kelly found the food by following her own random path, but will abandon it for the stronger path laid out by Gary and Cindy.

That’s the basic principle. More ants will stumble upon the successful trails of others. Short paths get reinforced while long paths decay.

Path optimisation

Whenever an ant follows a trail, it might deviate from it. It might need to avoid obstacles like other ants. Trails are fuzzy because they’re based on ‘smell’. Some of these deviations happen to be shortcuts.

This leaves pheromones on a shorter path, and shorter paths take less time to travel along. So, the pheromones on those shorter trails have less time to decay. Therefore shorter paths are more likely to be followed by other ants and get reinforced in the process.

Effects like these cause the ants to work towards a shorter path between nest and food over time.

And so, a bunch of ants start walking between their nest and your picnic basket in a single file line, stealing food in a coordinated manner. They seem quite smart collectively by acting quite stupidly on their own. (One would expect major swarm intelligence at frat parties.)

The same sort of simple rules give rise to the examples mentioned at the beginning of this article. (Even the ant cities!)

Emergent swarm stupidity

There are loopholes in the ways ants behave. Take army ants, for example. These ants are blind. They start following nearby ants when they lose a foraging pheromone track. This can culminate in a fascinating disaster:

This is called an ant mill, a clear indicator that there is no CEO ant coordinating the whole. None of the individuals caught in the mill have a clue they’re not making progress. Unless they get some external stimulus they will keep walking till they starve. Any CEO would be giving his employees stern talkings-to when noticing the problem.

These mills can be huge. There have been cases where it took a single ant over 2.5 hours to walk the full circle — a circle with a diameter of hundreds of meters. Some people make a game of nudging ants in such a configuration (please don’t).

I’ll come back to this phenomenon near the end of this article.