Revolution in the colony: How Argentine ants selectively overthrow their queens

By David Han ’21

In many ant species, colonies are structured in a way that would be familiar to the rulers of feudal Europe: a large population of workers and soldiers labors under the control of a single queen. Unlike human monarchs, though, the ant queen is the sole reproductive individual in the colony, and in most cases has a dramatically longer lifespan than her workers. However, just as in human societies, significant variation in leadership dynamics and social order can occur between ant species.

One such outlier is the Argentine ant, which is one of the most widespread pest species in the world, having successfully colonized thousands of habitats with their densely-populated nests. These ants are polygynous, or have multiple queens in the same colony, which increases the rate of egg production compared to single-queen populations. While increasing colony size fast seems beneficial, polygyny has drawbacks; when resources are limited, multiple sources of leadership can lead to conflict, and the number of eggs each queen can produce drops the as the total number of queens increases.

Past research on Argentine ants has found that the worker population uses a particularly brutal strategy to handle the problem of queen overload. Each spring, as if by ritual, the workers rise up and execute roughly 90% of the queens in the nest, leaving only a handful remaining to keep the colony alive. Initially, scientists were puzzled by this shocking behavior; the revolutionary ants killed both unrelated queens and ones they had close kinship to indiscriminately, and there were no immediate signs of queen quality, such as weight and rate of egg laying, that significantly influenced the decisions either.

To figure out what influences the decisions made worker ants, a study published in ​Scientific Reports t​ his July examined factors that past work missed. Authors Silvia Abril and Crisanto Gomez decided to focus their work on a form of communication normally undetectable by humans: pheromone signalling. If humans spoke by spraying differently-scented perfumes at one another, that would be an approximation of how

ants tell each other what they’re doing, where they’re going, and what needs to be done to keep their colonies running.

In the case of the Argentine ants, pheromones produced by queens impact their interactions with workers and how many queens hatch from eggs; when pheromone levels are too high, new queens are blocked from hatching, which prevents new colonies from being founded, and when they are too low, too many queens hatch and cannot all be cared for by the colony. For this reason, Abril and Gomez decided to look at factors that impact pheromone production, such as the number of pheromone-producing queens in a colony, the temperature, and the specific chemicals the queens were making.

When the scientists examined lab-grown colonies of the ants at different temperatures, they found that worker ants in hotter nests and ants in nests with more starting queens both killed off a greater percentage of their queens. This might happen because both changes move the queen pheromone concentration in the nest away from the “sweet spot” at which the right number of new queens can hatch. In hot nests, queens struggle to produce enough pheromones, while in nests with too many queens, pheromone levels spike.

A third factor that Abril and Gomez considered was the specific chemical “fingerprint” of the queens. By using a technique called gas chromatography, the levels of two specific pheromones that are associated with higher egg-laying rates were measured, and the pair found that the queens that survived produced the highest levels of both. For this reason, they concluded that the workers can sense which queens are

the most worthwhile to keep in the colony and may base their “execution” decisions off of that.

Although more work could definitely be done to pinpoint exactly why Argentine ants regulate their queen population this way, this story challenges common narratives about how ant social systems are controlled; while in some ways the queen lives the high life in the colony, in the end workers may really run the show.