How do ant colonies crown a queen? Scientists found out

In ant colonies, caste means everything. A queen grows wings, lays eggs, and lives a life of reproduction. A worker stays wingless, handles all the labor, and rarely reproduces. But how do ants end up with such different fates when they all start out the same?

For a long time, scientists knew that both genes and the environment played a role in shaping ant castes. What wasn’t clear was how the two interacted – or whether body size and caste could be separated.

A new study from scientists at The Rockefeller University shows that body size plays a key role in determining caste. Larger ants tend to develop queen-like traits, while smaller ones become workers.

But genes also set the rules by determining when, and if, a particular size tips the balance toward becoming a queen.

Queens aren’t just bigger ants

A queen is not just a large worker ant. That distinction is critical. Queens don’t just grow bigger – they sprout wings, develop complex eyes, and grow large ovaries to lay eggs. Workers don’t.

And yet, in many ant species, queens and workers come from the same genetic background. It’s one of the clearest examples of how the same genome can lead to very different outcomes.

This kind of flexibility is called developmental plasticity. Female ants, in particular, are a great model to study how environmental cues – like food, temperature, or social surroundings – shape the development of an organism.

Some scientists had suggested that in ants, body size and caste might be decoupled, just like in fruit flies and other insects where environmental signals influence specific traits without necessarily affecting overall size. But ants seem different. In most cases, caste traits and body size go hand in hand.

“It’s a hotly debated topic, at least among ant biologists,” said Patrick Piekarski, a postdoctoral researcher at Rockefeller’s Laboratory of Social Evolution and Behavior.

“We wanted to resolve this question, by determining whether it is possible to uncouple size from something like ovary development or eye development.”

Meet the clonal raider ant

The team used a special kind of ant to get answers: the clonal raider ant, Ooceraea biroi. These ants reproduce asexually, producing genetically identical offspring. They also go through their life stages in sync, which makes them perfect for controlled experiments.

The clonal raider ant doesn’t have traditional queens. Instead, some individuals – called intercastes – develop queen-like features.

They’re larger, have more developed ovaries, rudimentary eyes, and even the beginnings of wings. These intercastes serve as stand-ins for queens in the study.

“We work with this unusual ant species because, otherwise, we would not be able to control the genotype of the ant,” explained Daniel Kronauer, the Stanley S. and Sydney R. Shuman Professor at Rockefeller.

“With clonal raider ants, we can do the equivalent of a massive identical twin study to determine how the environment impacts the adult phenotype.”

Environment sets the stage for queen ants

The first part of the study tested how much the environment affects caste.

The researchers kept the ants’ genetics the same and changed their surroundings. They varied food supply, temperature, and even the types of ants that cared for the larvae.

Each of these changes impacted how the ants developed – but only by influencing their final size. For example, less food meant smaller ants, which almost always became workers.

But if a larva reached a certain size, it still developed queen-like features, even under harsh conditions.

Size, it turned out, was the switch.

Genes still pull the strings

Then the team flipped the question. What happens when ants have different genes but grow up in the same environment? Here’s where it got interesting.

Ants from a genetic line labeled “M” grew smaller on average than the ones from line “A.” But when M ants and A ants were the same size, M ants were more likely to develop queen-like traits.

In other words, genes didn’t just affect how big an ant got – they changed the significance of that size.

“An interesting illustration of that is that you have certain genotypes that start to look more queen-like at smaller body sizes,” Piekarski says.

Two paths, same size

The big takeaway? Size still predicts caste, but genes determine the rules for what size counts as “queen material.”

Some ants are genetically wired to tip into queen-like development at smaller sizes. Others need to grow bigger before that switch is flipped.

“If some environmental factor affects caste, it will affect size too. It can’t induce change in one and not the other,” commented Piekarski.

“As far as we can tell, no matter which environmental variable you manipulate, the relationship between ant body size and caste remains unchanged and is instead genetically encoded.”

Evolution of social roles

Understanding how body size ties into caste is about more than measuring ants. For Kronauer, this research is part of a much bigger picture.

“One of our goals is to understand how an insect society functions,” Kronauer explains. “Studying how individuals differentiate in a colony can inform our understanding of the types of caste systems, from queens to workers to soldiers, that can evolve in the thousands of ant species out there.”

His lab sees ant colonies as superorganisms – living systems that are made up of many individuals, each playing a role.

The more we understand how those roles develop, the better we can grasp the biology of cooperation, division of labor, and even the evolution of complex societies.

“The brains of a queen and a worker are quite different, and this is correlated with striking differences in behavior,” Kronauer stated.

“Workers leave the nest to forage, take care of the larvae, build and expand the nest; the queen mostly just mates and lays eggs.”

Identifying how body size relates to becoming a queen ant or a worker opens the door to understanding how social roles, brain function, and colony dynamics develop and evolve together.

The full study was published in the journal Proceedings of the National Academy of Sciences.

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