A slight tweak in the activation of a single protein could determine whether some ants become lowly workers or reproductive queens, according to a new study.

While most ant species are born into a particular caste in their colony, with little to no ability to climb the social ladder, Jerdon's jumping ant (Harpegnathos saltator) is somewhat different.

When a queen of this species dies, workers in the colony duel over who gets to take her place. The ants that are ultimately victorious become what is known as 'gamergates'. Putting aside their foraging work, these individuals instead order other ants around and spend their time reproducing and laying eggs, essentially taking over the queen's role.

Previous studies have shown the transition to a gamergate comes with changes to the ant brain, including different gene expressions, different hormone levels, and different cellular compositions. These reigning ants also live about five times longer than other ant workers.

But what exactly triggers this incredible midlife transformation? Up until now, it's been mostly a mystery.

"Animal brains are plastic; that is, they can change their structure and function in response to the environment," explains molecular biologist Roberto Bonasio of the University of Pennsylvania.

"This process, which also takes place in human brains – think about the changes in behavior during adolescence – is crucial to survival, but the molecular mechanisms that control it are not fully understood."

To explore the idea of brains plasticity further, Bonasio and his colleagues conducted a series of experiments on Jerdon's jumping ants, in which their neurons were exposed to different levels of hormones.

The team focused their attention on two hormones, in particular juvenile hormone III (JH3) and the ecdysteroid hormone 20-hydroxyecdysone (20E). Both are thought to regulate social behavior in ants and even some other social insects, like honeybees.

In the current study, when ant brains were injected with an analog of JH3, the authors found it decreased their hunting activity. On the other hand, when 20E was injected into the brain, it stimulated ovary activation.

The findings suggest there's something about the signaling pathways of these hormones that determines the switch from one social caste to another. But what is the switch itself?

Isolating specific neurons in the ant brains, researchers tested how this mechanism might work in further detail in the lab. Both social hormones ultimately seemed to influence the ant neurons by activating a single protein factor, known as Krüppel homolog 1 (Kr-h1).

Kr-h1 is sort of like a light switch, and the hormones are the fingers, flicking either the worker brain state or the gamergate brain state on or off.

Specifically, these hormones appear to impact what genes Kr-h1 ultimately binds to within the neuronal cell.

For the level of hormones seen in worker ants, Kr-h1 seems to bind and repress genes associated with gamergates. Meanwhile, for the level of hormones seen in gamergates, the protein seems to repress worker-based gene pathways.

When this protein is completely deleted from the neurons of ants, researchers found gamergate ants started acting like workers, and worker ants started acting like gamergates.

"We had not anticipated that the same protein could silence different genes in the brains of different castes and, as a consequence, suppress worker behavior in gamergates and gamergate behavior in workers," says Bonasio.

"We thought that these jobs would be assigned to two or more different factors, each of them only present in one or the other brain."

While the results certainly suggest there are two main social hormones controlling caste behavior in Jerdon's jumping ants and that this is done via a single protein factor, we will need further research to actually prove this is so.

What's more, the authors of the study want to investigate whether the role of Kr-h1 exists in other social organisms that share similar neurological hormones.

For now, however, the key message is that there appears to be multiple possible behaviors written into the ant genome, and gene regulation can impact which behavior ultimately takes over.

"In other words," says epigeneticist Shelley Berger from the University of Pennsylvania, "the parts of both Dr. Jekyll and Mr. Hyde are already written into the genome; everyone can play either role, depending on which gene switches are turned on or off."

The study was published in Cell.