Social parasites in evolutionary transition: Lasius (Chthonolasius) castor n.sp. and L. (Chthonolasius) pollux n.sp. – two hidden ant species with unusual biology

In the hidden world beneath our feet, ants build societies that are often more complex than human cities. Among the many ways these insects organize themselves, some species have evolved a strategy called social parasitism. In this arrangement, a queen does not start her colony by raising her own first generation of workers. Instead, she invades the nest of a different ant species, kills or displaces the resident queen, and forces the existing workers to raise her offspring. This is a well-known survival tactic for many ant species, particularly those in the genus Lasius. However, biologists have long believed that once a species adopts this parasitic lifestyle, it is locked into that path, unable to evolve into something else. The question of whether a parasite can change its ways and find a new, stable relationship with its host has remained a mystery, largely because observing these underground interactions is incredibly difficult.

A team of researchers has now uncovered a situation that challenges this long-held belief. By examining hundreds of ant nests across Europe, they discovered two new species of ants that appear to be in the middle of a major evolutionary shift. These ants, named Lasius castor and Lasius pollux, are not following the standard rules of social parasitism. Instead of simply taking over a host nest and replacing the host's workers with their own, they are forming long-term, mixed colonies where both species live and work together side by side. The researchers found that these new species have largely abandoned the traditional method of invading other ant species to start a family. Instead, they actively seek out specific relatives to live with, creating stable communities where both groups contribute workers. This discovery suggests that the path of evolution is not always a one-way street and that even highly specialized parasites might be capable of finding a new way to survive.

The study began with a massive effort to gather and analyze ant samples from across Central and Western Europe. The researchers collected nearly a thousand nest samples, a task that took decades of work from the team and their collaborators. Because these ants live underground and are very similar in appearance to other species, identifying them required high-powered microscopes and precise measurements of tiny physical features, such as the shape of their legs, the length of their hairs, and the structure of their waists. Using a method that treats these physical traits as a unique fingerprint for each species, the team was able to separate the new ants from their look-alikes. They found that Lasius castor and Lasius pollux are distinct species, each with its own specific body shape and hair patterns, but they are so similar to each other that they were previously mistaken for a single, unusual type of ant.

What makes these findings truly remarkable is how these ants build their colonies. In the traditional model of social parasitism, a new queen invades a host nest, kills the host queen, and the host workers raise the parasite's young until the parasite's own workers take over. The researchers found that this old-fashioned method is now very rare for Lasius castor and Lasius pollux. It happens in less than 6 percent of the cases they studied. Instead, in the vast majority of their nests—about 94 percent—these new ants live together with other species of the same subgenus. They do not kill the host queen or replace the host workers. Instead, they form mixed populations where the new species and their host relatives work together. In these shared nests, the new species makes up nearly 40 percent of the worker force, proving that they are fully functional members of the colony, not just freeloaders.

The researchers observed that these two new species have different preferences for their neighbors. Lasius castor tends to live in wetter, cooler environments and often partners with ants that like those same damp conditions. Lasius pollux, on the other hand, prefers drier, warmer steppe habitats and is usually found living with ants that thrive in those dry places. This suggests that the choice of partner is driven by the environment rather than a strict biological rule. The team also noted that the new ants actively search out these partners, rather than just stumbling upon them by chance. This active behavior points to a sophisticated relationship that goes beyond simple invasion.

Despite these clear observations, the researchers cannot yet say exactly what this relationship is called. It might be a form of permanent social parasitism, where the new species lives off the host but keeps its own workers. It could be a type of partnership called parabiosis, where two independent species share a nest and resources but keep their families separate. There is also a more complex genetic possibility that the new ants are not separate species at all, but rather a special genetic lineage that is passed down only through males, using the bodies of the host queens to reproduce. The paper argues that the most likely explanation is a transition toward permanent social parasitism, where the new species has evolved to live alongside its relatives without destroying them. However, the authors admit that the full picture is still unclear and that more genetic studies are needed to solve the puzzle.

The physical changes in these ants support the idea that they are evolving into something new. Compared to their relatives, Lasius castor and Lasius pollux have stockier bodies, shorter legs, and different shapes on their waists. These features look like the early signs of a parasite becoming more specialized, perhaps moving toward a state where it relies entirely on its host. Yet, unlike many parasites that lose the ability to work or care for their young, these ants have kept their full strength and ability to perform all the tasks a colony needs. This unique combination of traits suggests that evolution is currently in motion, capturing a moment where an ancient strategy is being rewritten. The discovery of these two hidden species offers a rare glimpse into how nature can find new solutions to old problems, showing that even in the rigid world of ant societies, change is always possible.