Ants and their slaves: Slavery isn't exclusive to humans

At last, I’ve summoned the courage to write about creatures that have long fascinated me, ones whose behavior has captivated me beyond the familiar worlds of physics and philosophy. I have to admit, the subject was delightful and worth every bit of research.

My interest in ants began with a strange article written in the 1970s by renowned biologist Edward Wilson about the phenomenon of slavery among ants, opening with this provocative sentence: "The institution of slavery is not unique to human societies. No fewer than 35 species of ants, constituting six interdependently evolved groups, depend at least to some extent on slave labor for their existence."

The article drew me in and left me with a flurry of questions. Slavery is a social phenomenon, deeply intertwined with the evolution of the human brain. So how could such small creatures, with limited cognitive and neurological capacity, construct an institution as complex as slavery?

Do ants know they’re enslaved? Is this a product of evolutionary necessity? Can the enslaved rebel?

We often admire ants for their organizational skills and collaborative behavior, traits that resemble our own social structures. These tiny creatures have evolved highly complex societies with remarkable coordination and efficiency. That’s what makes the existence of slavery among ants one of the most intriguing aspects of their world.

Ant colonies are built on a natural division of labor. Most include a queen, whose body is larger than that of other ants in the colony. In some species, the queen has wings that she uses for mating flights.

Her primary role is reproduction, laying thousands of eggs to ensure the survival of the colony. Fertilized eggs hatch into sterile female workers, typically in large numbers, while unfertilized eggs produce far fewer winged males.

The worker ants perform a range of tasks, such as foraging for food, tending to the young, feeding larvae, and defending the colony and queen. This division of labor enhances the colony’s ability to adapt to changing environmental conditions.

The history of ant slavery

In the 1740s, Swiss naturalist Charles Bonnet documented unusual ant behavior. One species was seen carrying the pupae of another species into its nest. But Bonnet didn’t fully grasp the implications. A few decades later, in 1810, Swiss entomologist Pierre Huber provided detailed observations of one ant species raiding another and taking its pupae.

Huber recognized that these captives were used as workers in the raiders’ colony. He’s credited with offering the first clear description of this behavior. Charles Darwin later discussed the phenomenon in “Origin of Species” (1859), seeing it as an example of natural selection and the evolution of complex insect behavior.

Today, we know that certain ant species raid the nests of others, killing adult defenders and capturing the immature ants—eggs, larvae, or pupae—and bringing them back to their own colony to serve as workers.

This behavior is believed to have evolved as a strategy for expanding a colony’s labor force by exploiting other colonies, thereby freeing the slave-making ants from performing routine tasks themselves.

Darwin suggested that ant slavery developed gradually. Some species began with opportunistic raids to steal brood for food. Occasionally, some of the captives would survive and grow up as workers in the new colony. Over time, they were kept and accepted for their usefulness.

Edward Wilson proposed an alternative explanation. In his experiments, he placed different non-enslaving Leptothorax ants in close proximity, closer than they would naturally occur, and observed that the larger colonies would attack and enslave the smaller ones.

This led him to interpret slavery as the result of territorial conflict rather than a mere food-resource competition.

Pheromones and assimilation

Ants maintain their social order through a complex chemical communication system called pheromones. These signals, secreted from glands in their bodies, help them coordinate foraging, raise alarms, mark paths, and recognize nestmates. Pheromones enable efficient group behavior despite their simple brains.

Pheromones also play a crucial role in slavery. Ants identify members of their colony largely through pheromones. Enslaved ants, once captured, are exposed to the pheromones of their captors and eventually adopt them. Only then are they accepted into the colony.

If the enslaved ants fail to assimilate, they are usually killed. But because they grow up surrounded only by their captors’ pheromones, they don’t perceive themselves as enslaved. Their assimilation is aided by genetic proximity. Most slave-making ants target species closely related to them, making it easier for the captives to adopt the colony’s scent.

Strategies of enslavement

Although only about 50 of the known 15,000 ant species engage in slavery, scientists have noted that these species use a range of strategies to capture and integrate their slaves. Some specialize in attacking only one species, while others are more opportunistic.

In some cases, slavery has driven evolutionary changes. For instance, the Amazon ant (Polyergus) has become completely dependent on slaves. The warriors of this species have powerful jaws designed for combat but are incapable of performing basic colony tasks. They rely entirely on their slaves for feeding and grooming. If a disease wipes out the enslaved workers, the dominant ants starve to death.

In contrast, certain species of Formica ants, which include several slave-making types, can survive without slaves. These ants can perform all necessary tasks but will enslave others when given the chance. This more flexible approach allows them to adapt to varying conditions.

Ant slavery falls under a broader behavioral category called social parasitism, where one species exploits the social behavior of another for its own benefit, often causing harm in the process. Unlike traditional parasites that live on or inside their hosts, social parasites exploit behaviors like labor and caretaking.

Edward Wilson believed social parasitism, exemplified by ant slavery, could evolve into even deeper dependencies. One example is Strongylognathus, found in Asia and Europe. While most species in this genus launch aggressive raids, testaceus lost its fighting ability over time. Instead, its queen invades the host colony and cohabits peacefully with the host queen. Both lay eggs, but only the host species' workers perform labor.

Remarkably, the testaceus offspring do nothing. They are fed and cared for entirely by the enslaved workers. Scientists suspect the parasitic queen manipulates the host colony into adopting her through pheromones.

Rebellion in the ranks

But can enslaved ants even resist? Evolutionary biologist Susanne Foitzik observed that kidnapped workers of the Temnothorax species often kill a significant portion of their captors’ brood, even though they would raise young normally in their own colonies. She sees this as a form of resistance that may weaken the slavers’ ability to conduct future raids. This behavior isn’t unique to ants.

Some parasitized birds, like cowbirds that lay eggs in other species’ nests, face similar resistance. Hosts have been known to smash parasitic eggs.

In her lab, Foitzik found that most of the slavers’ brood didn’t survive. Enslaved workers tended to the eggs and larvae but killed the pupae. On average, they killed 80% of young slaver queens and 60% of worker pupae.

To ensure this wasn’t a laboratory artifact, she maintained control colonies of enslaved ants that had never been raided. Over 90% of their brood survived. Despite their limited cognitive abilities, ants’ slavery behavior reflects a broader truth. Biological communities can evolve complex social strategies.

In evolutionary terms, ant slavery may help dominant species survive and expand. However, it also carries the risk of overdependence, and sometimes, rebellion from within.

A version of this article first appeared in Arabic on February 8, 2025