The emergent complexity of ant societies is one of the most fascinating phenomena in the natural world: how do these tiny creatures form such intricate social networks? These networks are so nuanced that the colony itself is sometimes referred to as an organism—or “superorganism”—in its own right, with individual ants as its component parts.
A paper published this month in the journal PNAS Nexus examines how the behavior of ants is affected by social contagion. Social contagion is the process by which a certain behavior can spread throughout a group, resulting in what’s called a “mass behavior.”
Social contagion is common among all manner of social animals, from ants and fish to birds and humans. But while it can be beneficial when it leads to co-operation and collective action, the study points out that the mass behaviors it creates can also have “catastrophic outcomes such as mass panic and stampedes.” As such, the positive reinforcement of social contagion is generally counterbalanced in animal societies by what the authors call “reverse social contagion.”
Social contagion stems from an individual’s urge to imitate an activity that they see being carried out by their neighbors, while reverse social contagion arises when individuals are less likely to do something if they see their neighbors already doing that same thing.
This prevents situations where entire groups all end up carrying out the same activity, regardless of that activity’s utility.
As the paper points out, negative outcomes of social contagion are strikingly rare amongst ants, suggesting that that reverse social contagion plays an important role in ant societies. To quantify how reverse social contagion regulates ant behavior, the researchers examined individual ant activity amongst 12 colonies of harvester ants. These colonies varied in size from several dozen ants to several hundred.
The experiment set out to determine how the size of a colony influenced the activity level of its workers: if ant behavior depended only on positive social contagion, more ants would be expected to be active in a larger colony, as they would have more opportunities to observe a given behavior in their fellow ants.Fig. 1. Illustration of the concepts of social contagion (top) and reverse social contagion (bottom). (top) An inactive ant interacts with an ant engaged in a foraging task: through social contagion (for example, caused by active recruitment), it also begins foraging. (bottom) Two ants engaged in foraging interact: through reverse social contagion (for example, caused by steric exclusion), one of them ceases their activity to become inactive. Image credit: Isabella Muratore.
The study found that just because the colony was larger, did not mean that more ants were exhibiting the same behavior. Because different groups engaged in a variety of behaviors, observations suggest that reverse social contagion was also at play.
This also forms a stark contrast to human societies, where the level of individuals’ activity tends to increase more quickly as a society’s population grows.
The announcement accompanying the paper uses the general example of food gathering to illustrate the difference: if an ant sees multiple fellow workers gathering food, it saves its own energy for another task that might be more beneficial to the colony. If a human sees their neighbors all gathering food, however, they worry there might be none left for them—a worry that tends to become more pressing as population increases.Fig. 7. a) Illustration of an urban settlement composed of individuals who act for their own benefits: each person transforms their cost of movement (measured in some form of currency depending on their means of transport, 𝐶 / 𝑁 ) into their own social interactions (proxied by the average connectivity, ⟨ 𝑘 ⟩ = 2 𝐸 / 𝑁). b) Illustration of a social colony of insects acting as a superorganism: each insect adjusts its energy expenditure, 𝐵 / 𝑁, in response to its average connectivity so that it will increase its expenditure in response to reductions in connectivity. Image credit: Anna Sawulska. As Simon Garnier, the lead author on the paper and an Associate Professor of Biological Sciences at the New Jersey Institute of Technology, explains in the announcement,
“Human behavior is often driven by personal gain, [but] ants … tend to prioritize the needs of the colony over their own. This has huge implications for understanding the differences between the organization of human and social insect societies.”
That, of course, is a bit of a generalization, as there are plenty of human societies that value the collective over individual interests, but that may be a question of sociology and culture, rather than behavioral science.
Regardless, the authors draw a fascinating conclusion: the oft-heard metaphors about ant colonies being “superorganisms” are actually pretty accurate. “This work,” the paper concludes, “suggests that the appropriate atomic unit for an ant is its colony—and not itself as a single organism.”
Illustration of the concepts of social contagion (top) and reverse social contagion (bottom). (top) An inactive ant interacts with an ant engaged in a foraging task: through social contagion (for example, caused by active recruitment), it also begins foraging. (bottom) Two ants engaged in foraging interact: through reverse social contagion (for example, caused by steric exclusion), one of them ceases their activity to become inactive. Credit: Isabella Muratore, PNAS Nexus (2024). DOI: 10.1093/pnasnexus/pgae246
In the world of social creatures, from humans to ants, the spread of behaviors through a group—known as social contagion—is a well-documented phenomenon. This process, driven by social imitation and pressure, causes individuals to adopt behaviors observed in their peers, often resulting in synchronized mass actions: Think of stampedes, or standing ovations. . .
One such mechanism is reverse social contagion. In reverse social contagions, increased interactions between individuals engaged in a behavior lead to a higher likelihood of them stopping that behavior, rather than engaging in it.
In a paper published in PNAS Nexus, researchers led by Maurizio Porfiri—NYU Tandon Institute Professor of Professor of Biomedical Engineering, Mechanical and Aerospace Engineering, and Civil and Urban Engineering, as well as the director of its Center for Urban Science and Progress (CUSP)—describe this unique phenomenon in colonies of harvester ants (Pogonomyrmex californicus) in order to understand the energetic consequences of highly integrated social behavior.
"Ants colonies reduce their energy spending per individual as the colony grows, similar to the size-dependent scaling of metabolic costs in birds and mammals discovered by Kleiber almost a century ago," said Porfiri. "To date, a convincing explanation of how this collective response emerges is lacking."
Utilizing tracked video recordings of several colonies, they discovered that individual ants did not increase their activity levels in proportion to the colony size. This was a curious finding, because larger colonies means more interactions between their members, and more opportunities for reinforcing behaviors.
To decode this behavior, the team—who also includes Pietro De Lellis from the University of Naples, Eighdi Aung and (Tandon alum) Nicole Abaid from Virginia Tech, Jane S. Waters from Providence College, and Santiago Meneses and Simon Garnier from the New Jersey Institute of Technology—applied scaling theories typically used to study human settlements.
They derived relationships linking colony size to interaction networks and activity levels, hypothesizing that reverse social contagion was at play. Their hypothesis was supported by respirometry data, which revealed a potential connection between ant activity and metabolism.
Imagine you are an ant, and you see one of your fellow workers foraging for food. If you are governed by social contagion, you might also begin foraging so you don't look lazy. But the energy you expend foraging might not be worth it if one ant can efficiently gather the food.
In this case, reverse social contagion tells you to kick your feet up and relax while you let your compatriot do the work, because you'll need your energy later for another task. In this way, restraining social contagion makes the colony more efficient.
The study draws a fascinating parallel between insect colonies and human cities. In both systems, social interactions influence energy expenditure, but in opposite directions. Insect colonies exhibit hypometric scaling—activity levels do not increase proportionally with colony size. In contrast, human cities show hypermetric scaling, where energy expenditure grows faster than the population size.
"Human behavior is often driven by personal gain," says Simon Garnier, Associate Professor of Biological Sciences at NJIT and senior author on the paper. "Ants, on the other hand, tend to prioritize the needs of the colony over their own. This has huge implications for understanding the differences between the organization of human and social insect societies." Unlike humans, ants manage their energy as a colony rather than individually, somehow displaying a cooperative response. This study shows that ants use reverse social contagion to regulate their overall activity and energy use.
Essentially, when many ants are busy with a task, some will stop to prevent the entire colony from overworking. This behavior aligns with scaling laws and metabolic patterns seen in other biological systems.
In simpler terms, think of an ant colony as one big organism where every ant's actions are coordinated for the colony's benefit, not just their own.
Future research will look into how exactly these ants communicate and manage their energy so efficiently.
This research not only sheds light on the regulatory mechanisms in ant colonies but also offers insights into the broader principles of social regulation across species. As we continue to explore these parallels, we may uncover more about the fundamental dynamics that govern both natural and human-made systems.
"This is the first step we are taking to understand and model energy regulation in ant colonies," said Porfiri.
"Is energy regulation accompanied by improved performance for the collective?
Can we design algorithms for robot teams inspired by ants that can maximize performance and minimize energy costs?
Can we learn some lessons for our city transportation networks?
These are just some of the questions we would like to address next."
More information: Maurizio Porfiri et al, Reverse social contagion as a mechanism for regulating mass behaviors in highly integrated social systems, PNAS Nexus (2024). DOI: 10.1093/pnasnexus/pgae246
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