Let’s play a game. It’s called “Viking horde or army ants?”
a) Hundreds of thousands of these individuals were seen fanning out over the South American forest, devouring everything in their path.
b) These quickly depopulated dozens of villages in 9th century England.
c) These show deference to a silent and immobile queen.
d) A popular novel ends when the heir to a family is brutally destroyed by these invaders.
e) This group is known for enslaving foreigners.
Answers: a) army ants; b) Viking horde; c) army ants; d) army ants; e) both.
How did you do?
As the game suggests, groups of ants and groups of humans don’t behave all that differently, despite being on opposite ends of the evolutionary tree. As Jurgen Kurths of the Potsdam Institute for Climate Impact Research puts it, “While the single ant is certainly not smart, the collective acts in a way that I’m tempted to call intelligent.”
Collective intelligence in animals accounts for the highly complex societies and behaviors of animals like, ants, birds, and fish, even when the collective’s individual animals may lack planning power on their own.
The army ants Eciton burchellii are nomadic, so a high level of coordination is necessary on a daily basis—every time they need to eat, they need to move. They will organize themselves into a uniform swath often tens of meters across and multiple meters deep, then move in unison. Feeding mainly on other arthropods—not newborn babies, as in Gabriel Garcia Marquez’s One Hundred Years of Solitude—, they gobble up whatever hasn’t gotten out of the way (including other ants), and when they’re done, they set up camp. While many ants tunnel or create habitations out of plants and other materials, these army ants form bivouacs out of their own bodies. Anchoring themselves to trees or logs, they hook their feet and mouths together, creating a sack for their fellow ants to live in, and thereby exposing only a fraction of the ants’ bodies to the elements (Kronauer 1-11).
The species Polyergus rufescens coordinate for a different purpose. Like the Eciton burchellii, they descend upon other colonies en masse. But instead of eating the other species of ants they come across, they kidnap them and turn them into slaves who forage and raise the host colony’s young (Kronauer 54-59).
All of the coordination involved in these various activities requires near-instantaneous communication, and for creatures who don’t vocalize, that requires some combination of instinct and adaptation. Most ants communicate through pheromones, particularly when scavenging for sustenance: a group of ants will sally forth in a haphazard manner to look for food and, when one finds it, she returns to the colony with a sample. As other ants follow her back to the food source, more pheromones are dropped, thereby drawing more ants to the food trail. That’s why you’ll see more and more ants marching from the outdoors into your kitchen when there’s an available food source and ants nearby to find it.
In addition to pheromonal communication, individual ants use ‘rules of thumb’ to guide their immediate responses to circumstances. Harvester ants live in the desert where pheromone trails wouldn't last in the dry heat, yet their colonies still take an intelligent approach to collecting food. Research from the lab of Stanford University's Deborah Gordon, including a study by our own Bob Schafer, found that ants wait for successful foragers to return before sending out new foragers. This shows that ants follow simple rules to flexibly respond to the available amount of food (they won’t send out more ants to forage if there’s nothing to find) without having access to information about the entire colony. As Gordon describes it, “A colony is analogous to a brain where there are lots of neurons, each of which can only do something very simple, but together the whole brain can think.”
The ant species E. hamatum responds to the environment in a particularly remarkable way: they spontaneously build bridges across impassable situations using their own bodies. And, they do it such that they will target the narrowest part of an expanse first, creating the minimum viable bridge for the other ants to pass over before paving over wider areas with their bodies. Incredibly, these ants are blind. They coordinate their actions by attending to the density of the crowd: when they bump into each other more frequently than usual, they understand they need to choose a different path. If an alternate path is unavailable, they forge one, which might include a bridge.
Not unlike ants’ bridge-building, starlings’ complex flocking behavior is honed by individuals to benefit the entire murmuration (that’s the beautifully descriptive and presumably onomatopoeic collective noun for “starling”) even when the individual has only a partial picture of the whole. Starlings’ en masse acrobatics create safety not only in numbers—a murmuration can have up to a million birds— but by confusing predators. The birds also likely share information while flocking. Murmurations twist and loop as one, the starlings coordinating to such an extent as to maintain well-defined edges.
Just how starlings do this has been a subject of speculation since Roman times and prompted physicists Andrea Cavagna and Irene Giardina of Italy’s Sapienza University to conduct a multi year experiment. It showed that “rather than keeping track of every neighbor within a certain distance, each bird tracks and responds to a finite number of its closest neighbors — seven in the case of starling flocks.” This number accounts for the birds adjacent to it: if each bird maintains a certain distance from each adjacent neighbor, the flock will bend and fold more or less simultaneously. When one bird notices a falcon approaching, it will turn, causing an instantaneous cascade of movement as its neighbors follow suit.
Open questions remain about the remarkable reaction times starlings display, but recent research into schooling fish offers some insight into how underwater decision-making occurs. Collective decision making in fish also results from an avalanche or cascade of action based on neighbors’ movements. And while some decisions were random—as was sometimes the case for golden shiners, a “jumpy” fish who was studied for its startle behavior—it seemed that information-based decision-making was key to creating consensus among schooling fish. A study designed to sort random decision-making from informed decision-making showed that two groups of golden shiners trained to expect food in different color coded environments came to the decision that benefited more of the fish most often, guided by fish occupying what appeared to be leadership positions.
In animals like ants, birds, and fish, collective intelligence (or collective computation) has long fascinated human observers. After all, their colonies’ complexity is difficult to comprehend relative to the individuals’ simplicity. But if we consider these animals to be cognitively inferior to ourselves, they are nevertheless adept at living cooperatively. Understanding how to improve our own communal existence is one aim of the researchers whose work is described in this post. Another research goal several studies cited was the potential to create swarm intelligence among robots. If that notion makes you, like me, more than a little nervous, you may be comforted to learn that such robots would ostensibly be used to enter natural disaster areas and self-direct their responses when they encounter difficult terrain or situations. Or, you might remain terrified at the prospect of semi-sentient robots, no matter their moral disposition.
By Aimee Fountain
Kronauer, Daniel JC. Army Ants: Nature’s Ultimate Social Hunters. Harvard University Press, 2020.