Media release

From: Frontiers

Swarm intelligence directs longhorn crazy ants to clear the road ahead for sisters carrying bulky food

Scientists show how collective form of understanding emerges from simple actions of unintelligent worker ants

Scientists studied the obstacle-clearing behavior of longhorn crazy ants, where a subset of workers temporarily specializes in removing tiny objects blocking the path between the nest and large food items. Experiments revealed that serial clearing behavior can be triggered by a single pheromone mark, which happened to be deposited near an obstacle by a forager recruited to a large food item. Clearing mostly occurs in the context of collective transport, which typically stalls in front of obstacles. The authors concluded that obstacle-clearing is a form of ‘swarm intelligence’ which emerges at the colony level, and which does not require understanding by individual ants.

Among the tens of thousands of ant species, incredible ‘intelligent’ behaviors like crop culture, animal husbandry, surgery, ‘piracy’, social distancing, and complex architecture have evolved. Yet at first sight, the brain of an ant seems hardly capable of such feats: it is about the size of a poppy seed, with only 0.25m to 1m neurons, compared to 86bn for humans. Now, researchers from Israel and Switzerland have shown how ‘swarm intelligence’ resembling advance planning can nevertheless emerge from the concerted operation of many of these tiny brains. The results are published in Frontiers in Behavioral Neuroscience.

“Here we show for the first time that workers of the longhorn crazy ant can clear obstacles from a path before they become a problem – anticipating where a large food item will need to go and preparing the way in advance. This is the first documented case of ants showing such forward-looking behavior during cooperative transport,” said Dr Ehud Fonio, a research fellow at the Weizmann Institute of Science in Israel, and the corresponding author of the study.

‘I can see all obstacles in my way’

The researchers were inspired when they made a fascinating chance observation in nature: individual crazy ant workers used their mandibles to pick up and carry away tiny gravel pebbles near groups of workers cooperating to transport large insect prey.

“When we first saw ants clearing small obstacles ahead of the moving load we were in awe. It appeared as if these tiny creatures understand the difficulties that lie ahead and try to help their friends in advance,” said Dr Ofer Feinerman, a professor at the Weizmann Institute, and the study’s final author.

Fonio et al. designed a suite of 83 experiments to study this obstacle-clearing behavior on a single crazy ant ‘supercolony’ on the Weizmann Institute’s campus. For pebbles, they used plastic beads with a diameter of 1.5 millimeter (half the body length of the ants) to block the ants’ route. For prey, they used pellets of cat food, of which the ants are fond.

Triggered into clearing mode by pheromones

Like many ant species, crazy ants are known to alert their sisters to the presence of large food items by laying odor trails: running erratically (hence their ‘crazy’ name), they touch the ground with the tip of their abdomen every 0.2 seconds to deposit a tiny droplet of a pheromone. This pheromone swiftly attracts other workers to the food. But here, the scientists found this pheromone to play a key role in clearing behavior as well.

Their observations showed that workers were most prone to clear beads that lay approximately 40mm away from food towards the direction of the nest. They moved these beads for up to 50mm before dropping them, away from the route leading back to the nest. The record holder cleared 64 beads in succession.

Such clearing behavior always occurred when the pellet was whole, but rarely when it was divided into crumbs. This distinction seemed adaptive, as the observations showed that crumbs were always carried home by single workers, who would simply walk around any beads in their path. Intact pellets, however, always prompted ‘cooperative’ transport by multiple workers, who typically remained stalled by a grid of beads until these were cleared.

That the beads were a real hindrance was also clear from the time that cooperative transport took to pass through a 5cm by 7cm tunnel: this was 18 times longer when the passage was filled with beads than when it was free of obstacles.

Further observations also revealed that workers didn’t need to be in contact with the food to start clearing behavior: they were prompted to do so by pheromones deposited by foragers. A single mark that happened to be near a bead was sufficient to put a worker in ‘clearing mode’, after which they would actively look for more beads to clear.

‘Awe-inspiring’

“Taken together, these results imply that our initial impression was wrong: in reality, individual workers don’t understand the situation at all. This intelligent behavior happens at the level of the colony, not the individual. Each ant follows simple cues – like fresh scent marks left by others – without needing to understand the bigger picture, yet together they create a smart, goal-directed outcome,” concluded Dr Danielle Mersch, formerly a postdoctoral researcher at the same institute.

“We find this to be even more awe-inspiring than our initial guess,” said Feinerman.

“Humans think ahead by imagining future events in their minds; ants don’t do that. But by interacting through chemical signals and shared actions, ant colonies can behave in surprisingly smart ways – achieving tasks that look planned, even though no single ant is doing the planning. These ants thus provide us an analogy to brains, where from the activity of the relatively simple computational units, namely neurons, some high cognition capabilities miraculously emerge.”