Ants are social insects that live in colonies. Individual ants perform specific tasks for the colony, like supplying food, cleaning the nest, defending the colony, etc. There’s no ‘control room’ telling which ant what to do, yet they seem to know exactly what to do and when.
Consider the foragers – worker ants that bring food into the colony. The amount of food they carry perfectly matches the colony’s total hunger. How do they figure this? Scientists have puzzled over this question for some time. According to a new study, the trick might be in the way other ants eat the food the foragers carry.
While the ants accomplish complicated tasks, they use simple rules to decide what to do at each step. When they carry more food, they simply move deeper into the nest; when they’re carrying less than a certain amount, they leave to look for more food.
Researchers at the Weizmann Institute of Science, Israel, conducted the study.
Better together
How could an aggregate of simple decisions give rise to complex behaviour? Danny Raj M. studies collective phenomena at the Indian Institute of Science, Bengaluru. According to him, one technique researchers have used to study this question is to identify some “emergent” behaviour and then investigate the rules that ants follow to give rise to that behaviour.
For example, ant colonies can compare different sites before building their nests. Individual ants fail to do this on their own whereas, together, they turn into competent site inspectors. So researchers first study their behaviour during an inspection, then observe how individual ants contribute to this phenomenon, and finally deduce the rules that govern this behaviour.
Similarly, although only a few ants in a colony are foragers, they sense and sate the colony’s hunger. Foragers bring food from a wild source and distribute it to ants in the nest. They store the food they collect, such as honeydew or nectar, in their ‘crop’ – a pouch above the stomach – and feed other ants mouth-to-mouth. They fetch more food when the colony is more hungry. They leave in search of more food when the stock in the colony drops below a certain threshold. All this continues until the colony has been fed.
Informed decision-making
Individual ants are not so smart, or they could be if they invested more energy than they’re known to do. But they achieve these tasks by responding to some cues in their physical environment and what other ants are doing in that environment.
For example, ants leave substances called pheromones on their way to the food source so other foragers from the colony can follow the pheromone trail – instead of searching for the source from scratch.
Previously, researchers have found that foragers decide when to exit the nest based on personal information and local information. Personal information is based on their memories of previous interactions; local information is that from the ants around them, and could be in the form of direct communication with their antennae or indirect communication that involves physical cues.
However, these rules of foraging behaviour are complex and vary by species, and there are more than 15,000 of them.
Carpenter ants
A 2012 study concluded that individual foragers determined when to exit the nest for more food by tracking the rate at which other foragers entered the nest. In a 2018 study, Dr. Feinerman and his colleagues reported that the rate at which foragers exit a nest depends on how much food a forager has and how hungry the colony as a whole is.
The team studied a hungry colony of carpenter ants (genus Camponotus). In such a colony, ants eat more per mouth-to-mouth feeding interaction with foragers, which means foragers have few interactions with their colony-mates. In a less hungry colony, ants eat less per interaction, which means foragers spend more time distributing the food.
So foragers need to ‘compute’ the colony’s hunger level based on the number of interactions they need to unload the food they’re carrying.
Agent-based model
In the new study, the researchers used an agent-based model to check an idea that could explain forager movement in ant colonies in the simplest way possible.
In the model, ants were points that could move on a surface, or a plane – like the space in which ants move in the wild. This plane has a nest entrance, other ants in the space defining the nest, and a region outside the nest. The ants are then made to take a step into the nest or toward the exit, depending on their intent in a specific state.
The model also simulated encounters with other ants inside the nest to mimic feeding interactions.
After simulating these conditions for several cycles, the researchers calculated the frequency of ants exiting the nest.
Based on the resulting data, they suggested that foragers simply move deeper when they carry more food.
So when the colony is hungry, ants consume food from the forager faster. This causes the foragers’ food stock to dwindle quickly, reducing the time they spend in the nest before leaving. But when the colony isn’t hungry, the ants eat less, and the amount of food that foragers carry remains higher for longer. So the foragers move deeper into the nest and interact with more ants, and leave the nest less often for more food.
When the amount of food in the crop drops below some threshold, the foragers exit.
What if...?
“This explanation reduces the cognitive load required for ants to give rise to the emergent phenomenon,” Dr. Raj said.
But he added that while the mechanism elucidated in this study could play out in normal circumstances, it could change if the circumstances changed. For example, would ants behave the same way if, say, the number of foragers decreases drastically and the colony suddenly becomes more hungry?
We don’t know, but we have a start.
Joel P. Joseph is a freelance science journalist and researcher.
