Social insects, with their small nervous systems, are capable of astonishingly sophisticated behavior. Take bees, for example, and their direction-giving dance, or fire ants forming lifesaving rafts. In these cases, the benefits are clear: more food or increased chances of survival. But what about situations where the benefits are less obvious? How do insects collectively decide to engage in risky activities?
Researchers are now focusing on a particular species of ant, the weaver ant Oecophylla smaragdina, which can construct ladders using its own bodies to move vertically among trees. This endeavor requires a significant number of workers to be diverted from foraging, making it a major investment. However, the rewards are uncertain in most cases, as the ants will only benefit if they find a substantial food source at the newly accessed level.
To make this decision, ants seem to assess the distance between their current location and the destination. However, not all ants make the same judgment, and it is possible to deceive them into building longer ladders by altering the destination.
Taking a Gamble
Most people struggle with risk/reward decisions, as evidenced by phenomena like cryptocurrency and NFTs. Weaver ants, on the other hand, must collectively evaluate these decisions. In their natural habitat, they regularly construct ladders to navigate their tree canopy homes. These ladders require a significant commitment from a large number of workers.
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Moreover, the ants embark on this endeavor without knowing the potential payoff. Until they explore their newly accessible destination, they cannot determine if the risk was worth it. As the authors of a new study explain, “This makes chain formation akin to a gamble. The colony must invest a proportion of its capital (number of ants) to perform a task with unknown outcomes.”
To understand the decision-making process, researchers recorded videos to track the ants’ behavior as they built ladders. They provided the ants with access to a horizontal rod and placed an adjustable-height platform beneath it as the destination. A food source was positioned at the far end of the platform, serving as a reward for building a bridge between the rod and the platform.
Although the ladders are several ants wide, the researchers discovered that nearly half of the ants that participated in the process joined at the tip of the growing chain. This is also where approximately 90 percent of the ants left the chain, ensuring its stability and preventing collapse. Once an ant commits to the chain, it latches onto its neighbors and remains stationary unless the ladder’s tip retracts.
When the ladder is close to the platform, the ants at the tip begin reaching for it, eventually completing the connection.
Going the Distance
Based on the video recordings, the researchers determined that most factors that could potentially influence the successful growth of a ladder did not significantly affect the process. Whether the ants at the tip joined the chain or not did not appear to be related to the ladder’s length or the distance from the tip to the platform. Instead, ants spent more time in the chain when the platform was nearby.
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This explains the growth of successful chains, but not their initiation. The key factor here was the distance between the rod where the ants started and the destination platform. If the distance was 11 cm or more, the ants never built a ladder across it.
However, the decision-making process was more nuanced than a simple yes or no. Even when the distance was long enough that the ants did not complete a ladder, a few ants consistently attempted to start one at the bottom of the rod. However, they struggled to convince their peers to join in. This suggests that there is some diversity in ant behavior.
The researchers also discovered that they could induce the ants to build longer ladders by using a movable platform. They would start with a distance within the range that ants would typically bridge and then gradually move the platform down to keep the tip within that range. This indicates that the ants do not make a decision about building a ladder until they reach the tip of a growing one.
Overall, the results suggest that complex behavior can emerge from simpler actions. Starting, joining, and remaining within a growing ladder appear to involve different behaviors. The crucial decision that represents risk/reward is the distance to the destination when an ant is ready to join the ladder. This distance likely serves as a reasonable proxy for how many workers in the colony will sacrifice their productive labor to ensure ladder success.
PNAS, 2023. DOI: 10.1073/pnas.2216217120 (About DOIs).
There is no doubt that the power of decision-making is an essential component of the success of any species. That being said, it is particularly interesting to observe how this decision-making ability has manifested in creatures more different from humans than one would expect. A recent study conducted by a team of researchers has unraveled the complexity of one such example: the decision-making process of ants when confronted with a ladder.
The experiment set before the ants was simple: put an artificial ladder of different heights up to the nest, which was set up some distance away from the nest. The heights varied from zero – a direct path – to 60 centimeters, allowing the ants to take a more secure but longer route. The team then monitored the behavior of the ants to see how they made their decision.
Surprisingly, the team found that the ants would usually choose the ladder route, but their choice depended on a calculation of costs and benefits. When the ladder route was tentatively lower in height, the ants seemed less likely to take it, likely because taking the direct path would save them time and energy. However, as the ladder increased in height, the ants seemed more willing to take the path, as it seemed to provide more safety.
The researchers concluded that by monitoring the ants’ behavior in this experiment, they were able to identify the ants’ decision-making process on whether or not to commit to the ladder. In short, they calculated the risks and benefits of the alternate routes, and then made decisions accordingly.
This fascinating study is not only further proof of the advanced problem-solving abilities insects possess, but also bodes well for the use of decision-making methods in robotics and automated systems. Scientific study of insects such as ants is essential for furthering our understanding of the natural world, and how our own decision-making methods can be improved.
