When decisions are tough, it pays to think as a colony

Individual ants do well on easy decisions, but tackling tough ones takes a group.

In this experiment, ants have been color coded to help track them. Credit: Arizona.edu

There can be many benefits to living in a group: better protection from predators, more individuals available to gather or hunt food, and no need to travel far to find a mate. Additionally, living among many other conspecifics enables collective decision-making. Instead of relying on just one individual’s intelligence or experience, a group of animals can integrate information from all its members, improving estimates and increasing accuracy. That’s why large groups of people do much better than lone guessers when asked to estimate the number of marbles in a jar, and why the “ask the audience” lifeline on Who Wants to be a Millionaire is so appealing.

But is the “wisdom of crowds” always an improvement over the choices made by one or just a few? A new study in PNAS examines decision-making in ant colonies and finds that the outcome of some group decisions may depend on how challenging the problem is.

Temnothorax ants nest in a variety of locations, from soil to rotting wood, but they prefer nests that are dark (a dim nest may signal to ants that the walls are strong and sound). An international group of scientists from Arizona State University and Uppsala University used this preference to test the ants’ decision-making process. The researchers presented ants with two different potential nest sites made of balsa wood: one that was quite dark and one that was brighter, but the brightness of this second nest varied across trials. In some trials, the correct choice was obvious since the “bright” nest was very well-lit. In other trials, the light levels of the two nests were very similar, making the decision tougher for the ants.

If the ants’ decisions were perfect, they would choose the dim nest each time. However, the researchers found that the probability of choosing the best nest depended on two variables: how difficult the task was and how many ants were involved.

When the nests were drastically different in quality, ants working on their own chose the better nest most of the time, as did those working in colonies; individual ants actually chose the better nest slightly more often than the groups did. But when the decision was difficult due to subtle differences in the nests’ brightness, large groups fared much better than either solitary ants or small groups.

These results may be due to differences in the ways that individuals and colonies assess nest quality. When on their own, ants visit each nest multiple times and then make a decision. The more times they visit each nest, the better their decision tends to be. However, this is an expensive proposition for an insect—both in terms of time and energy—and solitary ants often give up and end up making a decision too early.

For large groups of ants, it isn’t necessary for all the ants in the colony to even visit each nest once. Instead of being based on individual comparisons, the colony’s decision is based on positive feedback and a quorum rule. The better the nest, the more likely ants are to recruit others to that site; once ants start encountering others there at a certain rate, a quorum is reached and the rest of the colony members simply end up drawn to the new nest.

When the choice is easy, even lone ants can make the right decision fast, and there is little advantage to extra social information gleaned from the rest of the colony. In fact, in these circumstances, random fluctuations can cause a group of ants to reach a quorum at an inferior nest, so colonies may fare even worse than individuals when the decision is simple. But when nest differences are subtle, individuals in a colony share the burden of assessing both nests and end up making the right decision more quickly and more often.

These results have obvious parallels to other animals, since species as diverse as cockroaches, fish, honeybees, and humans engage in collective decision-making. This certainly isn’t the first study to find that two heads aren’t always better than one, but it’s one more step on the way to understanding how groups come to decisions. Clearly, there can be both costs and benefits to making choices as a group, and the outcome may very well be based on the circumstances. Since collective decision-making is integral to the success—or failure—of bodies such as Congress, the United Nations, executive boards, and juries, we all have a pretty big stake in figuring out how to maximize the benefits and minimize the costs.

PNAS, 2013. DOI: 10.1073/pnas.1304917110 (About DOIs).