Collective nest site choice by ant colonies

How does a colony of insects cooperate to create a collective intelligence? We study this question in Temnothorax ants, a powerful model system for analyzing collective behavior. Colonies are small enough that all workers can be individually marked, and they thrive in glass-walled nests that allow detailed video analysis of every social interaction. Colonies show strong preferences for important nest attributes and can pick the best of several sites, even when few ants visit more than one site. We have found that the decision emerges from competitive recruitment at each site, organized by a minority of active ants. When one of these finds a site, her decision to bring nestmates there depends on how good she thinks it is, a simple rule that leads to stronger recruitment at better sites. The ants amplify this difference with a quorum rule that accelerates recruitment once a site’s population has reached a threshold density. This decision algorithm is both robust and flexible; by quantitatively changing their behavior, the ants can use the same rules to emphasize either speed or accuracy of decision-making, depending on the urgency of their situation.

Our current research uses this system to answer key questions about the adaptive design of decision mechanisms. These questions include:

How does colony size affect decision performance?

The number of subunits is potentially a key influence on the function of any cognitive system, whether colonies made up of individual ants or nervous systems made up of neurons. With ant colonies, however, we can experimentally manipulate the number of individuals in controlled experiments and look for effects on decision accuracy, speed and other aspects of performance.

Do colonies show ‘personalities’?

Animal personalities, or behavioral syndromes, can emerge in individuals when mechanistic contraints lead to behavioral correlations across contexts. We are looking for such syndromes at the group level, by asking whether some colonies are consistently cautious and prudent and others more bold or reckless. If so, we will explore how these colony-level traits emerge from differences in individual behavior.

Are colonies rational decision-makers?

Rationality, or consistency in how a decision-maker values options, is a fundamental assumption of classical optimization models of animal behavior. However, apparently irrational behavior has been seen in a variety of animals. This may reflect cognitive limitations or it may indicate that animals rely on cognitive rules that are highly effective in their evolved settings, but not in others. The analysis of when and how irrationality appears can reveal underlying cognitive mechanisms and the evolutionary and ecological forces that shaped them. We are applying these ideas to ants by asking whether colonies, and the individuals that compose them, show consistent departures from rationality in certain circumstances.

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