Abstract
Both a single ant and the colony to which it belongs can make decisions, but the underlying mechanisms may differ. Colonies are known to be less susceptible than lone ants to “choice overload”, whereby decision quality deteriorates with increasing number of options. We probed the basis of this difference, using the model system of nest-site selection by the ant Temnothorax rugatulus. We tested the applicability of two competing models originally developed to explain information-processing mechanisms in vertebrates. The Tug of War model states that concurrent alternatives are directly compared, so that choosing between two alternatives takes longer than accepting a single one. In contrast, the Sequential Choice Model assumes that options are examined in parallel, and action takes place once any option reaches a decision criterion, so that adding more options shortens time to act. We found that single ants matched the Tug of War model while colonies fitted the Sequential Choice model. Our study shows that algorithmic models for decision-making can serve to investigate vastly different domains, from vertebrate individuals to both individuals and colonies of social insects.
Highlights
The social insects offer some of the clearest examples of collective cognition, where group members share the processing of information about their environment[1,2,3]
Two classes of models vary in how decisions depend on the nature of the threshold(s), and in observable predictions: the threshold may require an accumulated difference between competing streams, or each stream may act in parallel, the faster one triggering a decision once it reaches the threshold. The former class of models is known as the Tug of War (ToW), because of the proposed direct interaction between options; it may be most appropriate for animals that must often choose the best among a set of simultaneously present options. The latter model is known as the Sequential Choice Model (SCM), because it assumes a biological decision mechanism adapted to situations where animals mainly meet isolated opportunities and decide whether to pursue them or not[28]
The ToW model predicts a shift in the opposite direction, but it makes no specific prediction for whether the added time should differ between options
Summary
The social insects offer some of the clearest examples of collective cognition, where group members share the processing of information about their environment[1,2,3]. Collective cognition allows a colony to distribute the burden of option assessment across many individuals, minimizing “choice overload.” This phenomenon, first identified in humans, is the worsening of decision quality with increasing numbers of options due to a decision-maker’s limited information-processing capacity[15,16]. Two classes of models vary in how decisions depend on the nature of the threshold(s), and in observable predictions: the threshold may require an accumulated difference between competing streams, or each stream may act in parallel, the faster one triggering a decision once it reaches the threshold The former class of models is known as the Tug of War (ToW), because of the proposed direct interaction between options; it may be most appropriate for animals that must often choose the best among a set of simultaneously present options. If an animal encounters multiple options simultaneously, the SCM proposes that the decision is reached through a horse race, in which the “choice” results from which option reaches the action threshold earlier, with better (or at least preferred) options reaching the threshold earlier, on average (for details, see Fig. 1)
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