Abstract
Decision-making animals can use slow-but-accurate strategies, such as making multiple comparisons, or opt for simpler, faster strategies to find a ‘good enough’ option. Social animals make collective decisions about many group behaviours including foraging and migration. The key to the collective choice lies with individual behaviour. We present a case study of a collective decision-making process (house-hunting ants, Temnothorax albipennis), in which a previously proposed decision strategy involved both quality-dependent hesitancy and direct comparisons of nests by scouts. An alternative possible decision strategy is that scouting ants use a very simple quality-dependent threshold rule to decide whether to recruit nest-mates to a new site or search for alternatives. We use analytical and simulation modelling to demonstrate that this simple rule is sufficient to explain empirical patterns from three studies of collective decision-making in ants, and can account parsimoniously for apparent comparison by individuals and apparent hesitancy (recruitment latency) effects, when available nests differ strongly in quality. This highlights the need to carefully design experiments to detect individual comparison. We present empirical data strongly suggesting that best-of-n comparison is not used by individual ants, although individual sequential comparisons are not ruled out. However, by using a simple threshold rule, decision-making groups are able to effectively compare options, without relying on any form of direct comparison of alternatives by individuals. This parsimonious mechanism could promote collective rationality in group decision-making.
Highlights
Animals need to make choices between multiple available options at many stages in their life histories, such as during mate selection, foraging, or when deciding where to shelter or to build a nest
Recruitment latency was measured as time from discovering a nest to first leading a tandem run to that nest and data are only available for ants which recruited by tandemrunning
In a simulation presenting the two nests separately the pattern is dramatically different, with significantly higher recruitment latencies for the poor nest (Fig. 4c). This clearly demonstrates that differences in recruitment latency can be generated when only one nest is present, as a sideeffect of the threshold rule
Summary
Animals need to make choices between multiple available options at many stages in their life histories, such as during mate selection, foraging, or when deciding where to shelter or to build a nest. The fitness benefits of choosing the best option mean that decision-making strategies will be subject to natural selection. Animals may be best served by using a simpler ‘rule of thumb’, which reduces sampling time, but still ensures the option they choose is good enough. One simple but effective strategy is sequential search in which the animal keeps searching until it finds an option that exceeds a threshold of acceptability [2,7]. This kind of fixedthreshold strategy is used in foraging [8], mate choice [9] and refuge selection [10]. An intermediate strategy is to allow thresholds to be influenced by experience of previous options; in effect this would cause a ‘sequential comparison’
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