A Simple Threshold Rule Is Sufficient to Explain Sophisticated Collective Decision-Making

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.

Variability in individual assessment behaviour and its implications for collective decision-making

“…Specifically, if the quality assessments of individuals are influenced by experience, this would stop workers from being locked into repeatedly rejecting a nest that they deemed poor, even though it constituted the only viable option. Furthermore, as supported by previous experimental and modelling results, experiential influence would also help to ensure that even when multiple options of very high quality were present, the best nest would still be preferred [20,36].…”

“…Previous work has shown that ants can choose between nests without directly comparing them, and thus save both time and cognitive effort [9]. The exact mechanism underpinning such decentralized nest choice remains unclear, but it has been hypothesized that workers may have considerable variation in their individual quality thresholds, each accepting a nest option only if it meets their individual 'quality standards' [20,21]. For this system to be effective, however, differing worker quality assessments would need to be flexibly distributed within colonies, in order to ensure the best possible emigration decisions, and avoid the pitfalls of becoming locked into suboptimal choices.…”

“…Yet, while adaptive threshold distributions have been indicated through modelling to be plausible [20,23], to date, there remains a paucity of empirical evidence to support the theory.…”

“…Chiefly, this emerges from evidence showing that when assessing a nest site, ants spend longer within the nest if they deem it to be of higher quality, while attempting to leave if it is of lower quality [9,22,24]. Furthermore, although the prerecruitment latency of ants is shorter for higher-quality nests [20], they spend a greater proportion of this latency inside the new nest, if it is of better quality [25]. It has been hypothesized that such a trend exists, as by spending longer within a nest, workers may better contribute to a quorum threshold prior to recruiting [24,26,27].…”

Local Termination Criteria for Stochastic Diffusion Search: A Comparison with the Behaviour of Ant Nest-Site Selection

“…Scout B will then make an independent assessment of the nest, and will either reject it and keep searching, or accept the nest and spend some time in it, before returning home and recruiting a further scout. By this positive feedback process, a good quality nest will accumulate ants [34], [36]. Different ants appear to have differing thresholds for starting recruitment to a nest; this means that even low quality nests can attract some ants, but scouts will accumulate more quickly and to a higher level at higher quality nests [39] [35].…”