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A simple threshold rule is sufficient to explain sophisticated collective decision-making.

Robinson EJ, Franks NR, Ellis S, Okuda S, Marshall JA - PLoS ONE (2011)

Bottom Line: 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.This parsimonious mechanism could promote collective rationality in group decision-making.

View Article: PubMed Central - PubMed

Affiliation: School of Biological Sciences, University of Bristol, Bristol, United Kingdom. Elva.Robinson@yccsa.org

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.

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Arenas used in emigration experiments, showing nest locations.Dashed outline = old destroyed nest; shading = good nests. (A) Good nest 120 cm from old nest; poor nest 30 cm [26]. (B) Equidistant good and poor nests 45 cm from old nest [20]. (C) Three equidistant new nests 36 cm from old nest. Dashed lines indicate points at which tandem-runs were recorded.
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pone-0019981-g001: Arenas used in emigration experiments, showing nest locations.Dashed outline = old destroyed nest; shading = good nests. (A) Good nest 120 cm from old nest; poor nest 30 cm [26]. (B) Equidistant good and poor nests 45 cm from old nest [20]. (C) Three equidistant new nests 36 cm from old nest. Dashed lines indicate points at which tandem-runs were recorded.

Mentions: A colony emigration by rock ants begins with scouts searching for and assessing new sites. Successful scouts recruit nest-mates using tandem running, in which an informed ant leads another to the new site [17]. When the number of ants at a site reaches a ‘quorum threshold’, the ants switch to rapid transport behaviour to carry the brood, queen and remaining nest-mates to the new nest [18]. Ant colonies discriminate between nest sites on the basis of a range of attributes including cavity dimensions, light level and entrance width [16], [19]. Three possible mechanisms by which the colony collectively chooses the better of two nests have been proposed: i) comparison ii) recruitment latency iii) threshold rule. First, ants could use a comparison strategy, in which ants that have visited both nests would compare their qualities and recruit nest-mates only to the better one [20]. Second, ants visiting just one nest could determine how long they hesitate before recruiting nest-mates (recruitment latency) based on the nest quality [20]. A combination of these two mechanisms has been used as the basis of several decision models [18], [21]–[24]. Although lone T. rugatulus ant workers are capable of comparing the attributes of nest-sites which are very close together [25], the evidence for individual ants making direct comparisons between nests during colony emigration is weak [26], and furthermore, ant colonies are able to choose a distant good nest over a nearby poor nest, when recruitment latency differences would be expected to be cancelled out by travel time (Fig. 1a) [26], [27]. The third possible mechanism of choice is a sequential search strategy, based on a very simple rule of thumb, a ‘threshold rule’, by which an ant assesses a nest against her own fixed quality threshold, and either accepts the nest and begins recruitment, or rejects it and continues searching [26]. Quality-dependent recruitment latency has been demonstrated empirically only when colonies were presented with a single new nest and were not required to make a choice [20], [22], [24], [28]. Robinson et al. [26] hypothesized that these apparent recruitment latency effects could emerge as a by-product of a threshold rule, because ants that find a low-quality nest will tend to reject it and continue searching, whereas ants that find a high quality nest will tend to accept it and begin recruitment. In addition, Robinson et al. [26] hypothesized that the apparent comparison phenomenon in which ants that have visited equidistant poor and good nests usually recruit only to the good nest (Fig. 1b; [20]) can be explained more parsimoniously with a threshold rule in which the poor nest is rejected and forgotten, then the good nest is discovered and accepted, rather than requiring individual ants to perform the more cognitively complex task of remembering and comparing the qualities of different nests. We aim to test the hypothesis that this simple parsimonious mechanism (the threshold rule) is sufficient to reproduce observed empirical patterns of collective decision-making.


A simple threshold rule is sufficient to explain sophisticated collective decision-making.

Robinson EJ, Franks NR, Ellis S, Okuda S, Marshall JA - PLoS ONE (2011)

Arenas used in emigration experiments, showing nest locations.Dashed outline = old destroyed nest; shading = good nests. (A) Good nest 120 cm from old nest; poor nest 30 cm [26]. (B) Equidistant good and poor nests 45 cm from old nest [20]. (C) Three equidistant new nests 36 cm from old nest. Dashed lines indicate points at which tandem-runs were recorded.
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Related In: Results  -  Collection

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pone-0019981-g001: Arenas used in emigration experiments, showing nest locations.Dashed outline = old destroyed nest; shading = good nests. (A) Good nest 120 cm from old nest; poor nest 30 cm [26]. (B) Equidistant good and poor nests 45 cm from old nest [20]. (C) Three equidistant new nests 36 cm from old nest. Dashed lines indicate points at which tandem-runs were recorded.
Mentions: A colony emigration by rock ants begins with scouts searching for and assessing new sites. Successful scouts recruit nest-mates using tandem running, in which an informed ant leads another to the new site [17]. When the number of ants at a site reaches a ‘quorum threshold’, the ants switch to rapid transport behaviour to carry the brood, queen and remaining nest-mates to the new nest [18]. Ant colonies discriminate between nest sites on the basis of a range of attributes including cavity dimensions, light level and entrance width [16], [19]. Three possible mechanisms by which the colony collectively chooses the better of two nests have been proposed: i) comparison ii) recruitment latency iii) threshold rule. First, ants could use a comparison strategy, in which ants that have visited both nests would compare their qualities and recruit nest-mates only to the better one [20]. Second, ants visiting just one nest could determine how long they hesitate before recruiting nest-mates (recruitment latency) based on the nest quality [20]. A combination of these two mechanisms has been used as the basis of several decision models [18], [21]–[24]. Although lone T. rugatulus ant workers are capable of comparing the attributes of nest-sites which are very close together [25], the evidence for individual ants making direct comparisons between nests during colony emigration is weak [26], and furthermore, ant colonies are able to choose a distant good nest over a nearby poor nest, when recruitment latency differences would be expected to be cancelled out by travel time (Fig. 1a) [26], [27]. The third possible mechanism of choice is a sequential search strategy, based on a very simple rule of thumb, a ‘threshold rule’, by which an ant assesses a nest against her own fixed quality threshold, and either accepts the nest and begins recruitment, or rejects it and continues searching [26]. Quality-dependent recruitment latency has been demonstrated empirically only when colonies were presented with a single new nest and were not required to make a choice [20], [22], [24], [28]. Robinson et al. [26] hypothesized that these apparent recruitment latency effects could emerge as a by-product of a threshold rule, because ants that find a low-quality nest will tend to reject it and continue searching, whereas ants that find a high quality nest will tend to accept it and begin recruitment. In addition, Robinson et al. [26] hypothesized that the apparent comparison phenomenon in which ants that have visited equidistant poor and good nests usually recruit only to the good nest (Fig. 1b; [20]) can be explained more parsimoniously with a threshold rule in which the poor nest is rejected and forgotten, then the good nest is discovered and accepted, rather than requiring individual ants to perform the more cognitively complex task of remembering and comparing the qualities of different nests. We aim to test the hypothesis that this simple parsimonious mechanism (the threshold rule) is sufficient to reproduce observed empirical patterns of collective decision-making.

Bottom Line: 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.This parsimonious mechanism could promote collective rationality in group decision-making.

View Article: PubMed Central - PubMed

Affiliation: School of Biological Sciences, University of Bristol, Bristol, United Kingdom. Elva.Robinson@yccsa.org

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.

Show MeSH
Related in: MedlinePlus