Quorum decision-making facilitates information transfer in fish shoals

Abstract

Despite the growing interest in collective phenomena such as ''swarm intelligence'' and ''wisdom of the crowds,'' little is known about the mechanisms underlying decision-making in vertebrate animal groups. How do animals use the behavior of others to make more accurate decisions, especially when it is not possible to identify which individuals possess pertinent information? One plausible answer is that individuals respond only when they see a threshold number of individuals perform a particular behavior. Here, we investigate the role of such ''quorum responses'' in the movement decisions of fish (three-spine stickleback, Gasterosteus aculeatus). We show that a quorum response to conspecifics can explain how sticklebacks make collective movement decisions, both in the absence and presence of a potential predation risk. Importantly our experimental work shows that a quorum response can reduce the likelihood of amplification of nonadaptive following behavior. Whereas the traveling direction of solitary fish was strongly influenced by a single replica conspecific, the replica was largely ignored by larger groups of four or eight sticklebacks under risk, and the addition of a second replica was required to exert influence on the movement decisions of such groups. Model simulations further predict that quorum responses by fish improve the accuracy and speed of their decision-making over that of independent decision-makers or those using a weak linear response. This study shows that effective and accurate information transfer in groups may be gained only through nonlinear responses of group members to each other, thus highlighting the importance of quorum decision-making.behavior ͉ collective decision-making ͉ schooling ͉ shoaling ͉ social A nimal groups, including humans, often exhibit complex dynamic patterns that emerge from local interactions among group members (1-4). This collective behavior is of particular interest when individuals with limited personal information use cues and signals provided by others to decide on a course of action (5, 6).It has been suggested that the accuracy of decision-making increases with group size (7). However, our understanding of exactly how behavioral interactions scale to collective properties, and the consequences of this process to individual survival, are limited because of the difficulty inherent in addressing the complicated feedbacks that arise from repeated social interactions (1, 4, 8): individuals both create and are influenced by their social context (9, 10). In many social interactions, it may not be possible to identify which individuals, if any, possess pertinent information. Simply copying the behavior of others indiscriminately may lead to cascades of information transfer where the nonadaptive behavior of single animals or small numbers of individuals is reproduced by many other individuals at no benefit to the copiers (11-13).Recent advances in understanding collective decision-making have mostly come from studies of eusocial and gregarious insects (6,(14)(15)(1...

Leadership, consensus decision making and collective behaviour in humans

“…However, in contrast to some recent studies (e.g. Ward et al 2008;Franks et al 2009;Sumpter & Pratt 2009 did not find evidence that consensus decisions followed a quorum decision rule. Kerth et al (2006) found that Bechstein's bats, Myotis bechsteinii, can make group roost decisions that follow a majority rule.…”

Decision accuracy in complex environments is often maximized by small group sizes

“…Recent experiments in simplified laboratory environments have demonstrated that the wisdom of crowds can improve decision accuracy across a variety of contexts, including avoiding a replica predator [22,23] and discriminating between conspecific phenotypes [24], suggesting that this may be an important driver favouring the evolution of sociality. Nonetheless, it is evident in nature that many organisms make decisions alone, and that many social organisms live in relatively small groups [25], seemingly failing to take advantage of the informational benefits of large group size.…”

“…Consequently, although they may lack the capacity for numerosity (the ability to explicitly count or tally), or to explicitly cast a 'vote', organisms such as schooling fish do effectively perform majority consensus decision-making through their employment of simple and local social interactions [15,[22][23][24]39]. Furthermore, the presence of individuals lacking preferences has been shown to further promote majority voting by such animal groups [39].…”

Desert ants achieve reliable recruitment across noisy interactions

“…In general, as long as they are consistent with phenomenology, simple models of communication are preferred to more complex descriptions [12] because they are more elegant and depend on fewer assumptions. Such simple descriptions, often referred to as swarm models, have been highly useful in recounting collective patterns within a wide variety of species, including fishes, birds, locusts and other social insects [4,[13][14][15]. They use mathematical modelling to show how the observed large-scale experimental phenomena emerge from a small set of proposed behavioural rules.…”