A common rule for decision making in animal collectives across species

Abstract

A diversity of decision-making systems has been observed in animal collectives. In some species, choices depend on the differences of the numbers of animals that have chosen each of the available options, whereas in other species on the relative differences (a behavior known as Weber’s law), or follow more complex rules. We here show that this diversity of decision systems corresponds to a single rule of decision making in collectives. We first obtained a decision rule based on Bayesian estimation that uses the information provided by the behaviors of the other individuals to improve the estimation of the structure of the world. We then tested this rule in decision experiments using zebrafish ( Danio rerio ), and in existing rich datasets of argentine ants ( Linepithema humile ) and sticklebacks ( Gasterosteus aculeatus ), showing that a unified model across species can quantitatively explain the diversity of decision systems. Further, these results show that the different counting systems used by animals, including humans, can emerge from the common principle of using social information to make good decisions.

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

“…Each individual's voting strategy is to choose the low correlation cue with probability p and the high correlation cue with probability 1 2 p and to vote for the option indicated by that cue. Majority consensus determines the collective decision [15,18,38,39].…”

“…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].…”

“…Theoretical and empirical work has shown that the social interaction rules adopted by many organisms effectively integrate the opinions of the group, such that the resulting collective decision often closely mirrors the majority opinion [15,18,[22][23][24]38,39]. 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].…”

“…Environmental cues frequently covary with biologically meaningful features and consequently often serve as a potent source of information [4]: an odour can predict the presence of food, or the rustling of leaves can predict the presence of a predator, etc. A further reduction of uncertainty may be achieved by incorporating social cues into the decision-making process [4,[6][7][8][9][10][11][12][13][14][15]. It has been suggested that by pooling imperfect estimations, groups may achieve increased decision accuracy, the so-called wisdom of crowds [16][17][18][19][20][21].…”

Adversity magnifies the importance of social information in decision-making

“…Here we show that Bayesian estimation predicts superaggregation in adversity, which was inadvertently present in some decision-making models [30,31], and absent from others due to simplifying assumptions [26][27][28][29]. Furthermore, we find that many non-Bayesian decision-making models may also lead to super-aggregation in adversity, and discuss the conditions in which this will hold.…”