Group decision making in fission–fusion societies: evidence from two-field experiments in Bechstein's bats

Abstract

Group decisions are required when group coordination is beneficial, but individuals can choose between alternatives. Despite the increased interest in animal group decision making, there is a lack of experimental field studies that investigate how animals with conflicting information make group decisions. In particular, no field studies have considered the influence of fission-fusion behaviour (temporary splitting into subgroups) on group decisions. We studied group decision making in two wild Bechstein's bat colonies, which are fission-fusion societies of stable individual composition. Since they frequently switch communal roosts, colony members must regularly make group decisions over where to roost. In the two-field experiments, we provided marked individuals with conflicting information about the suitability of potential roosts. We investigated whether conflicting information led to group decisions that followed a 'unanimous' or a 'majority' rule, or increased colony fission. Individual behaviour suggests that bats considered both their own information and the behaviour of others when deciding where to roost. Group decisions about communal roosts reflected the information available to a majority of the bats roosting together, but conflicting information led to an increased fission in one colony. Our results suggest that fission-fusion societies allow individuals to avoid majority decisions that are not in their favour.

Leadership, consensus decision making and collective behaviour in humans

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Group size, grooming and fission in primates: A modeling approach based on group structure

“…When group size or within group competition for food increases, dis advantages may outnumber the advantages of group living, (Chapman et al, 1995;Janson and Goldsmith, 1995;Ron et al, 1994). As a consequence, group cohesion decreases and the group may split either temporarily (Kerth et al, 2006;Popa Lisseanu et al, 2008;Wittemyer et al, 2005) or irreversibly (Henzi et al, 1997a, b;Lehman et al, 2007). In social or pre social animals, irreversible fission is a common mode of group proliferation and dispersion.…”

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“…Analysis of gaze behavior across observers suggests that the greater collective integration benefits for visual search arise from an interaction between the foveated properties of the human visual system (high foveal acuity and low peripheral acuity) and observers' nonexhaustive search patterns, and can be predicted by an extended signal detection theory framework with trial to trial sampling from a varying mixture of high and low target detectabilities across observers (SDT-MIX). These findings advance our theoretical understanding of how to predict and enhance the wisdom of crowds for real world search tasks and could apply more generally to any decision-making task for which the minority of group members with high expertise varies from decision to decision.group decision rules | signal detection theory | ideal observer analyses | wisdom of crowds G roups of insects (1-4), fish (5-7), birds (8-10), mammals (11)(12)(13)(14), and primates (15-18) have been shown to aggregate their individual judgments into group decisions for various tasks (19,20). Although some groups seem to have leaders who make decisions alone on behalf of their groups (17,(21)(22)(23), it is difficult for individuals to outperform even simple aggregations of the entire group's individual judgments (4,7,9,10,19,(24)(25)(26).…”