Cohesion, order and information flow in the collective motion of mixed-species shoals.

Ashley J W Ward,T M Schaerf,E Crosato,M Prokopenko,A L J Burns,M M Webster,J T Lizier

doi:10.1098/rsos.181132

Ashley J W Ward, T M Schaerf + Show 5 more

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https://doi.org/10.1098/rsos.181132

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Abstract

Despite the frequency with which mixed-species groups are observed in nature, studies of collective behaviour typically focus on single-species groups. Here, we quantify and compare the patterns of interactions between three fish species, threespine sticklebacks (Gasterosteus aculeatus), ninespine sticklebacks (Pungitius pungitius) and roach (Rutilus rutilus) in both single- and mixed-species shoals in the laboratory. Pilot data confirmed that the three species form both single- and mixed-species shoals in the wild. In our laboratory study, we found that single-species groups were more polarized than mixed-species groups, while single-species groups of threespine sticklebacks and roach were more cohesive than mixed shoals of these species. Furthermore, while there was no difference between the inter-individual distances between threespine and ninespine sticklebacks within mixed-species groups, there was some evidence of segregation by species in mixed groups of threespine sticklebacks and roach. There were differences between treatments in mean pairwise transfer entropy, and in particular we identify species-differences in information use within the mixed-species groups, and, similarly, differences in responses to conspecifics and heterospecifics in mixed-species groups. We speculate that differences in the patterns of interactions between species in mixed-species groups may determine patterns of fission and fusion in such groups.

Highlights

Despite the frequency with which mixed-species groups are observed in nature, studies of collective behaviour typically focus on single-species groups
We found that single-species groups were more polarized than mixed-species groups, while single-species groups of threespine sticklebacks and roach were more cohesive than mixed shoals of these species
While there was no difference between the inter-individual distances between threespine and ninespine sticklebacks within mixed-species groups, there was some evidence of segregation by species in mixed groups of threespine sticklebacks and roach

Summary

Introduction

Despite the frequency with which mixed-species groups are observed in nature, studies of collective behaviour typically focus on single-species groups. Those groups that express high group cohesion and polarization may move more efficiently and maximize the benefits of grouping for individual group members [30] In addition to these measures, a particular feature of collective behaviour is the propagation of information across a group that occurs when a single individual initiates a change in direction and, with some small time delay, its near neighbours respond by adapting their trajectories to follow suit. Transfer entropy quantifies the reduction in uncertainty in predicting the updates of one time series (in this case, the temporal sequence of spatial movements of an animal) that can be achieved by knowing the past values of a second time series (a similar sequence derived from a second individual animal) It models the directed effect of one time series (the source) on another (the target), from which we can infer the information flow from source to target [31,32]. For example, strong wave-like sequences of information propagating across a group as the group undertakes collective turns [33,37]

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