Abstract
Collective motion describes the global properties of moving groups of animals and the self-organized, coordinated patterns of individual behaviour that produce them. We examined the group-level patterns and local interactions between individuals in wild, free-ranging shoals of three-spine sticklebacks, Gasterosteus aculeatus. Our data reveal that the highest frequencies of near-neighbour encounters occur at between one and two body lengths from a focal fish, with the peak frequency alongside a focal individual. Fish also show the highest alignment with these laterally placed individuals, and generally with animals in front of themselves. Furthermore, fish are more closely matched in size, speed and orientation to their near neighbours than to more distant neighbours, indicating local organization within groups. Among the group-level properties reported here, we find that polarization is strongly influenced by group speed, but also the variation in speed among individuals and the nearest neighbour distances of group members. While we find no relationship between group order and group size, we do find that larger groups tend to have lower nearest neighbour distances, which in turn may be important in maintaining group order.
Highlights
The collective movements of animal groups provide some of the most striking examples of self-organization in nature
We initially assumed that this arose primarily through the fish responding to the current in the river, rather than through local interactions within the groups, our data indicate that the group direction of travel relative to the current was only weakly related to group polarization
The existence of such a rule may be inferred from the clear, low encounter-frequency area of within one body length of the focal fish [28]
Summary
The collective movements of animal groups provide some of the most striking examples of self-organization in nature. Collective motion can 2 be broadly studied at two interdependent levels, the individual and the group. Group-level organization may be measured in terms of the morphology of the group, the patterns of distribution of animals within the group and the overall co-ordination of group members. The shape of an animal group emerges through self-organization, mediated by environmental factors. Polarized groups may function to increase the perceptual confusion experienced by a predator encountering such groups [9], and may facilitate the spread of information between individuals [6,10,11]. Theoretical and empirical studies of collective motion have emphasized factors such as increase in speed and density in generating group polarization [12,13,14,15]. Despite breakthroughs in our understanding of these aspects of global organization in animal groups, it remains a priority to give broad consideration to the many potential factors that may be involved in this process in free-ranging conditions
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