Collective Behaviour without Collective Order in Wild Swarms of Midges

Abstract

Collective behaviour is a widespread phenomenon in biology, cutting through a huge span of scales, from cell colonies up to bird flocks and fish schools. The most prominent trait of collective behaviour is the emergence of global order: individuals synchronize their states, giving the stunning impression that the group behaves as one. In many biological systems, though, it is unclear whether global order is present. A paradigmatic case is that of insect swarms, whose erratic movements seem to suggest that group formation is a mere epiphenomenon of the independent interaction of each individual with an external landmark. In these cases, whether or not the group behaves truly collectively is debated. Here, we experimentally study swarms of midges in the field and measure how much the change of direction of one midge affects that of other individuals. We discover that, despite the lack of collective order, swarms display very strong correlations, totally incompatible with models of non-interacting particles. We find that correlation increases sharply with the swarm's density, indicating that the interaction between midges is based on a metric perception mechanism. By means of numerical simulations we demonstrate that such growing correlation is typical of a system close to an ordering transition. Our findings suggest that correlation, rather than order, is the true hallmark of collective behaviour in biological systems.

Geometry of thresholdless active flow in nematic microfluidics

“…These active components can be subcellular (such as microtubules powered by molecular motors, and actomyosin networks [2,3]), synthetic (e.g., self-propelled colloids [4] or interacting microrobots), or, alternatively, living organisms [5][6][7][8], such as birds, fish [9], microorganisms [10,11], or insects [12]. Hybrid systems composed of motile rod-shaped bacteria placed in nontoxic liquid crystals have also been realized recently [13].…”

Collective selection of food patches in Drosophila

“…Research on arthropod swarms (e.g. midges, Attanasi et al, 2014;acari, Mailleux et al, 2011;ants, Sasaki et al, 2013;bees, Seeley et al, 2012), fish schools (Ward et al, 2011), bird flocks (Ballerini et al, 2008), mammal troops (Strandburg-Peshkin et al, 2015) and human crowds (Moussaïd et al, 2011) shows how consensus decisions are typically reached via mechanisms of quorum sensing and networks of feedback loops that disproportionately amplify or reduce the responses of individuals to social stimuli (Camazine et al, 2001;Couzin, 2009;Sumpter, 2010). Through transfer and collective processing of social information, grouped individuals can make faster and/or more accurate decisions than isolated conspecifics, an emergent property of collective decisions known as 'swarm intelligence' (Krause et al, 2010;Sumpter and Pratt, 2009).…”

Intermittent collective dynamics emerge from conflicting imperatives in sheep herds

“…So far, only a handful of quantitative datasets have been gathered for large animal groups (19-21). Most of them have focused on elementary cases where the prevailing biological imperative seems to be group cohesion, either to gain protection from potential predators, such as for the spontaneous collective motion exhibited by starling flocks (19,22) and some fish schools (23-25), or for reproductive purposes, as in swarms of midges (21,26).One important and, so far, often neglected aspect of collective motion is the existence of individual-level behavioral shifts, which, in turn, may trigger a transition at the collective level. For instance, in many species of fish, groups regularly alternate between a swarming state, in which fish simply aggregate with a low level of polarization, and a schooling state, in which individuals are aligned and move in the same direction (27,28).…”

“…So far, only a handful of quantitative datasets have been gathered for large animal groups (19-21). Most of them have focused on elementary cases where the prevailing biological imperative seems to be group cohesion, either to gain protection from potential predators, such as for the spontaneous collective motion exhibited by starling flocks (19,22) and some fish schools (23-25), or for reproductive purposes, as in swarms of midges (21,26).…”

“…Deciphering these interactions, their relation with the patterns emerging at the collective level, and their connections with the physiological and ecological constraints peculiar to each group-living species is crucial to understanding the evolution of collective phenomena in biological systems (16)(17)(18). So far, only a handful of quantitative datasets have been gathered for large animal groups (19)(20)(21). Most of them have focused on elementary cases where the prevailing biological imperative seems to be group cohesion, either to gain protection from potential predators, such as for the spontaneous collective motion exhibited by starling flocks (19,22) and some fish schools (23)(24)(25), or for reproductive purposes, as in swarms of midges (21,26).…”