Loffredo Emanuele, Venturelli Davide, Giardina Irene
Laboratory of Physics of the École Normale Supérieure, CNRS UMR 8023 and PSL Research, 24 rue Lhomond, 75231 Paris Cedex 05, France.
SISSA-International School for Advanced Studies and INFN, via Bonomea 265, 34136 Trieste, Italy.
Phys Biol. 2023 Apr 11;20(3). doi: 10.1088/1478-3975/acc5cc.
Living groups move in complex environments and are constantly subject to external stimuli, predatory attacks and disturbances. An efficient response to such perturbations is vital to maintain the group's coherence and cohesion. Perturbations are often local, i.e. they are initially perceived only by few individuals in the group, but can elicit a global response. This is the case of starling flocks, that can turn very quickly to evade predators. In this paper, we investigate the conditions under which a global change of direction can occur upon local perturbations. Using minimal models of self-propelled particles, we show that a collective directional response occurs on timescales that grow with the system size and it is, therefore, a finite-size effect. The larger the group is, the longer it will take to turn. We also show that global coherent turns can only take place if i) the mechanism for information propagation is efficient enough to transmit the local reaction undamped through the whole group; and if ii) motility is not too strong, to avoid that the perturbed individual leaves the group before the turn is complete. No compliance with such conditions results in the group's fragmentation or in a non-efficient response.
生物群体在复杂环境中移动，不断受到外部刺激、捕食攻击和干扰。对这些干扰做出有效反应对于维持群体的连贯性和凝聚力至关重要。干扰通常是局部的，也就是说，最初只有群体中的少数个体能察觉到，但却能引发全局反应。椋鸟群就是这样，它们能非常迅速地转向以躲避捕食者。在本文中，我们研究了在局部干扰下何时会发生全局方向变化。通过使用自驱动粒子的极简模型，我们表明集体方向反应发生的时间尺度会随着系统规模的增大而增长，因此这是一种有限尺寸效应。群体越大，转向所需的时间就越长。我们还表明，只有在以下条件下才能发生全局连贯转向：其一，信息传播机制足够有效，能够毫无衰减地将局部反应传递到整个群体；其二，运动性不能太强，以避免受干扰的个体在转向完成前就离开群体。不满足这些条件会导致群体分裂或反应低效。
