University of Colorado, U.S. Mechanical Engineering Department, Material Science and Engineering Program, Boulder, Colorado, United State of America.
PLoS Comput Biol. 2022 Feb 17;18(2):e1009869. doi: 10.1371/journal.pcbi.1009869. eCollection 2022 Feb.
Collective living systems regularly achieve cooperative emergent functions that individual organisms could not accomplish alone. The rafts of fire ants (Solenopsis invicta) are often studied in this context for their ability to create aggregated structures comprised entirely of their own bodies, including tether-like protrusions that facilitate exploration of and escape from flooded environments. While similar protrusions are observed in cytoskeletons and cellular aggregates, they are generally dependent on morphogens or external gradients leaving the isolated role of local interactions poorly understood. Here we demonstrate through an ant-inspired, agent-based numerical model how protrusions in ant rafts may emerge spontaneously due to local interactions. The model is comprised of a condensed structural network of agents that represents the monolayer of interconnected worker ants, which floats on the water and gives ant rafts their form. Experimentally, this layer perpetually contracts, which we capture through the pairwise contraction of all neighboring structural agents at a strain rate of [Formula: see text]. On top of the structural layer, we model a dispersed, on-lattice layer of motile agents that represents free ants, which walk on top of the floating network. Experimentally, these self-propelled free ants walk with some mean persistence length and speed that we capture through an ant-inspired phenomenological model. Local interactions occur between neighboring free ants within some radius of detection, R, and the persistence length of freely active agents is tuned through a noise parameter, η as introduced by the Vicsek model. Both R and η where fixed to match the experimental trajectories of free ants. Treadmilling of the raft occurs as agents transition between the structural and free layers in accordance with experimental observations. Ultimately, we demonstrate how phases of exploratory protrusion growth may be induced by increased ant activity as characterized by a dimensionless parameter, [Formula: see text]. These results provide an example in which functional morphogenesis of a living system may emerge purely from local interactions at the constituent length scale, thereby providing a source of inspiration for the development of decentralized, autonomous active matter and swarm robotics.
集体生命系统经常实现单个生物体无法单独完成的合作涌现功能。在这种情况下，红火蚁（Solenopsis invicta）的筏通常被用来研究，因为它们能够形成完全由自身身体组成的聚合结构，包括类似于系绳的突起，这些突起有助于它们探索和逃离被洪水淹没的环境。虽然在细胞骨架和细胞聚集体中也观察到类似的突起，但它们通常依赖于形态发生素或外部梯度，这使得局部相互作用的孤立作用仍未得到很好的理解。在这里，我们通过受蚂蚁启发的基于代理的数值模型展示了蚂蚁筏中的突起如何由于局部相互作用而自发出现。该模型由一个紧凑的结构代理网络组成，代表相互连接的工蚁单层，这些工蚁漂浮在水面上，赋予了蚂蚁筏的形状。在实验中，这个层会不断收缩，我们通过以应变率[Formula: see text]对所有相邻结构代理进行成对收缩来捕捉这种收缩。在结构层的顶部，我们模拟了一个离散的、格点化的、可移动代理层，代表自由蚂蚁，这些蚂蚁在漂浮的网络上行走。在实验中，这些自主行走的自由蚂蚁具有一定的平均持久长度和速度，我们通过受蚂蚁启发的现象学模型来捕捉这些特性。在检测半径[Formula: see text]内，相邻的自由蚂蚁之间会发生局部相互作用，自由活动代理的持久长度通过由 Vicsek 模型引入的噪声参数[Formula: see text]进行调整。R 和[Formula: see text]都被固定以匹配自由蚂蚁的实验轨迹。根据实验观察，当代理在结构层和自由层之间转换时，筏就会发生履带式运动。最终，我们展示了如何通过增加蚂蚁的活动（由无量纲参数[Formula: see text]来描述）来诱导探索性突起生长的阶段。这些结果提供了一个例子，说明生命系统的功能形态发生可能纯粹来自组成长度尺度的局部相互作用，从而为去中心化、自主的主动物质和群体机器人技术的发展提供了灵感来源。
