script_cache_simulations.R from Temporary prey storage along swarm columns of army ants: an adaptive strategy for successful raiding?

While pillaging the brood of other ant colonies, <i>Eciton</i> army ants accumulate prey in piles, or caches, along their foraging trails. Widely documented, these structures have historically been considered as by-products of heavy traffic or aborted relocations of the ants' temporary nest, or bivouac. However, we recently observed that caches of the hook-jawed army ant, <i>Eciton hamatum</i>, appeared independently from heavy traffic or bivouac relocations. In addition, the flow of prey through caches varied based on the quantity of prey items workers transported. As this suggested a potential adaptive function, we developed agent-based simulations to compare raids of caching and non-caching virtual army ants. We found that caches increased the amount of prey that relatively low numbers of raiders were able to retrieve. However, this advantage became less conspicuous—and generally disappeared—as the number of raiders increased. Based on these results, we hypothesize that caches maximize the amount of prey that limited amounts of raiders can retrieve, especially as prey colonies coordinately evacuate their brood. In principle, caches also allow workers to safely collect multiple-prey items and efficiently transport them to the bivouac. Further field observations are needed to test this and other hypotheses emerging from our study.

当劫掠其他蚁群的幼虫时，行军蚁属（Eciton）的军团蚁会沿着觅食路径将猎物堆积成堆，即猎物缓存（caches）。这类结构已有大量文献记录，长期以来被认为是蚁群密集往来的副产物，或是其临时宿营巢（bivouac）搬迁中止的结果。然而我们近期观察到，钩颚军团蚁（Eciton hamatum）的猎物缓存并非由密集往来或宿营巢搬迁所产生。此外，经猎物缓存流转的猎物量会随着工蚁运输的猎物数量变化而改变。由于这提示缓存可能具有适应性功能，我们开发了基于智能体的模拟（agent-based simulations），以对比存在猎物缓存与不存在猎物缓存的虚拟军团蚁的劫掠行动。我们发现，当劫掠者数量相对较少时，猎物缓存能够提升可回收的猎物总量。但随着劫掠者数量增加，这一优势会逐渐减弱，最终基本消失。基于上述结果，我们提出假说：当劫掠者数量有限时，猎物缓存能够最大化可回收的猎物总量，尤其是在猎物蚁群协同疏散幼虫的场景下。从理论上讲，猎物缓存还能让工蚁安全收集多件猎物，并高效地将其运至宿营巢。后续仍需开展野外观测，以验证本研究提出的这一假说及其他衍生假说。