Anatomy of a superorganism -- structure and growth dynamics of army ant bivouacs

TL;DR

This study uses CT imaging to analyze the internal structure and growth dynamics of army ant bivouacs, revealing their heterogeneous composition and insights into their self-assembly rules, which could inform engineered self-assembling systems.

Contribution

The paper provides the first detailed internal structural analysis of army ant bivouacs using CT scans, uncovering their heterogeneity and growth patterns, and discusses implications for bio-inspired self-assembly.

Findings

Bivouacs have a thick shell and a less dense interior with empty spaces.

Ants do not carry more than about eight times their weight.

Bivouac size is not limited by the physical constraints of individual ants.

Abstract

Beyond unicellular and multicellular organisms, there is a third type of structural complexity in living animals: that of the mechanical self-assembly of groups of distinct multicellular organisms into dynamical, functional structures. One of the most striking examples of such structures is the army ant bivouac, a nest which self-assembles solely from the interconnected bodies of hundreds of thousands of individuals. These bivouacs are difficult to study because they rapidly disassemble when disturbed, and hence little is known about the structure and rules that individuals follow during their formation. Here we use a custom-built Computed Tomography scanner to investigate the details of the internal structure and growth process of army ant bivouacs. We show that bivouacs are heterogeneous structures, which throughout their growth maintain a thick shell surrounding a less dense interior that contains empty spaces akin to nest chambers. We find that ants within the bivouac do not carry more than approximately eight times their weight regardless of the size of the structure or their position within it. This observation suggests that bivouac size is not limited by physical constraints of the ants' morphology. This study brings us closer to understanding the rules used by individuals to govern the formation of these exceptional superorganismal structures, and provides insight into how to create engineered self-assembling systems with, for instance, swarms of robots or active matter.