  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.
