To overcome obstacles and survive harsh environments, fire ants link their bodies together to form self-assemblages such as rafts, bridges and bivouacs. Such structures are examples of self-assembling and self-healing materials, as ants can quickly create and break links with one another in response to changes in their environment. Because ants are opaque, the arrangement of the ants within these three-dimensional networks was previously unknown. In this experimental study, we applied micro-scale computed tomography, or micro-CT, to visualize the connectivity, arrangement and orientation of ants within an assemblage. We identified active and geometric mechanisms that ants use to obtain favorable packing properties with respect to well-studied packing of inert objects such as cylinders. Ants use their legs to push against their neighbors, doubling their spacing relative to random packing of cylinders. These legs also permit active control of their orientation, an ability ants use to arrange themselves perpendicularly rather than in parallel. Lastly, we found an important role of ant polymorphism in promoting self-aggregation: a large distribution of ant sizes permits small ants to fit between the legs of larger ants, a phenomenon that increases the number of average connections per ant. These combined mechanisms lead to low packing fraction and high connectivity, which increase raft buoyancy and strength during flash floods.
P.C.F. developed the cyanoacrylate fixation technique, helped with experiments, wrote software and did analysis and statistics on the data, developed the new measurement metrics used in this work, as well as wrote the final version of the paper. N.J.M. performed experiments, obtained and maintained ant colonies, wrote the first version of the paper, and identified connections to outside work in other fields. A.L. performed micro-CT scanning and provided significant expertise needed to both obtain the data and design the experiments to work with the available equipment. D.L.H. provided ideas, quality control, and editing of the manuscript.
The authors declare no competing financial interests.
This work was supported by the US Army Research Laboratory and the US Army Research Office Mechanical Sciences Division, Complex Dynamics and Systems Program, under contract number W911NF-12-R-0011.
Supplementary material available online at http://jeb.biologists.org/lookup/suppl/doi:10.1242/jeb.093021/-/DC1
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