Fire ants, Solenopsis invicta, link their bodies together to form structures such as rafts, bivouacs and bridges. However, these structures are in a constant state of danger due to exposure to outside forces. This study investigate how they their self-healing abilities.

We press two ant aggregations together and measure the forces to pull them apart. As the group size increases, the contribution of each ant decreases. This phenomenon, known as the Ringelmann effect, or social loafing, has previously been shown for cattle and humans. Below is the data for the ants compared to other experiments.

(B) Testing rigs to measure the strength of groups of ants (C) Four ants held apart by 1 cm undergoing ants a tensile test simulatenously (D) Relation between contribution per individual and number of individuals in a group. Green hollow circles correspond to tensile force in tug-of-war by Ringlemann, red hollow squares correspond to loudness of clapping, black hollow triangles correspond to loudness of shouting, blue curve corresponds to tensile force of group of ants in contact, and red curve corresponds tensile tests of group of ants not in contact, each separated by a distance of 1 cm. Contribution is normalized with respect to that of a single individual.

We rationalize this effect with an agent-based simulation which exhibits the Ringelmann effect of ants that periodically make and break links with each other, but grip with higher probability if the ants are stretched. Over time, ants compensate for the Ringelmann effect by building more links. To show this, we performed 2D ant tensile tests with simulations to support the experiments. Below is a figure comparing the two.

Ringelmann effect in experiments and simulations. Experiments are shown in (A–C), simulations in (D–F). (A) The ant tensile test. (B) Relationship between tensile force and elongation for a group size of 20 ants. To reduce the noise, we use a running average of 30 points. The fluctuations indicate the rearrangement of connections during the test. (C) The tensile strength, or maximum force, as a function of group size. The best fit line of power 0.37 is provided. (D) Simulated ants in a tensile test. Green ants are fixed to the bottom row, red ants are fixed to the top row which is moving upwards, and all other ants are colored to be distinguishable. Lines represent the body springs. (E) The relationship between force and elongation for 120 ants. (F) Maximum force of a tensile test as a function of the number of non-fixed ants. A best fit line of power 0.40 is provided.

We use tensile testing to measure the strength of ant clusters as a function of group size and contact time. We find that each ant contributes less to the group as the group size increases, a phenomenon known as the Ringelmann effect. To understand the causes for this phenomenon, we employ a two-dimensional simulation. This simulation relied on the following rule: ants randomly make and break connections, but grip each other with higher probability when stretched. The resulting simulation showed results similar to experiments. When two clusters are joined together, fire ants will self-heal into one connected structure. Using modeling, we found that the rate of connections made is proportional to the number of ants not yet connected. The rate at which new connections are made decreases with time, until saturation.