Meeting Abstract

P1.18  Friday, Jan. 4  Effect of moisture content on nest construction activity of fire ants MONAENKOVA, D*; GRAVISH, N; . GOODISMAN, M; GOLDMAN, D; Georgia Institute of Technology dmonaen@physics.gatech.edu

Fire ants (Solenopsis invicta) build large underground nests, which provide them with a living space protected from overheating, dehydration and predators. Field studies and laboratory experiments have revealed that one of the important environmental factors affecting nest building activity is soil moisture content. In this work we use x-ray computed tomography to study the growth in 3D of nest networks as a function of soil wetness. Because capillary cohesion in wet soils leads to the competition between tunnel stability and the labor-intensity of the excavation, we expect to find an optimal moisture content, which allows the most effective nest construction. We prepared digging containers (2.8 cm diameter by 11.5 cm deep plastic tubes) with a simulated soil of 240±30 µm glass particles. The prepared moisture content W (defined as the ratio between mass of water in the soil to mass of dry soil) varied from 0 to 0.2. Fifty fire ant workers were placed in the enclosed digging region and allowed to dig for 18 hours. We found that ants constructed tunnels in all moisture levels. However, maximum tunnel depth, H, was significantly affected by W. The minimum H was observed at two saturation extremes: W=0 (H=5.1 ±1.6 cm) and at W=0.2 (H=4.4 ±1.1 mm). The minimum tunnel depth at W=0.1 (H=11.5 cm) was at least two times greater than at either W=0 or W=0.2 for all tested colonies (p <0.0001). The increase in H mirrors the dependence of the soil cohesion on W and we therefore conclude that the tunnel stability is a key factor influencing the digging strategy of fire ants.