Nest complexity, group size and brood rearing in the fire ant, Solenopsis invicta

Colonies of the monogyne social form of the red imported fire ant, Solenopsis invicta, may contain half a million workers per nest. Spatially organizing such a large group within the confines of a single nest requires a pragmatic architectural design. The morphology of field mounds and subterranean nest chambers of S. invicta were determined using several cast methods. The above-ground portion of a fire ant nest, the mound, consisted of a dense matrix of narrow tunnels. Below ground level, subterranean shafts and nodes were excavated through the grass root system (10-20 cm), connecting the mound tunnels to the subterranean chambers. Chambers averaged similar to 5 cm(2) and appeared at depths from 10-80 cm below ground level. The orientation of mound tunnels and subterranean shafts was more often vertical than horizontal, probably facilitating the diurnal micro-migration of colony members back and forth from the subterranean chambers to the mound. The mean size of groups in the subterranean chambers of field nests was remarkably small relative to colony size (similar to 200 individuals per chamber: 50% workers, 50% brood). In laboratory experiments, neither crowding nor nest partitioning affected brood rearing. The possibility that small group size and nest complexity generate localized or time-released communication signals is discussed.