Foundress survival and brood production in the desert seed-harvester ants Pogonomyrmex rugosus and P. barbatus (Hymenoptera, Formicidae)

Summary:

A three-factor laboratory experiment (2 species × 2 soil types × 4 moisture levels) demonstrated that water is the most important factor effecting colony establishment and growth in the two ecologically equivalent sister species of desert seed-harvester ants Pogonomyrmex rugosus and P. barbatus. All experimental indices, percent gyne survival, gyne live wet mass, percent of gynes producing workers, and mean number of individuals (workers, larvae, and pupae), increased from low to moderate moistures and then levelled off. Soil texture also displayed a significant effect as gyne wet mass was 5 - 10% higher across treatments in soils that had a higher clay content. The only species effect was that P. barbatus gynes were smaller but produced more workers and brood than those of P. rugosus. That P. rugosus survives longer under arid conditions was demonstrated by a significantly higher number of nests with live workers in the driest treatment; number of live gynes displayed a similar pattern but differences were not significant. Additionally, P. barbatus produced fewer workers than P. rugosus in the driest treatment, while this number was higher than P. rugosus at all other moisture levels. Metabolic rate was higher and workers eclosed sooner in P. barbatus compared to the larger P. rugosus.