Decision making by young queens of the harvester ant Messor semirufus while searching for a suitable nesting site

Field observations on the behavior of young queens of the harvester ant Messor semirufus, reveal a considerable variance among individuals in the time span between landing after nuptial flight, and the start of nest digging. During that interval, which can last from a few minutes to almost 2 h, the queens wander around while being exposed to many dangers, especially to the risk of predation. Furthermore, queens were observed digging in places where other queens have just passed through and declined to dig. We hypothesized that M. semirufus queens are performing a sequential sampling of the environment, to deduce its quality distribution, before making the decision to dig. To test this hypothesis, we conducted laboratory experiments to examine the influence of two factors, soil hardness and queen density, on digging decisions. The results indicate that these factors indeed affect the walking and searching time until digging. Furthermore, it turns out that the young queens are able to perceive and react to changes, either an improvement or a deterioration, in soil hardness and in queen density: their digging decision is stimulated by an improvement, and impeded by a deterioration in these factors, relative to initial condition.