Social organization of nest emigration inLeptothorax (Hym., Form.)

Abstract

The investigated species of the genusLeptothorax have small colonies and nest in fragile nest sites. Thus they depend on frequent nest emigrations, if the ecological conditions are no longer favorable. Field observations of nest sites ofL. rugatulus are reported and experiments show that even slight physical disturbances easily release nest emigrations. Six species (L. muscorum, L. rugatulus, L. longispinosus, L. curvispinosus, L. acervorum andL. crassipilis) use both tandem running and carrying behavior during nest emigration. Tandem running is strictly limited to the first phase of nest emigration, whereas carrying behavior is the predominant technique (84%). This is the first case discovered in ants where two different recruitment techniques are employed for different purposes during one recruitment task. Workers which are recruited by tandem running inspect the new nest site. If they accept it, they join the scout force and lead or carry nestmates to the new nest by themselves. Thus the recruitment of workers by tandem running serves to increase the number of active scouts and «movers» («recruitment of recruiters»). The efficiency is considerably higher in the species which use this combined technique than inL. nylanderi, a species in which tandem in only the opposite direction could be recorded. These reverse tandems do not increase the number of scouts. It is not yet proven which workers «decide» whether or not still more new recruiters are to be recruited. A hypothesis is discussed that the severity of the ecological conditions inside the old nest affects the behavioral threshold of workers to respond to the signals of a tandem calling nestmate.