INTRASPECIFIC USURPATION OF INCIPIENT FIRE ANT COLONIES

Brood raiding, or reciprocal stealing of brood, is common among incipi ent colonies of the fire ant Solenopsis invicta. Paradoxically, worker s from a colony that loses its brood during a raid often abandon their nest and join the winning colony. Queens abandoned by their workers m ay then migrate from their original nest site and attempt to forcefull y usurp another incipient colony by displacing that colony's queen or queens. This study examined factors that influence the success of usur pation attempts. Queens attempting to usurp a nearby colony after labo ratory brood raids were successful in less than 30% of trials. Usurpat ion attempts were more successful if workers familiar to the migrating queen were present in the nest, as would happen if a queen were to fi nd the colony to which her workers had moved. Cross-fostering experime nts showed that this effect was due to familiarity rather than related ness. Usurpation attempts were less successful if they were delayed by 16 h. The probability of usurpation success was not reduced by doubli ng the number of defending workers in the invaded colony. However, col onies founded by three queens were almost always able to resist usurpa tion attempts. These results support the hypothesis that workers aband on their natal colony after losing a brood raid to increase the likeli hood that their queen can usurp the colony to which they migrate. Thes e results also provide the first evidence that colonies of ants founde d by several queens are better able to resist usurpation attempts than colonies founded by a single queen.