Group defense by colony-founding queens in the fire ant Solenopsis invicta

Abstract

Mutualistic associations among nonkin can form when animals in groups have a greater chance of overcoming challenges presented by the environment than do solitary animals. Colony founding by small groups of unrelated queens, a habit documented in several species of ants, is often interpreted as a mutualistic interaction selected by intense competition among incipient colonies. However, many new colonies in these species are founded in areas where their chief enemies are mature ant colonies, rather than other newly founded colonies. In this study, we tested whether group nest-founding in the fire ant Solenopsis invicta improved the ability of queens to survive attacks by mature colonies. In the laboratory, queens in groups of three were more likely than solitary queens to survive attacks by workers of the native fire ant Solenopsis geminata. When newly mated queens were established experimentally in the field, workers from mature S. invicta colonies caused the majority of queen deaths. Queens in groups of two, but not in groups of four, had higher survival rates than did solitary queens during the period between colony establishment and the appearance of the first workers. The advantage of cooperative defense approximately counterbalanced the disadvantages caused by competition within foundress associations of two to three queens. Previous studies have shown that colonies founded by multiple queens produce larger worker populations than colonies founded by solitary queens; however, experimentally increasing worker number in incipient colonies had no effect on colony survival in the field.