GROUP DEFENSE BY COLONY-FOUNDING QUEENS IN THE FIRE ANT SOLENOPSIS-INVICTA

Mutualistic associations among nonkin can form when animals in groups have a greater chance of overcoming challenges presented by the enviro nment than do solitary animals. Colony founding by small groups of unr elated queens, a habit documented in several species of ants, is often interpreted as a mutualistic interaction selected by intense competit ion among incipient colonies. However, many new colonies in these spec ies are founded in areas where their chief enemies are mature ant colo nies, rather than other newly founded colonies. In this study, we test ed whether group nest-founding in the fire ant Solenopsis invicta impr oved the ability of queens to survive attacks by mature colonies. In t he laboratory, queens in groups of three were more likely than solitar y queens to survive attacks by workers of the native fire ant Solenops is geminata. When newly mated queens were established experimentally i n the field, workers from mature S. invicta colonies caused the majori ty 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 th e disadvantages caused by competition within foundress associations of two to three queens. Previous studies have shown that colonies founde d by multiple queens produce larger worker populations than colonies f ounded by solitary queens; however, experimentally increasing worker n umber in incipient colonies had no effect on colony survival in the fi eld.