Pleometrotic colony foundation in the ant Crematogaster scutellaris (Hymenoptera: Formicidae): better be alone than in bad company

In this study, we investigated whether pleometrosis occurs in Crematogaster scutellaris (OLIVIER, 1792) and which advantages it may provide. A survey of C. scutellaris queens in aphid galls revealed that 19% of the occupied galls contained two or more queens (max. five). Moreover, the observed distribution of the number of queens per gall was not statistically different from that expected from a null model based on random allocation. Microsatellite analysis on naturally associated queens showed no evidence of relatedness among them. We then investigated the possible advantages of pleometrotic foundation, comparing the survival and brood production of queens spontaneously nesting alone or in pairs, in small and large nests. Paired queens never showed any form of cooperation in brood care and interacted aggressively or tried to avoid each other. Mortality of paired queens was always greater than that of solitary queens, and mortality was higher in small than large nests. Finally, the larger queen in a pair was more likely to survive the fights. The total number of eggs produced by queens in pairs was nearly twice that produced by isolated queens, although the total progeny production (larvae, pupae, and workers) was the same. This suggests significantly higher brood attrition in paired queens than in solitary ones, probably due to an increase in egg cannibalism or trophic egg production. Further, the time until first worker eclosion was the same for pleometrotic and haplometrotic queens. These results suggest that associative foundation in C. scutellaris is more likely to be the result of a random process, probably triggered by the need to escape from predation and abiotic stresses after the nuptial flight, than an active choice of queens. Additionally, the possible advantages of associative foundation, if any, should be very limited.