Phenotype and individual investment in cooperative foundress associations of the fire ant, Solenopsis invicta

Fire ant (Solenopsis invicta) queens founding a colony with unrelated nest mates potentially face a trade-off. Increased individual investment enhances worker production, colony survival, and growth. However, increased investment may reduce a queen's probability of surviving fights that invariably arise after worker eclosion. Indeed, previous studies showed that queens lose less weight (a measure of investment) when initiating colonies with cofoundresses than when alone, and that within associations the queen losing more weight is more likely to die. In this study, we tested whether queens adjust weight loss to social environment and fighting ability and whether restraining weight loss directly increases survival prospects. Experimental manipulation of colonies showed that reduced investment by queens within associations is primarily a response to the presence of a nest mate and not simply a response to per-queen brood-care demands. Differences in head width were associated with relative and combined weight loss of cofoundresses, as well as with queen survival. In contrast, the investment strategies of queens were not significantly influenced by their nest mates' initial weight. Similarly, manipulation of the queens' relative weight by feeding and exposure to contrasting social environment (queens kept alone or in groups) did not significantly affect survival. These results indicate that head width differences or correlated phenotypic attributes of fighting ability influenced both investment strategies and survival probability of queens. That queens with larger heads invested less energy into brood rearing and were more likely to survive reveals more selfish interactions among cofoundresses than has previously been assumed and casts some doubts about the idea that group selection must be invoked to account for the maintenance of cooperation in foundress associations of ants.