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 enhance s worker production, colony survival, and growth. However, increased invest ment may reduce a queen's probability of surviving fights that invariably a rise after worker eclosion. Indeed, previous studies showed that queens los e less weight (a measure of investment) when initiating colonies with cofou ndresses than when alone, and that within associations the queen losing mor e weight is more likely to die. In this study, we tested whether queens adj ust weight loss to social environment and fighting ability and whether rest raining weight loss directly increases survival prospects. Experimental man ipulation of colonies showed that reduced investment by queens within assoc iations is primarily a response to the presence of a nest mate and not simp ly a response to per-queen brood-care demands. Differences in head width we re associated with relative and combined weight loss of cofoundresses, as w ell as with queen survival. In contrast, the investment strategies of queen s were not significantly influenced by their nest mates' initial weight. Si milarly, manipulation of the queens' relative weight by feeding and exposur e to contrasting social environment (queens kept alone or in groups) did no t significantly affect survival. These results indicate that head width dif ferences 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 li kely 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.