Social insects provide ideal systems for investigating how kinship and ecological factors affect cooperation and conflict. In many ant species, unrelated queens cooperate to initiate new colonies. However, fights between queens break out after the eclosion of the first workers, leading to the death of all but one queen. Queens within associations potentially face a trade-off. On one hand, a queen should restrain her investment in brood production and care if this helps her to maintain fighting ability. On the other hand, a queen may benefit by increasing her contribution to brood production if having more daughter workers than her cofoundresses enhances her chances of taking over the colony. Increased investment is also beneficial because a large brood enhances colony survival. Using microsatellites, we determined the maternity of workers (adults and larvae) at the time of queen execution in the fire ant, Solenopsis invicta. Differential mass loss by initially equal nestmates affected survival, with the queen losing less body mass being more likely to survive. Surprisingly, the queen which lost less body mass, that is the one which provided the lowest energy investment, was the one which achieved higher maternity. A control experiment indicated that interactions among queens are responsible for this differential partitioning of reproductive and investment tasks between nestmates. The finding that the queen most likely to win the fights is the one with above-average maternity may explain why workers apparently do not attempt to influence the outcome of fights.