Variation in social structure alters queen body mass and productivity in the desert seed-harvester ant Messor pergandei

Parent-offspring conflict theory predicts conflict between parents and their offspring over per-offspring resource investment. Across the range of the desert seed-harvester ant, Messor pergandei, daughter queens use three different social strategies during colony founding that are expected to alter the optimal level of parental investment. To test whether social strategy variation is associated with shifts in body mass, we surveyed queen live mass over 3 years at 25 sites that spanned the range of behavioral strategies and founding group sizes. To test whether reduction in parental investment into individual offspring negatively impacts their productivity, queens were individually isolated and allowed to produce a single worker cohort under common garden conditions. Queen live mass was highly variable, from 24 mg on average at the site with the lightest queens to 1.5 times that size, 37 mg, at the site with the heaviest queens. As predicted by parent-offspring conflict over investment, solitary colony founding sites contained the heaviest queens, followed by secondary monogyny. Polygynous queens were lightest, with a strong negative relationship between group size and live mass. Reductions in body mass had a negative effect on queen productivity across all queen social types; however, queens from sites where queen-queen aggression is typical were significantly more efficient at brood rearing, resulting in lower mass loss during founding per unit offspring biomass. This may represent an adaptation to queen competition to gain a strength advantage over potential rivals.