Sexual size dimorphism and sex ratios in bees and wasps

When parental investment is estimated from the body weight of offspring, the sex investment ratio increases with increasing sexual size dimorphism across species of bees and wasps. This relationship was recently found in ants and was hypothesized to be artifactual and occur because body weight overestimates parental investment in the larger sex (females) proportional to the degree they are larger than the smaller sex (males). Analyses using an apparently unbiased alternative to body weight in estimating parental investment in solitary wasps support this hypothesis. The analyses also show, however, that increase in the sex investment ratio with increasing sexual size dimorphism may not be entirely artifactual. In solitary wasps, it is hypothesized that actual covariance between sexual size dimorphism and the sex investment ratio will occur if mothers produce small males when those males compete primarily with brothers for access to mates.