Adaptive colony sex ratios in primitively eusocial bees.

Colony-level predictions about sex-ratio optimization in eusocial Hymenoptera are different from the ones that apply to the population level. A recent empirical study on the sweat bee Halictus rubicundus has revealed a distinct pattern in the colony sex ratio of the summer brood: eusocial colonies produced more female-biased sex ratios and non-eusocial colonies produce more male-biased sex ratios. These data are consistent with theoretical hypotheses as put forward by Trivers and Hare and several later authors. When interpreted in the light of these theoretical contentions, the sex-ratio variation in Halictus rubicundus appears to be adaptive for workers, replacement queens and - under reasonable additional assumptions - also for foundresses.