SEX-RATIOS, LOCAL FITNESS ENHANCEMENT AND EUSOCIALITY IN THE ALLODAPINE BEE EXONEURA-RICHARDSONI

Previous studies of a facultatively eusocial allodapine bee, Exoneura richardsoni Rayment, indicated that high levels of cooperative nesting among close relatives seem to be maintained by benefits that lead to increases in per capita brood production. These traits could lead to l ocal fitness enhancement, which in turn could select for female-biased sex ratios. We show here that sex investment ratios in this species a re female-biased in small colony sizes, becoming progressively male-bi ased in larger colonies, consistent with expectations for local fitnes s enhancement, but not explainable by alternative models. Our results support previous suggestions that local fitness enhancement can lead t o sex ratio bias in primitively social Hymenoptera, but differ from pr evious studies by suggesting that patterns of bias could lower selecti ve thresholds for sib-directed altruism in small colonies, but have an opposing effect in large colonies.