FEMALE-BIASED SEX-RATIOS IN A FACULTATIVELY SOCIAL BEE AND THEIR IMPLICATIONS FOR SOCIAL EVOLUTION

Montane populations of the Australian allodapine bee, Exoneura bicolor , are characterized by high levels of cooperative nesting and strongly female-biased sex ratios. A conspecific population from heathland sho ws much lower levels of cooperative nesting and lower levels of female bias. In both habitats, sex-ratio bias is greatest in the smallest br ood sizes and becomes successively less biased in larger broods. Parit y is approached in the largest heathland colonies, but not for any bro od-size category in montane areas. Adult intracolony relatedness is mo derately high for colonies in both reused and newly founded nests in t he montane habitat, but probably low or zero for newly founded nests i n heathland. Colony efficiency, measured as the number of brood per ad ult, increases with colony size in both habitats, suggesting that coop eration between females increases mean female fitness. It is argued th at patterns of sex allocation are consistent with nonlinear fitness-re turn models, in which the mean reproductive value of daughters increas es with the number of daughters produced in a brood. Such increases pr obably arise from a number of social interactions, including cooperati ve brood defense, increased task efficiency, and lower per capita cost s in nest construction. The term ''local fitness enhancement'' is intr oduced here to describe these effects collectively. The female-biased ratios should lower selective thresholds for sib-directed altruism, at least in the earlier stages of colony development. It is argued that local fitness enhancement facilitates eusociality in allodapine bees a nd could also play a role in other haplodiploid taxa, provided coopera tive nesting largely involves sisters, colony efficiency increases wit h colony size, and optimal colony sizes are only achieved after two or more generations after founding.