In many social parasite ants, the worker caste is lost and host workers care for the parasite brood so that the sex ratio in reproduction is unable to be influenced by worker control. Instead, the queen is expected to bias the sex ratio to maximize her own inclusive fitness. Effects of factors other than worker control on sex ratio also can be evaluated. In this study, we examined primary and secondary sex ratios in the inquiline parasitic ant Vollenhovia nipponica. The following three hypotheses were proposed and tested in particular: (1) the primary sex ratio is female-biased, being determined by local mate competition among males, (2) the proportion of female eggs is higher in the early period of oviposition because females eclosed earlier are predicted to obtain higher mating and colony-founding success, and (3) the secondary sex ratio is not different from the primary sex ratio because parasite broods are not controlled by host workers. In 18 colonies, the sex ratio of eggs produced by V. nipponica queens was observed by counting cell chromosome numbers of eggs. Furthermore, new females and males in the colonies were collected to evaluate the secondary sex ratio. Although the primary sex ratio was female-biased over the observation period, female egg proportion was not different between the early and late periods. Moreover, no difference was observed between the primary and secondary sex ratios, indicating that the broods were not influenced. The female-biased sex reproductive ratio was determined by queens that biased the primary sex ratio.
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