Evidence that workers recognize unfertilized queen-laid eggs for male production in stingless bees

The behavioural variation in bee workers raises questions about the reproduction in eusocial bees, as well as possible triggers for ovarian activation and oviposition. However, there are no data on possible differences between queen-laid eggs found alone or together with worker-laid eggs in a brood cell. This study evaluated the ploidy of eggs from queens found together with worker-laid eggs in the brood cells of the stingless bee Scaptotrigona xanthotricha. In the brood cells with two eggs, there was a difference in size between them, with the smaller one being a similar size to those found in brood cells with only one egg. The frequency of diploid eggs in brood cells with one egg was 96.6%, whereas, in those with two or more, it was 39.3%. These results suggest that egg fertilization can be identified by workers, because when there is a worker-laid egg, those from the queen are often haploid, supporting a prediction from Hamilton's theory of kin selection.