Reproductive regulation in an orchid bee: social context, fertility and chemical signalling

In social insects, the communication of social status helps individuals evaluate each other's reproductive potential, thus reducing conflict. Queens communicate their status through chemical signals, and the responses of workers to these signals include the suppression of ovarian activation. In most species of primitively social insects, dominant individuals indicate their status through aggressive behaviour, which also inhibits reproduction in workers. In some species, which lack queen-worker dimorphism, chemical signalling may act synergistically with agonistic interactions to establish the division of labour between females. Here, we investigated which mechanisms are involved in reproductive regulation in the orchid bee Euglossa melanotricha. Our long-term observations showed that dominant females monopolized egg laying and were able to recognize the eggs of subordinates. The overt aggression towards subordinates affected the egg-laying behaviour of these females but did not inhibit their ovarian development. We found that dominants maintained their monopoly on reproduction even after their experimental removal. When subordinates were removed, the productivity of the nest was reduced significantly, indicating clear benefits of the division of labour between females. We then analysed the chemical cuticular profile of the females and found that variation in the composition of hydrocarbons reflected the social status of the different individuals. The results of this study suggest that chemical signals evolved as honest signals and that workers restrain themselves from reproducing. This reduces reproductive options but increases selection pressures on the workers to obtain indirect fitness.