Researchers have found that pheromones play a key role in reproduction and social status in the buff-tailed bumblebee colony. Specifically, sterile female workers seem to advertise their infertility with extra pheromones in an attempt to ward off harassment from competing bees.
The queen buff-tailed bumblebee (Bombus terrestris) lives for one cycle between winters, and in that time, the colony goes through two distinct social phases centering on her reproduction. First, in the pre-competition phase, the queen collects nectar and lays infertile female worker eggs to help her tend to the brood. At this time, reproduction in the colony is exclusive to the queen. In the second, competition phase, the queen begins laying male and female worker eggs which have the ability to reproduce. Mature female workers compete with the queen and among each other for reproduction.
According to Etya Asalem from Tel Aviv University and colleagues, it is during the competition phase that female workers develop a social status based on the reproductive qualities. According to the study, of all the female workers in the colony, approximately 68% developed ovaries and 37% actually laid eggs. And thanks to high egg-laying rates of the queen and her ability to destroy competing worker eggs, 95% of the hatched male offspring in the colony were descendents of the queen.
The scientists found, however, that there was another factor at play. During the competition phase, some of the female workers which actually had ovaries secreted pheromones called octyl esters, which inhibited oviposition. In other words, these pheromones only occurred in non-competing female workers with ovaries, and these bees were unable to oviposit, or lay eggs, while the octyl esters were in their system, say the authors.
In addition, there was a close relationship between social isolation and octyl esters secretion. That is, the female workers which were secreting octyl esters were also socially isolated from the group, the authors say, as if to advertise to the queen and other competing females that they were not a threat. Asalem and colleagues suggest that this sterility-specific secretion enables some of the female workers to care for the colony and the brood, in place of reproduction, to allow the colony to continue functioning amidst the reproductive free-for-all of the competition phase.
As Asalem explained in the report published this week in the Proceedings of the Royal Society B, “Advertising sterility … may protect the non-reproductive workers from attack and harassment, and thus enhance social harmony by maintaining some division of tasks in the seemingly chaotic [competition phase].”
Amsalem, E., Twele, R., Francke, W., & Hefetz, A. (2009). Reproductive competition in the bumble-bee Bombus terrestris: do workers advertise sterility? Proceedings of the Royal Society B: Biological Sciences, -1 (-1), -1–1 DOI: 10.1098/rspb.2008.1688