Aggression or ovarian development as determinants of reproductive dominance in Bombus terrestris: Interpretation using a simulation model

Insect societies generally manifest reproductive division of labor with one or a few fecund females (queens) and sterile helpers (workers). This is generally mediated by behavioural hierarchy that reduces the cost of within-group aggression. Aggressive behaviour and ovarian development are thus two co-occurring correlates of reproductive skew, but their mutual influence remains unclear. Here we examined the interplay between these factors by constructing a simulation model, based on the biology of Bombus terrestris, to test whether aggression levels are determined by the level of ovarian development or vice versa. We approached this question using virtual groups composed of either equally or unequally matched workers, as well as supplementing the simulation data by experimental data using actual bee groups. The simulation revealed that previous encounter experience (win/lose) rather than mutual assessment of reproductive status determines and maintains hierarchy in B. terrestris. Furthermore, although bumblebee workers largely vary in their aggression tendencies, pre-existing differences between nest-mates were not obligatory for establishment of the hierarchy. The initially formed linear hierarchy became polynomial after 10 days, clearly separating between egg-laying workers and helpers. This was consistent with the results obtained for actual bee groups. We propose that aggressive behaviour and the outcome of previous encounters are major determinants of ovarian development and not vice versa. Workers may gain a head start in the competition due to variation in their innate aggressive tendencies, or by fortuitously winning their first encounters. Small differences in aggressiveness between workers are amplified to initially form a short-term linear hierarchy that subsequently turns to polynomial hierarchy, creating the most basic phenomenon of social groups, reproductive division of labor.