Professor Brock University, Department of Biological Sciences, Canada
Background/Question/Methods
Being dominant is important for increasing an individual’s fitness in social contexts. For many organisms, dominant individuals are the most aggressive, yet most interactions between individuals are characterised by tolerance. This is often explained as subordinates conceding to dominants without the need for costly aggression. Eastern carpenter bees (Xylocopa virginica) are facultatively social, meaning individuals live either solitarily or socially. Social nests display linear dominance hierarchies with high reproductive and ergonomic skew towards primary (dominant) females. Fighting inside nests before the onset of foraging activity suggests that aggression underlies the formation of dominance hierarchies. Primary and secondary females were categorized by their foraging behaviours and collected from nests. Primary females were the most active foragers, compared to secondaries that only foraged occasionally. Non-nestmate females were paired in circle tubes to observe their interactions. These dyads were categorized by the status of both females; two primary females (primary dyad), one primary and one secondary female (mixed dyad), or two secondary females (secondary dyad). Given that there are no pre-existing hierarchies between non-nestmates aggression is likely to still be an important factor for their interactions. As such primary dyads should display higher aggression and lower tolerance compared to mixed and secondary dyads.
Results/Conclusions
Aggressive and tolerant encounters were rare across dyads, accounting for 9.1 and 11.5% of all behaviours respectively. All dyad categories showed similar numbers of aggressive and tolerant behaviours, as well as similar latency times to first aggression and tolerance. While mixed and secondary dyads tended to be more active, instances of extreme aggression were concentrated in primary dyads. These findings suggest that while aggression may be important to establishing dominance it is not the only factor behind dominance hierarchies. Moreover, the similar degree of tolerance and aggression across all dyad categories supports that tolerance might be just as important to X. virginica as aggression in the formation of dominance hierarchies. Given that tests were performed on individuals without pre-existing dominance relationships this shows that escalating to extreme aggression is generally avoided by females. This might be explained by the high cost of nest construction, making nest sites a limiting resource for this species. As such tolerating and cooperating with rivals for access to nests is likely extremely important and results in aggressive interactions being avoided as a result.
