Phased contests allow rapid hierarchy formation in paired bumble bee workers

A primary approach to establishing hierarchy is through contests, in which conflicts are settled using various behavioural patterns. Although dominance ranks in a hierarchy are determined by quality-related innate factors, how the inequality of competitors is assessed and triggers the formation of a hierarchy by means of behavioural interactions remains unclear. Here, we report the sequence of aggressive behaviours that mediates the onset of hierarchy formation in eusocial bumble bee workers by continuously monitoring pairwise contests. Sequential analysis characterized the dynamics of the behavioural repertoire during hierarchy formation in Bombus terrestris; this was organized in phases associated with a divergence in dominance rank and included mutual pumping (a dorsoventral abdominal movement), bilateral physical contact and unilateral strikes. Escalated bilateral aggression served as a mutual assessment strategy and caused a rapid divergence in rank in evenly matched contests. Head butting was subsequently performed by dominants, suggesting it is a badge of higher rank. The behavioural progression was also similar in two other Bombus species despite interspecific variation in behavioural patterns. Size asymmetry reduced the duration of bilateral aggression, while age advantages over callow workers eliminated bilateral aggression. These results suggested potential roles of mutual assessment by ritualized fighting in hierarchy formation in queenless bumble bee societies.