We analysed the social and reproductive behaviours of two Polistes species, the usually monogynousP. gallicusand the potentially polygynousP. dominulus, in laboratory cages. We induced digynic associations in both species in order to investigate the proximal factors responsible for the differences in reproductive strategies and to determine which traits were involved in the change-over from the one type of nest foundation to the other in the course of evolution. In both species, an encounter between two reproductive females led to a fight ending in either the flight or the submission of one of the wasps and then, in the latter case, in the establishment of a two-females-founding nest. Encounters on a well developed nest led to violent fighting inP. gallicus, sometimes ending in death or mutilation, while joining a nest initiated by another female was possible inP. dominulus. The relationships between associated foundresses were clearly of the dominance/ subordination type in both species. Some behavioural differentiation accompanied the hierarchical organization. However, subordinate wasps ofP. gallicuswere able to perform both dominance behaviour and oophagy, which usually occur in the dominant individuals. This refusal of the subordinate role sometimes led to the decline of the brood, and of the colony itself. When placed in a multi-foundress situation, a monogynous species such asP. gallicusshows behaviour which is intermediate between strictly territoriality and a truly polygynous behaviour. In the normal conditions of the species life, this intermediate behaviour cannot be said to be an evolutionarily stable strategy because it is accompanied by mutual, lasting differential oophagy that results in a decrease in the production of descendants.
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