To leave or to stay: direct fitness through natural nest foundation in a primitively eusocial wasp

Dispersing from the natal nest to found new nests is an avenue for gaining direct fitness for workers in some primitively eusocial insects, especially in species with a perennial nesting cycle where males are present throughout the year. Such nest foundation is difficult to study in nature or in small laboratory cages. Hence, we have investigated the dynamics of nest foundation by workers of the primitively eusocial wasp Ropalidia marginata inside closed walk-in cages so that we could locate and observe every event of nest foundation. Starting with nine parent nests we observed the dispersal of female workers that initiated 9 single-foundress and 20 multiple-foundress nests. Wasps congregated outside their parent nests and engaged in dominance-subordinate interactions before initiating multiple foundress nests. The most dominant wasps of such aggregations became queens, and among the others, some joined the new nests as cofoundresses to become workers while the others remained in the parent nests. Solitary foundresses never participated in such off-nest aggregations. Solitary foundresses and future queens of multiple foundress nests engaged in self-feeding behaviour outside their parent nests, a behaviour not performed by wasps that did not initiate new nests. Queens of new nests gained immediate direct fitness. Although the cofoundresses continued to gain only indirect fitness, they are expected to have a higher probability of gaining direct fitness in the future as compared to the corresponding probability in their much larger parent nests. These findings underscore the importance of direct fitness in the evolution of cooperation in primitively eusocial insects.