Making the right choice: how Crematogaster scutellaris queens choose to co-found in relation to nest availability

One of the main tasks a freshly mated ant queen has to face is to find a safe and suitable nest site to start a new colony. Colony foundation by associated queens, also known as pleometrosis, has been described for several ant species and, under specific selective pressures, represents an alternative to independent colony foundation. Despite most newly mated queens of the common Mediterranean acrobat ant Crematogaster scutellaris generally adopting independent colony foundation inside tree trunks, both field and laboratory studies have demonstrated that the formation of pleometrotic groups may occur, particularly inside lignified aphid galls on poplar or oak trees. These associations typically end with the survival of only one queen after the foundation phase, and the benefits they may provide remain unclear. In this study, we investigated how queen density and different nest availability may promote the formation of pleometrotic associations in C. scutellaris. We found that occupied nests are not actively sought after by queens, as hypothesised in previous studies, but might be accepted when they are the only safe refugia available. Moreover, the tendency to form groups increases as queen density increases, and nest availability is a limiting factor. Finally, we found no evidence that the size of the queen affects whether to join an already occupied nest.