Group or solitary dispersal: worker presence and number favour the success of colony foundation in ants
Dispersal strategies are highly variable. Any strategy is associated to costs and benefits, and understanding which factors favour or disfavour a strategy is a key issue in ecology and evolution. Ants exhibit different dispersal and colony foundation strategies. Some species have winged queens that disperse solitarily and far by flight, and that found new colonies alone. Others have apterous queens that disperse with workers over short walking distances, and found new colonies as a group (colony fission). The putative benefits conferred by workers have been little studied and quantified, because comparing the costs and benefits of solitary vs. group dispersal and foundation is difficult when comparing different species. We did this using the ant Myrmecina graminicola, one of the few species that use both strategies. Young queens were mated and allowed to found new colonies in the laboratory, with either zero, two or four workers. We monitored the survival and growth of foundations over one year. The presence of workers increased both survival and growth, with more workers yielding higher growth. These results show the benefit of dispersing and founding in a group. The presence of few workers (as little as two workers) was sufficient to provide benefits, suggesting group foundation does not require a dramatic decrease in the number of propagules produced in M. graminicola. Our results support the hypothesis that the two strategies coexist along a competition-colonization trade-off, where solitary foundation offers a colonization advantage while group foundation has a competitive advantage.