Colony-founding success of pleometrosis in a fungus-growing termite Odontotermes formosanus

Abstract

Cooperative colony foundation (pleometrosis) has evolved in independent groups of social insects and enhances the growth rate and survival of a colony, but it is associated with a cost because intracolonial aggression may be higher in pleometrotic than in haplometrotic colonies. We studied 647 colony-founding groups consisting of one to four pairs of reproductives of Odontotermes formosanus (Shiraki) in our laboratory to examine the benefits and costs of pleometrosis by analyzing its effects on nest excavation, degree of aggression, mortality, and colony growth. The results indicate that pleometrosis significantly reduced the time required for searching for a nesting site and increased the tunnel depth. Furthermore, the nest-excavation success rate was increased 35–50% in pleometrotic colonies. Pleometrosis also increased the fecundity and colony-survival rates. However, it increased individual mortality, probably because the reduction in the available space per individual intensified intracolonial aggression, particularly when more than three pairs of reproductives founded the colonies. Overall, the laboratory bioassays revealed that although pleometrosis benefited the colony-founding success of small groups (two pairs), it was costly for large groups (three pairs). The results are consistent with those of a field survey of O. formosanus, which showed that small pleometrotic groups (three or four reproductives) exhibit longevity, whereas large groups (more than four reproductives) are rare in the colony-founding stage. Hence, we conclude that the colony-founding success of pleometrosis depends on group size: pleometrosis benefits a small group but is costly for a large group. The evolution of cooperative colony foundation (pleometrosis) in social insects is believed to have been favored because of its beneficial effects on the survival and growth of colonies. In this study, we demonstrated the costs and benefits of pleometrosis in colonies of O. formosanus, a fungus-growing termite. Pleometrosis benefits survival in the following three ways: improving performance in nest excavation, preventing colony collapse following the death of a single reproductive, and increasing the growth rate. However, colony-founding success depends on group size. Pleometrosis benefits small groups (two reproductive pairs) more than it does large groups (three reproductive pairs). We hypothesize that pleometrosis in termites evolved after eusociality was achieved because it represents a trait that is common in fungus-growing termites.