Fig trees are pollinated by fig wasps, which also oviposit in female flowers. The wasp larvae gall and eat developing seeds. Although fig trees benefit from allowing wasps to oviposit, because the wasp offspring disperse pollen, figs must prevent wasps from ovipositing in all flowers, or seed production would cease, and the mutualism would go extinct. In Ficus racemosa, we find that syconia (‘figs’) that have few foundresses (ovipositing wasps) are underexploited in the summer (few seeds, few galls, many empty ovules) and are overexploited in the winter (few seeds, many galls, few empty ovules). Conversely, syconia with many foundresses produce intermediate numbers of galls and seeds, regardless of season. We use experiments to explain these patterns, and thus, to explain how this mutualism is maintained. In the hot summer, wasps suffer short lifespans and therefore fail to oviposit in many flowers. In contrast, cooler temperatures in the winter permit longer wasp lifespans, which in turn allows most flowers to be exploited by the wasps. However, even in winter, only in syconia that happen to have few foundresses are most flowers turned into galls. In syconia with higher numbers of foundresses, interference competition reduces foundress lifespans, which reduces the proportion of flowers that are galled. We further show that syconia encourage the entry of multiple foundresses by delaying ostiole closure. Taken together, these factors allow fig trees to reduce galling in the wasp-benign winter and boost galling (and pollination) in the wasp-stressing summer. Interference competition has been shown to reduce virulence in pathogenic bacteria. Our results show that interference also maintains cooperation in a classic, cooperative symbiosis, thus linking theories of virulence and mutualism. More generally, our results reveal how frequency-dependent population regulation can occur in the fig-wasp mutualism, and how a host species can ‘set the rules of the game’ to ensure mutualistic behavior in its symbionts.

Summary 1.The mechanisms that prevent competition (conflict) between the recipient and co-operative actor in co-operative systems remain one of the greatest problems for evolutionary biology. Previous hypotheses suggest that self-restraint, dispersal or spatial constraints can prevent direct competition for local resources or any other common resources, thereby maintaining stable co-operation interactions. In this study, we use the obligate fig-fig-wasp Mayr) show that the number of viable seeds of figs is positively correlated with the number of pollinator offspring when the number of vacant female flowers is high while the foundress number is low (two foundresses). Meanwhile, they are negatively correlated when the number of vacant female flowers is low and the number of foundresses is increased manually (eight foundresses). The correlation coefficient between viable seeds and wasp offspring (galls) depends on vacant female flower availability. 3. Our data suggest that the interaction between figs and fig wasps is conditional, and that they co-operate when local resource availability is plentiful but are in conflict when local resource availability is limited. The self-restraint, dispersal and spatial heterogeneity previously hypothesized in maintaining stable co-operation cannot sufficiently prevent the symbionts from utilizing more local resources at the expense of the recipients. The conflict, which can disrupt the co-operation interaction, exists after the local resource is saturated with symbionts. The repression of symbiont increase, therefore repressing the utilization of local resources in the conflict period, is crucial in the maintenance and evolution of co-operation.

Researchers test whether and how onymity, as opposed to anonymity, promotes cooperation in a social dilemma experiment.