Fitness costs for fig wasps that fail to pollinate their host Ficus perforata

Abstract

Mutualisms are of fundamental ecological importance, but risk breaking down if one partner stops paying the costs yet still takes the benefits of the interaction. To prevent such cheating, many mutualisms have mechanisms that lower the fitness of uncooperative symbionts, often termed host sanctions. In mutualisms where the interacting partners are species-specific, we would expect to see coevolution of the levels of host sanctions and partner cooperation across species-pairs. In the mutualism between fig trees and their species-specific pollinating fig wasps, host sanctions vary greatly in strength, and wasp cooperation levels vary accordingly. Here I show experimentally that in Panamanian Ficus perforata (section Urostigma, Americana) there are fitness costs for wasps that do not pollinate. These fitness costs are caused by a combination of abortions of unpollinated figs and reduced proportion of wasp larvae that successfully develop to adults. The relative fitness of wasps that do not pollinate compared to wasps that pollinate is 0.59, leading to the intermediate sanction strength 0.41. Next, by screening pollinators of F. perforata I found that 1.9% of wasp individuals in natural populations failed to carry pollen. Across five actively pollinated Neotropical fig species and their pollinators, fig species with stronger host sanctions had fewer uncooperative wasps, as would be expected if sanctions promote cooperation.