Evolution of egg dumping in a subsocial insect.

Egg dumping, or abandonment of eggs and young to the care of other conspecifics, frees individuals from costs of maternal care while potentially imposing energetic and ecological costs on egg recipients. It is not clear, however, that egg dumping necessarily represents selfish manipulation of egg recipients, and in some ecological contexts, recipients may benefit from enlarged broods. Thus, egg dumping may either be mutually beneficial for dumpers and recipients or entail costs for dumpers that are compensated by other means, such as improving reproduction of genetically related egg recipients. Here I use field experiments to test the relative importance of manipulation (i.e., "parasitism"), mutualism, and kin selection in the evolution of egg dumping in the tingid lace bug Gargaphia solani. In support of mutualism and kin selection, I found that reproduction of egg recipient G. solani benefits from brood enlargement, most likely because eggs and gregarious nymphs find safety in greater numbers. But contrary to both parasitism and mutualism, egg dumper reproduction was not improved by offspring abandonment. Indeed, dumpers laid smaller clutches than recipients, and dumpers did not convert a survival advantage into greater future reproduction. Genetic analyses of a natural G. solani population revealed, however, that dumpers are related to their egg recipients. Moreover, Hamilton's rule showed that egg-dumping G. solani earn sufficient indirect genetic benefits for kin selection to favor the behavior. Thus, egg dumping in some species may be kin-selected cooperation rather than parasitism or mutualism.