Extrafloral nectaries (EFNs) allow plants to engage in mutualisms with ants preventing herbivory in exchange for food. EFNs occur scattered through the plant phylogeny and likely evolved independently from herbivore-created wounds subsequently visited by ants collecting leaked sap. Records of wound-feeding ants are, however, anecdotal. By surveying 38,000 trees from 40 species, we conduct the first quantitative ecological study of this overlooked behavior. Ant-wound interactions were widespread (0.5% of tree individuals) and occurred on 23 tree species. Interaction networks were opportunistic, closely resembling ant-EFNs networks. Fagaceae, a family lacking EFNs, were strongly overrepresented. For Fagaceae, ant occurrence at wounds correlated with species-level leaf damage, potentially indicating that wounds may attract mutualistic ants, supporting the hypothesis of ant-tended wounds as precursors of ant-EFNs mutualisms. Given the commonness of herbivore wounds, wound sap as steadily available food source might furthermore help to explain the overwhelming abundance of ants in (sub)tropical forest canopies.