Plant species with larger extrafloral nectaries produce better quality nectar when needed and interact with the best ant partners.

Few studies have explored the phenotypic plasticity of nectar production on plant attractiveness to ants. Here, we investigate the role of EFN size on the productivity of extrafloral nectar in three sympatric legume species. We hypothesised that plant species with larger EFNs i) have higher induced nectar secretion after herbivory events, and ii) are more likely to interact with more protective, i.e., dominant, ant partners. We target 90 plants of three Chamaecrista species in the field. We estimated EFN size and conducted field experiments to evaluate any differences in nectar traits before and after leaf damage to investigate the phenotypic plasticity of nectar production across species. We censused ant species feeding on EFNs over time. Plant species increased nectar descriptors after leaf damage, but in different ways. Supporting our hypothesis, C. duckeana, with the largest EFN size, increased all nectar descriptors, with most intense post-herbivory induced response, taking its place as the most attractive to ants, including dominant species. EFN size variation was an excellent indicator of nectar productivity across species. The higher control over reward production in plants with larger-sized EFNs reflects an induction mechanism under damage that reduces costs and increases the potential benefits of indirect biotic defences.