Eversible tentacle organs in caterpillar–ant communication: do they signal partner quality in polyommatinae lycaenid butterflies?

In mutualistic associations, lycaenid butterfly caterpillars trade nectar secretions against protective services by ants. Eversions of paired abdominal tentacle organs (TO) have been suggested to honestly signal nectar secretion capacity of caterpillars to their ant visitors. Using data from 1561 staged encounters between larvae of nine West Palaearctic Polyommatinae species and worker ants of three species, I show that within experimental series, significantly positive correlations between TO eversion and nectar secretion rate emerged only sporadically (12 of 50 series). In a meta-analysis integrating over all tested species combinations and experimental conditions, the relationship between nectar secretion and TO eversion rate was weak (r2 = 6.6%), though significantly positive. This association between myrmecophilous behaviours was not stronger in experiments with feeding mature larvae than in prepupal non-feeding larvae; however, the latter delivered distinctly more nectar. Relationships between nectar secretions and TO eversions were independent of the density of ant partners available and did not vary consistently between lycaenid species showing different levels of myrmecophily. When mean values of nectar secretions per experimental series were related to the respective mean TO eversion rates, a clear positive relationship only emerged among intimately ant-associated species. In moderate myrmecophiles average tentacle activity was unrelated to mean nectar delivery. Overall, these experiments yielded only weak support for the reliable-signalling hypothesis. I propose that TO eversions rather serve as complementary dimension of multimodal communication between partners. ‘Apparent honesty’ may then emerge if caterpillars achieve optimal ant attendance by concomitantly increasing nectar secretion and TO eversion rates.