Myrmecochorous plants use chemical mimicry to cheat seed-dispersing ants

P>1. Deceptive species are a characteristic trait of insect-plant mutualisms. Deception of pollinators is essential for reproduction in diverse plant species; however, mimicry in seed dispersal is almost unknown. Myrmecochory, the dispersal of diaspores by ants, is a mutualism in which the elaiosome, a lipid-rich seed appendage, works as an ant reward. This coevolved interaction may be exploited by cheating, non-rewarding diaspores that mimic elaiosomes. 2. Our study assesses the impact of the morphology and fatty acid composition of diaspores on ant dispersal in rewarding and cheating myrmecochores and non-myrmecochorous species. Four elaiosome types (defined by Sernander 1906) were included in our research: the 'Viola odorata type', with a clearly defined elaiosome, the similar but diplochorous 'Euphorbia type' with an additional ballistic dispersal of diaspores, and the 'Puschkinia' and 'Hepatica' types, which we classify as 'cheaters' as they mimic ant rewards by chemical cues. 3. We tested the influence of morphology, chemical composition of diaspores and presence of rewards on removal rates of diaspores, dummies impregnated with seed extracts and pure fatty acids and scent-standardized diaspores in a population of Formica polyctena ants with 10 European spring ephemerals, mostly Ranunculaceae. The main free fatty acids of diaspores were identified by gas chromatography and mass spectrometric analysis. 4. Removal rates of diaspores and dummies treated with seed extracts were significantly influenced by their fatty acid composition. Discriminant function analysis confirmed oleic acid as the main trigger substance eliciting seed-removal behaviour by ants in most species. Oleic acid content and removal rates differed among elaiosome types and were highest in rewarding myrmecochores, lower in cheaters and smallest in non-myrmecochorous seeds. Removal rates of scent-standardized diaspores did not differ among mutualists and cheaters suggesting a selective pressure for morphological adaptation and were lowest in non-myrmecochorous species, thus corroborating an influence of diaspore morphology on ant-removal patterns. 5. For the first time, we demonstrate chemical mimicry in myrmecochory as a means of seed dispersal. Evolutionary stability of the ant-seed mutualism is maintained by the ability of ants to discriminate between certain types of rewards and to respond with differential diaspore distribution rates.