Gender-biased nectar targets different behavioural traits of flower visitors

Floral nectar is a chemically complex aqueous solution within which several secondary metabolites have been identified that affect attractiveness for pollinators. Understanding preferences and aversions to nectar quality in flower visitors is crucial since this may influence the patterns of insect floral visitation with consequences on the plant fitness. We hypothesise that nectar chemical variation through different floral sexual phases may affect the number of insect visits that each phase receives. The study was realized on a population of Echium vulgare L. growing in a natural area close to Bologna. Nectar was collected from functionally male and female flowers to investigate its chemical composition through the HPLC technique. A total of 200 min of behavioural observations on foraging insects was also carried out. Variation in nectar traits has been detected for the amino acid spectrum. The proportion of protein amino acids appeared to be significantly higher in male-phase flowers. This may explain the significantly higher number of visits on male flowers than expected observed for all bee taxa (except Hoplitis adunca females). Functionally male flowers presented higher concentrations of phenylalanine, whilst proline was highly represented in functionally female flowers. Since a recent study demonstrated that hymenopterans can oxidize proline at a high rate for ATP production, we can hypothesise that the quality of nectar offered by the two sexually distinct floral phases targets different insect behavioural traits and likely ensures an optimal pattern of visit among flower sexes, which are unequally distributed within and among individuals in the population.