Do Peacock butterflies (Inachis io L.) detect and prefer nectar amino acids and other nitrogenous compounds?

The preferences for nectar amino acids, urea and ammonium ions of peacock butterflies, Inachis io, were tested experimentally. Females clearly preferred a mimic of Lantana camara nectar containing amino acids to an otherwise similar plain sugar solution, whereas males did not discriminate between these test solutions. Neither males nor females discriminated between the full mixture of amino acids in a mimic of L. camara nectar and similar test solutions containing only the single amino acids arginine or proline. Furthermore, the butterflies were not able to detect methionine in the test solutions. Both sexes detected and preferred ammonium ions in test solutions but showed no response to urea. These results support the hypothesis that butterflies can select for high amino acid concentrations in floral nectar. However, it seems unlikely that they select for particular amino acids. The rather unspecific response of I. io males to the nectar constituents tested may result from their relatively low demand for nitrogen for spermatophore and sperm production, while their high activity may make energy supply (i.e. sugar) more important. The preference for ammonium ions suggests that I. io could also acquire nitrogen from ammonium-contaminated soil by puddling, as has been shown for sodium in swallowtail butterflies.