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Butterfly-pollinated flowers offer nectar with higher amino acid concentrations than most flowers pollinated by other animals, and female butterflies of some species prefer to consume amino acid-rich nectar. However, for over 30 years, there has been an ongoing discussion about whether nectar amino acids benefit butterfly fitness. A clear positive effect was only shown for the nectar-feeding Araschnia levana, and females of the fruit-feeding Bicyclus anynana also increased offspring quality when they were fed amino acids as adults. Thus, severe doubts remain about the general significance of these single positive results. We therefore tested a further species from a phylogenetically different butterfly subfamily, the small heath (Coenonympha pamphilus L., Satyrinae), taking into account feeding conditions over the whole life cycle of this species. C. pamphilus females receiving nectar amino acids as adults, irrespective of larval food quality, produced heavier larvae and also increased the hatching success of their eggs over the oviposition period. Furthermore, females raised under nitrogen-poor larval conditions tended to use nectar amino acids to increase the number of eggs laid. Thus, C. pamphilus females used nectar amino acids primarily to increase their offspring quality, and secondly tended to increase offspring quantity, if larval resources were scarce, showing a resource allocation pattern differing from both B. anynana and A. levana. Our study supports the old postulate that nectar amino acids generally enhance butterfly fitness.
