Oxidation and allocation of nectar amino acids during butterfly flight

Flying animals face extreme energetic demands, relying mainly on
carbohydrates and lipids, with occasional contributions from proteins and
amino acids. In nectar-feeding species such as butterflies and
hummingbirds, sugars are the primary fuel, yet the extent to which
nectar-derived amino acids support flight versus other functions remains
unclear. Using 13C-labelled nectar, we tracked the metabolic fate of
sugars and amino acids during flight in Pieris rapae butterflies. We found
that proline and glycine, two abundant nectar amino acids, were oxidized
alongside sugars. We also compared females subjected to low- versus
high-intensity flight. High flight intensity females incorporated less
glycine into tissues, implying greater diversion toward energy use during
flight. In contrast, they deposited more threonine – an essential amino
acid – into their abdomens, prioritizing reproduction and storage. These
findings reveal the role of nectar-derived nutrients in supporting
locomotion and reproduction, while showing how nectar use can modulate
trade-offs between flight and fecundity.