Are all butterflies equal? Population-wise proboscis length variation predicts flower choice in a butterfly

Abstract

Intraspecific morphological variation fundamentally influences individual resource exploitation. In plant–pollinator systems, variation in floral morphologies and pollinator mouthparts may affect pollinators' resource use. This relationship has frequently been studied across species, but hardly ever at the intraspecific level in natural circumstances. We studied flower visits of clouded Apollo butterflies, Parnassius mnemosyne . (1) We investigated whether proboscis (mouthpart) length variability influenced individual nectar plant choice within a single population. We hypothesized that flower depths would constrain butterflies’ flower visits via their proboscis lengths. (2) We studied whether individual proboscis length constrained feeding on the sticky catchfly, Silene viscaria , a species with ample nectar and the deepest corolla among the plants visited. We hypothesized that individuals observed visiting S. viscaria had longer proboscides than those not observed on this nectar source. We captured clouded Apollos, then measured proboscis length. We surveyed the population daily, identified marked individuals and recorded feeding on nectar plant species. We compared proboscis length to the flower depth of the six most-visited nectar plants and investigated whether individual visits on flowering plants were related to proboscis length. We found large intrapopulation variation in proboscis length, and high intra- and interspecific variation in flower depth of the six nectar plants. Flower depth of S. viscaria largely overlapped with proboscis length, while the other five plants had shorter flowers. Individuals with longer proboscides visited S. viscaria flowers more often than those with short proboscides. These results indicate the importance of morphological variation in the interaction between plants and pollinators. We provide the first evidence that individual variation in mouthpart length affects lepidopteran foraging in natural circumstances. We suggest that interactions between species in plant–pollinator systems are partially based on individually different continuous traits, rather than on well-defined discrete traits of different taxa as implied by the pollination syndrome hypothesis. • Clouded Apollo butterflies' mouthpart lengths show high population-wise variability. • Similar variability is found in the visited nectar sources' corolla depth. • Butterflies with long proboscides visit nectar plant with deepest flowers more often. • Intraspecific variation is important to understand plant-pollinator interactions. • Intraspecific variation's impact does not support pollination syndrome hypothesis.