Flower Selection and the Dynamics of Lipid Reserve in Two Nectarivorous Butterflies

Abstract

This study examines flower choice by two syntopic butterfly species, Agraulis vanillae (Nymphalidae) and Phoebis sennae (Pieridae), and the energy consequences of foraging selectivity. Foraging selectivity is influenced by proboscis of P. sennae is °~1 cm longer than that of A. vanillae, it is able to forage at long—corolla flowers (three Ipomoea spp.) inaccessible to A. vanillae. Relative to other nectar sources exploited by the two species, these Impomoea species are highly rewarding energetically. Phoebis sennae increases its energy intake relative to A. vanillae by exercising greater discrimination between high— and low—reward nectar sources, avoiding several species with small nectar volumes that are avidly visited by A. vanillae. Consequently, the average foraging profit of P. sennae is nearly 4 times that by A. vanillae. These differences in foraging selectivity energy intake are closely related to energy storage and depletion rates, measured as age—related changes in gross lipid contents of adult butterflies. Adult activities in the less selective species, A. vanillae, are fueled to a greater degree with lipid reserves set aside at metamorphosis; A. vanillae emerge from pupae with 13—16% of dry mass allocated to lipids, and these lipid stores decline at a variable rate throughout adult life. By contrast, in P. sennae lipid accumulated adult feeding provides the largest proportion of energy stores. P. sennae emerge from the pupa with only 6% of the dry mass allocated to lipids, and accumulate lipid stores to levels as high as 40% of dry mass through most of adult life, depleting these lipid reserves only in late adulthood. Further, A. vanillae show significant sexual dimorphism in lipid content throughout life, with females containing greater quantities, while P. sennae show no such dimorphism. These results are discussed with respect to Boggs' model concerning the allocation of larval—derived resources to reproduction.