ALLOCATION TO REPRODUCTION IN A HAWKMOTH: A QUANTITATIVE ANALYSIS USING STABLE CARBON ISOTOPES

Abstract

There is great interest in the importance of nectar nutrients to fecundity in the Lepidoptera, but nutrient allocation has been difficult to measure quantitatively. Here we trace the allocation of nectar nutrients in the hawkmoth Amphion floridensis using naturally occurring variation in plant stable carbon isotopes and thereby derive a descriptive model of carbon flow into eggs. Because 13C content (expressed as δ13C, the 13C:12C ratio relative to a standard) depends on photosynthetic mode, moths were fed sucrose solution made with either either C3 or C4 sugar (beet or cane), both of which were distinct from larval host plant. In addition, two of four experimental diets contained an amino acid supplement distinct in δ13C from either sugar or larval host plant. Females were hand fed daily from experimental diets, and their eggs were collected and analyzed for δ13C. Egg δ13C increased rapidly from a value resembling larval δ13C, and followed an asymptotic pattern of carbon incorporation. The presence of amino acids in the diet had no effect on either fecundity or egg δ13C. Because egg δ13C equilibrated at a value lower than δ13C diet, we invoke an allocation model in which carbon is contributed to eggs by two separate pools. One pool of carbon comes into isotopic equilibrium with adult diet, whereas the other does not, contributing carbon with an exclusively larval signature across a female's lifetime. Carbon fractional turnover rate and the relative contribution of the two pools were estimated by fitting the model to the data with nonlinear regression. The resulting model fitted the data well and indicated that 50–60% of egg carbon is derived from adult nectar sugars after the “mixing pool” has come into equilibrium. Thus, this study demonstrates that adult nectar sugars provide an important source of egg carbon and explores how properties of nutrient mixing and turnover can generate patterns of reproductive allocation.