Allocation to reproduction in a hawkmoth: A quantitative analysis using stable carbon isotopes

There is great interest in the importance of nectar nutrients to fecundity in the Lepidoptera, but nutrient allocation has been difficult to measure q uantitatively. Here we trace the allocation of nectar nutrients in the hawk moth Amphion floridensis using naturally occurring variation in plant stabl e carbon isotopes and thereby derive a descriptive model of carbon flow int o eggs. Because C-13 content (expressed as delta C-13, the C-13:C-12 ratio relative to a standard) depends on photosynthetic mode, moths were fed sucr ose solution made with either either C-3 or C-4 sugar (beet or cane), both of which were distinct from larval host plant. In addition, two of four exp erimental diets contained an amino acid supplement distinct in delta C-13 f rom either sugar or larval host plant. Females were hand fed daily from exp erimental diets, and their eggs were collected and analyzed for delta C-13. Egg delta C-13 increased rapidly from a value resembling larval delta C-13 , and followed an asymptotic pattern of carbon incorporation. The presence of amino acids in the diet had no effect on either fecundity or egg delta C -13. Because egg delta C-13 equilibrated at a value lower than delta C-13 d iet, we invoke an allocation model in which carbon is contributed to eggs b y two separate pools. One pool of carbon comes into isotopic equilibrium wi th adult diet, whereas the other does not, contributing carbon with an excl usively larval signature across a female's lifetime. Carbon fractional turn over rate and the relative contribution of the two pools were estimated by fitting the model to the data with nonlinear regression. The resulting mode l fitted the data well and indicated that 50-60% of egg carbon is derived f rom 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 tu rnover can generate patterns of reproductive allocation.