Dynamics of Reproductive Allocation from Juvenile and Adult Feeding: Radiotracer Studies

Nutrients used in reproduction may come from adult feeding or reserves stored from the juvenile stage. The dynamics of allocation from these sources are predicted to differ among nutrient types, depending on the relative availability of each nutrient type from adult and juvenile feeding. Using radiotracer techniques, I examined reproductive allocation of glucose and amino acids from adult and juvenile sources in two nymphalid butterflies, Euphydryas editha and Speyeria mormonia. The species used were intermediate in expected importance of adult nutrients to egg production, with abundant carbohydrates but few nitrogenous compounds available from the adult diet. As predicted, for compounds abundantly available in the adult diet, incoming nutrients were used in preference to stored nutrients. For compounds present in low amounts in the adult diet, juvenile reserves were used throughout adult life, although adult sources were used if available. Nutrients received by the female from the male at mating, although expected to be treated as stored reserves, were immediately used in egg production. Thus, restriction of adult or juvenile feeding may cause different nutrient types (e.g., carbohydrates, nitrogenous compounds) to become limiting to reproduction. These results are consistent with earlier allocation studies examining age-specific changes in body mass and reproductive effort, and the effects on fecundity of quantitative adult food reduction. The work has implications for understanding the evolution of nutrient types donated by males to females, the effects of a holometabolous lifestyle on age-specific fecundity, and the effects of using stored reserves vs. income in reproduction. The present results allow further predictions concerning effects of food supply perturbation on fecundity and, hence, population dynamics, and suggest ways in which species and individuals will differ in sensitivity to those perturbations.