| Description |
The metabolic load hypothesis states that herbivores will tend toward specialization to avoid the increased metabolic costs associated with detoxification of plant toxins; specialization allows the herbivore to evolve the oxidative machinery necessary to detoxify the chemical defenses of the hostplant it specializes on. Recent tests of this theory have been contradictory. My research was designed as a test of this theory, using the specialist herbivore Junonia coenia, the buckeye, and the generalist herbivore Vanessa cardui, the painted lady. Both species were fed artificial diets of one of three types: high concentration iridoid glycosides, low concentration iridoid glycosides, and a control diet with trace amounts of iridoid glycosides. Iridoid glycosides are toxins found in the Plantago lanceolata plant that Junonia coenia specialize on. The metabolic rates of the two species were then analyzed by calculating their nutritional indices and by measurements' of their carbon dioxide outputs. The results showed that the larvae of both species that were fed the high-concentration iridoid glycoside diet had significantly decreased relative growth rates. The standard metabolic rate, as measured by carbon dioxide output, failed to show any significant difference. These results could be explained either by an inability of the carbon dioxide-measuring apparatus to register significant differences in respiration or by an inadequacy of the metabolic load theory to accurately explain the specialization of certain herbivores. |
