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Nutritional Costs of a Plant Secondary Metabolite Explain Selective Foraging by Ruffed Grouse
Christopher G. Guglielmo, William H. Karasov and Walter J. Jakubas
Vol. 77, No. 4 (Jun., 1996), pp. 1103-1115
Stable URL: http://www.jstor.org/stable/2265579
Page Count: 13
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Plant secondary metabolites (PSMs) are commonly thought to deter vertebrate herbivores by being toxic or by reducing nutrient assimilation. An alternative, complementary hypothesis is that PSMs may influence herbivore forage selection at subtoxic levels by imposing high detoxication costs post absorption. Many studies of insect herbivores have been undertaken to measure the metabolic load of detoxication as it relates to host-plant specialization, but results have been equivocal and the subject of much debate. Some recent studies of vertebrate herbivores indicate that metabolism of PSMs can impose a cost by increasing nutrient losses due to conjugation of PSMs to endogenous materials, and by upsetting pH homeostasis. In this study, we demonstrate that detoxication costs in Ruffed Grouse are substantial, and are reduced by selective foraging. In winter, Ruffed Grouse feed preferentially on quaking aspen with relatively low levels of coniferyl benzoate (CB) in staminate flower buds. We collected aspen buds with low-and high-CB levels and conducted feeding trials with captive Ruffed Grouse that had been acclimated to an aspen bud diet. We measured nutrient utilization efficiencies and excretion of detoxication conjugates. Grouse assimilated 24% less energy from high- vs. low-CB buds. Using a nutritional model, we determined that the reduction of energy utilization efficiency was mainly due to dilution of the diet by CB, and not by digestive inhibition. As CB intake increased, grouse excreted more glucuronic acid and ornithine (two major detoxication conjugates), resulting in an energetic cost of 10% to 14% of metabolizable energy intake for low- and high-CB buds, respectively. Conjugation with the amino acid ornithine increased minimum nitrogen requirement by 68% to 90% for low- and high-CB buds, respectively. Ammonium excretion also increased with CB intake, indicating an upset of pH homeostasis. Thus, detoxication costs were relatively high and increased with higher CB intake. Ruffed Grouse preference for low-CB, high-protein aspen buds in nature appears to be related to lower utilization efficiency and higher detoxication costs associated with high CB concentrations. The importance of detoxication cost to herbivores must be more thoroughly evaluated and integrated into existing models of herbivore foraging behvior.
Ecology © 1996 Wiley