RESUMO
Lespedeza cuneata contains high levels of phenolics and is a common food plant of the meadow vole (Microtus pennsylvanicus);Penstemon digitalis contains substantial quantities of alkaloids and is a common food plant of meadow voles and prairie voles (M. ochrogaster). We investigated the palatability of these plants and the effects of their secondary compounds on the digestion efficiencies and growth of both species of voles. Voles ate very little of either plant when alternative food was present. Phenolics and alkaloids were extracted from the plants, incorporated into separate artificial diets, and fed to weanling voles for three weeks.Lespedeza phenolics reduced the growth of meadow voles but not prairie voles throughout the feeding trial. These compounds disrupted digestion, reducing protein digestibility by more than half.Penstemon alkaloids lowered only the initial growth rates of prairie voles and had no significant effect on meadow voles. Prairie voles tended to increase food consumption rates on both Lespedeza and Penstemon diets. This response offset some of the decrease in digested protein intake in the first case, and offset the increased metabolic cost of processing the diet in the latter case. Our results indicate that although Lespedeza phenolics and Penstemon alkaloids do influence consumption and digestibility of artificial diets, they do not greatly reduce the performance (i.e., growth or survival) of voles when consumed at levels generally observed for wild voles. However, these compounds probably do place an upper limit on the amount of fresh Lespedeza and Penstemon that can be consumed and contribute to the generalist feeding strategy of voles.
RESUMO
Very few studies have shown experimentally that plant chemical defenses actually reduce the performance of individual mammalian herbivores, much less the density of mammalian populations. We investigated the effects of representatives of three classes of plant phenoiics on the survival and growth of prairie voles by incorporating the compounds into artificial diets and feeding them to weanlings for three weeks. At low levels of protein, both quercetin (a flavonoid) and tannic acid (a hydrolyzable tannin) caused reduced growth rates; no effect occurred at high levels of protein. Quebracho (a condensed tannin) inhibited feeding and thus was lethal at all levels of protein. These results indicate that plant phenolics are likely to influence the performance and dynamics of natural populations of microtine rodents by reducing the quality of available forage. The hypothesis that the primary mode of action of the phenoiics is the reduction of digestibility of protein was not supported. The reduced growth caused by both quercetin and tannic acid could be attributed primarily to their toxicity. The effect of quebracho resulted from reduced intake (unpalatability).
