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Behavioral Circumvention of Plant Toxins

Pikas are a model system that provides a unique advantage for studying foraging behavior of herbivores -- they simultaneously select two disparate diets. One is consumed immediately in the summer, while the other diet is collected and stored for winter consumption.

My research demonstrated that the two diets did not differ nutritionally but did differ significantly in the concentration of a class of defensive chemicals, phenolics. The stored diet was three times higher in total phenolics and condensed tannins and six times more astringent than the summer diet. The high phenolic concentration in the stored diet was attributed to the abundance of a single plant species Acomastylis rossii, which was uncommon in the summer diet.

In laboratory digestion trials, I showed that this plant was toxic to pikas and reduced fiber digestion as well as protein digestion. Moreover, in laboratory tests, pikas preferred A. rossii individuals with artificially reduced phenolic concentrations. Why do pikas collect excessive quantities of A. rossii, a toxic plant, for their stored diet? Extensive field observations and experimental manipulations showed that pikas have the ability to behaviorally manipulate food quality through storage.

In field experiments, phenolics in the stored diet reached palatable levels by midwinter. Furthermore, the stored diet preserved significantly better than the summer diet; i.e., less biomass was lost to decomposers. Bioassays of plant extracts on lawns of soil bacteria demonstrated that only A. rossii had antibacterial qualities, thus A. rossii is probably responsible for the preservability of the winter diet. Thus, the strategy of storing high-phenolic plants allows pikas to exploit an abundant, but otherwise unusable food resource, toxic plants. Moreover, it does not require complex physiological machinery to process plant toxins.