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Plant-herbivore interactions play a significant role in the structure and functioning of ecosystems. Co-evolutionary theory suggests that plant defenses evolved due to herbivores and herbivore pressure can shape the genetic composition of their food resources. We used interactions between North American porcupines (Erethizon dorsatum) and trembling aspens ( Populus tremuloides) as a system to investigate this theory's important assumption that herbivores select food sources based on genetically controlled traits. We confirmed that porcupines exhibit intra-specific food selection and that this is linked to the genetic composition of the aspens. We also demonstrated that variation in phenolic glycosides and condensed tannins are strong components of this selection, thereby creating an important link between genetics, plant chemistry, and mammalian herbivory. We investigated potential impacts of porcupine herbivory on aspen using fluctuating asymmetry, however we did not detect any stress on heavily eaten trees, thereby questioning the validity of this tool for this study system.