Interactions between food chemistry and predation risk in fox squirrels

The role of plant defensive compounds has often been studied within the purview of consumer diet choice. However, consumers are often confronted with foods distributed within depletable patches. To investigate the complication of resource depletion, I merged a consumer-resource model of nutritional relationships between foods with the technique of giving-up densities for measuring foraging behavior in depletable food patches. Theory predicts that foods containing plant defenses that act as digestibility reducers (e.g., lignins and tannins) will be relatively less depleted under higher predation risk than will foods without defenses. In contrast, foods containing defensive toxins (e.g., alkaloids) that affect fitness directly, and not through diminished physiological uptake of energy, will be depleted without bias to predation risk or other foraging costs. I tested the theory using fox squirrels (Sciurus niger) feeding on sunflower seeds that were impregnated with either tannic or oxalic acid. Squirrels had their highest giving-up densities on oxalate-treated seeds and their lowest on water-treated (control) seeds. GUDs were also higher when patches were placed in riskier microhabitats. Moreover, risky microhabitats increased tannin GUDs relative to control GUDs but had no effect on oxalate GUDs. The former observation is consistent with defenses that act as digestibility reducers, and the latter is consistent with defensive toxins. These results indicate that the effects of plant defensive compounds on foraging behavior are complicated and require consideration of not only the potential effects of the compound, but also the environment to which the forager is exposed. Tannins influence squirrel foraging behavior, but the effects may not be seen or realized unless the forager is in a sufficiently risky environment.