Roles of Flavor and Reward Intensities in Acquisition and Generalization of Food Preferences: Do Strong Plant Signals Always Deter Herbivory?

Plants possess a variety of flavor intensities and nutritional qualities that influence diet selection by herbivores. Some studies suggest that herbivores prefer less intense flavors, either in nutritious or toxic foods. However, if flavor preferences are learned, and if they are influenced by flavor-postingestive feedback interactions, then herbivores should form preferences for either weak or strong flavors when they are followed by greater nutrient rewards. We conditioned two groups of lambs with intraruminal infusions of 30 g of starch while they consumed onion-flavored straw at concentrations of 0.5% (group 1) or 4% (group 2). On alternate days, lambs received infusions of 150 g of starch while they consumed onion-flavored straw at concentrations of either 4% (group 1) or 0.5% (group 2). When offered a choice of straw in both flavor concentrations, lambs preferred the concentration—high or low—associated with the higher dose of starch during conditioning (P < 0.05). When offered a choice of onion-flavored straw in four concentrations (0.25, 1, 2, and 5%), lambs in group 1 preferred the highest onion concentration (5%), whereas lambs in group 2 preferred lower onion concentrations (0.25, 1, and 2%) (P < 0.001). the same pattern of preference was maintained when lambs had no food (day 1; no preload), or an energy(day 2; barley preload) or a protein-rich (day 3; alfalfa–soybean preload) meal before testing (P > 0.05). Lambs with no experience of onion or starch (inexperienced lambs) preferred 0.5% onion-flavored straw to 4% onion-flavored straw (P < 0.001), and 1 and 2% onion-flavored straw to 0.5% and 5% onion-flavored straw (P < 0.05). Thus, strong flavors were initially avoided (inexperienced lambs), but they came to be preferred after their association with higher doses of starch (group 1). Collectively, our results suggest that herbivores acquire and generalize preferences for flavor intensities as a function of their previous experience with the quantitative relationship between flavor intensity and nutrient reward. This ability is critical for survival because flavors, nutrients, and toxins all vary in concentrations in different plant species and parts, as well as temporally within a species.