Mixing nutrients mitigates the intake constraints of a plant toxin in a generalist herbivore

The diversity and concentration of primary plant compounds, including nutrients and secondary metabolites, such as plant toxins differ spatially and temporally among food resources, creating a varied feeding environment for herbivores. Both nutrients and toxins influence foraging behavior, but their interactive effects remain poorly understood. Our aim was to determine how patterns of intake and feeding behavior are influenced by the nutritional composition of foods as modified by the presence of toxins and the capacity to choose between nutritionally different foods. We offered common brushtail possums, a generalist mammalian herbivore, a choice between 2 food mixtures, one high in protein (HP), the other high in carbohydrate (HC), or a no-choice diet formulated to reflect the mixture of protein and carbohydrate selected when animals had access to both HP and HC foods. We tested choice and no-choice dietary regimes in the presence or absence of the toxin, cineole, at a concentration known to limit intake by ∼50%. We found that possums could compensate for the effect of a high concentration of cineole on food intake if they could mediate dietary concentrations of the HP and HC foods. They appeared to do this by 1) selecting an increased proportion of dietary protein and 2) by continually adjusting the proportion of protein and carbohydrate eaten throughout a feeding night. Our results demonstrate that the protective effect of cineole against herbivory by possums is not a simple function of its concentration but rather depends strongly on the nutritional context in which it occurs.