Detoxification rates constrain feeding in common brushtail possums (Trichosurus vulpecula)

A central theory in mammalian herbivore-plant interactions is that feeding decisions depend on an animal's capacity to detoxify plant secondary metabolites (PSMs). Thus, if a herbivore could detoxify a PSM faster, it might eat more. This theory has been difficult to test because the mechanisms by which herbivores detoxify most PSMs are largely unknown. We investigated whether common brushtail possums alter aspects of their feeding in response to an increased capacity to detoxify benzoic acid, a common PSM. Animals detoxify benzoic acid primarily by conjugating it with glycine to form benzoyl glycine (hippuric acid). Therefore, adding glycine to a diet that contains benzoic acid provides a simple way to study how detoxification constrains feeding in mammals. Brushtail possums offered supplementary glycine metabolized benzoate faster and, in response, ate more. Furthermore, when given a choice, brushtail possums selected a diet containing both benzoate and glycine over diets with a high concentration of just one of these supplements. The possums' aversion to glycine may explain why their ability to select a favorable diet combination from feeders containing diets with each of the supplements seemed less acute: they preferred diets containing benzoate alone over those containing glycine and did not mix their diet even though it would have allowed them to eat more benzoate and thus more food. Administering the anti-emetic drug, ondansetron (a specific 5-HT₃ receptor antagonist), did not alter the amount eaten of diets containing benzoate, glycine, or both, suggesting that 5-HT₃ receptors do not regulate benzoate or glycine intake. The results show that common brushtail possums recognize changes in their detoxification capacity and alter their feeding response accordingly.