Do sheep use umami and bitter tastes as cues of post-ingestive consequences when selecting their diet?
Résumé
Grazing herbivores use their senses to select their diet. As tastes could reveal certain plant properties, we investigated whether sheep use them as cues to anticipate post-ingestive consequences. We focused on umami and bitter tastes reported to be associated with protein and toxin contents, respectively. We aimed to determine (1) lambs’ preferences for umami and bitter tastes offered together or with a control, and (2) lambs’ efficiency in learning positive and negative post-ingestive consequences associated with tastes coherently (i.e. umami taste → positive, bitter → negative) or incoherently (i.e. bitter → positive, umami → negative). The umami taste was induced by either 15 (MSG1) or 25 (MSG2) g monosodium glutamate per kg DM hay, whereas the bitter taste was induced by 2 (Quin1) or 3 (Quin2) g quinine per kg DM hay that were sprayed on an orchard grass hay. In the first experiment, 24 ewe lambs were tested on eight two-choice types: MSG1-Quin1, MSG1-Quin2, MSG2-Quin1, MSG2-Quin2, MSG1-Control, MSG2-Control, Quin1-Control, and Quin2-Control, in a Latin-square design. They always preferred the umami-tasting hay to the control and the bitter-tasting hay (P < 0.01) while they were indifferent to the bitter-tasting hay compared to control hay. After having checked that the lambs perceived the bitter taste induced by quinine (experiment 2) and that none of the substances used to modify the tastes induced their own post-ingestive consequences (experiment 3), we conducted the fourth experiment. Then, both coherent and incoherent groups decreased their preference for the taste associated with negative consequences compared to the taste associated with positive consequences, and aversion was greater in the coherent than in the incoherent group (0.04 ± 0.02 and 0.19 ± 0.08, respectively, P < 0.05). This difference could however be a residual effect of the large difference between initial preferences. After conditionings, to assess changes in individual tastes values, we re-tested the lambs’ preferences between umami- or bitter-tasting hay and control hay, and compared them with the initial preference patterns recorded in experiment 1. Preference for umami-tasting hay logically varied according to whether post-ingestive consequences were positive or negative, contrary to preference for bitter tasting-hay which was unaffected and close to umami value once negatively conditioned. Umami taste thus appears to have a positive value that can decrease after aversive conditionings, while bitter taste appears to have a negative value that made sheep suspicious about it even when associated with positive consequences.