Appraisal theories developed in cognitive psychology are used here to attempt to better understand emotional experiences in animals. We investigated whether lambs are able to form expectations and whether their emotional responses are affected by situations discrepant from the expectations they may have formed. Forty-five female lambs were trained to obtain a small or a large amount of food reward by performing an operant task (introducing their muzzle into a hole). Then, half the lambs were shifted to the large or the small reward (i.e. positive or negative shift respectively), while the remaining half continued to get the same amount of reward. Thereafter, the lambs previously submitted to a reward change were shifted back to their initial amount of reward (i.e. successive shifts) while the lambs previously maintained on the same amount of reward were subjected to extinction (no reward, thus a negative shift). Behavior, cortisol levels and cardiac activity were analyzed, and the treatments were compared with ANOVAs for mixed models. When the amount of reward delivered was decreased, the lambs showed more locomotor activity and performed the operant task at a higher frequency but less efficiently, and there was a decrease in the parasympathetic influence on their cardiac activity. These responses were exacerbated when the negative shift followed a positive one. Similar responses were observed under extinction, and these responses were more pronounced when animals were trained with a large amount of reward before extinction. In response to a positive shift, we noticed a decrease in the frequency of the attempted operant task; this occurred only when the positive shift followed a negative one. Variations in plasma cortisol were not consistent with changes in the amount of reward. This study shows that lambs evaluate a reward according to their previous experience with that reward. They are able to form expectations, and a discrepancy from these expectations influences emotional responses, especially in the case of a negative shift. Given the appraisal criteria used by lambs and the matching emotions, we can assume that the emotional response to a negative shift expressed by lambs could reflect the despair caused by frustration.