Abstract
During foraging decisions, animals often make irrational choices. The selective-value effect refers to the lack of preference for an option consisting of one highly preferred item plus one less preferred item compared to a single highly preferred item. A similar bias is the less-is-better effect, in which individuals prefer a single highly preferred item rather than an option that includes both a highly preferred item and a less preferred one. Here, we investigated the occurrence of these decisional biases in twelve tufted capuchin monkeys (Sapajus spp.) by exploring, for the first time in this genus, both the role of the relative value of the highly vs. less preferred food and the length of the intertrial interval. Overall, capuchins made rational choices by preferring the mixed option over the single one. However, on an individual level, some capuchins exhibited the selective-value effect. Mirroring previous findings in apes, both the relative value of the highly vs. less preferred food and the length of the intertrial interval modulated capuchins' preference for the mixed option. This study provides further insight into the factors influencing violations of rationality in primate food choices, suggesting that these decisions might reflect cognitive adaptations for optimizing sensory feedback and nutrient intake rather than being irrational.