Differences in Between-Reinforcer Value Modulate the Selective-Value Effect in Great Apes (Pan troglodytes, P-paniscus, Gorilla gorilla, Pongo abelii)

We investigated how apes allocated their choices between 2 food options that varied in terms of their quantity and quality. Experiment 1 tested whether subjects preferred an AB option over an A option, where the A item is preferred to the B item (e.g., apple + carrot vs. apple). Additionally, we tested whether the length of the intertrial interval (ITI) affected subjects' choices. Five orangutans, 4 gorillas, 7 bonobos, and 10 chimpanzees received 3 types of trials: preference (A vs. B), quantity (AA vs. A), and mixed (AB vs. A where A is the preferred food). We used 3 food items that substantially differed in terms of preference (carrots, apples, and pellets). Subjects showed no overall preference for the mixed option (AB) compared with the single option (A), even though they showed clear preferences during both the preference and quantity trials. The intertrial length had no effect on choice behavior. Experiment 2 further explored apes' choices by using 3 highly preferred food items (bananas, grapes, and pellets) in 6 orangutans, 4 gorillas, 8 bonobos, and 18 chimpanzees. Unlike the results of Experiment 1, apes generally chose the mixed option. Our results indicated that apes did not show a general "selective-value" effect but chose depending on the relative value of the food items involved. Subjects were more likely to select the mixed over the single option when (a) the mixed option was composed of items that were closer in value and (b) they were compared against the less valuable item forming the mixed option.