Relative food value determines apes' responses in a natural choice task (#550)
We investigated how apes allocated their choices between to food options that varied in terms of their quantity and quality. Exp. 1 tested whether subjects followed a “rational election” strategy by preferring the AB option over the 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 inter-trial interval affected subjects’ choices. Five orangutans, 4 gorillas, 7 bonobos and 10 chimpanzees received three types of trials: preference (A vs. B), quantity (AA vs. A) and rational-election (AB vs. A where A is the preferred food). We used three food items that substantially differed in terms of preference (carrots, apples, and pellets). Subjects showed no overall preference for the rational option (AB) compared to the non-rational option (A), even though they showed clear preferences during both the preference and quantity trials. Only orangutans showed some significant preferences in test trials but only in two out of six food combinations. The inter-trial length had no effect on choice behaviour. Exp. 2 further explored apes’ “rational election” by using three types of highly preferred food items (bananas, grapes and pellets) in 6 orangutans, 4 gorillas, 8 bonobos, and 18 chimpanzees. Unlike the results of Exp. 1, here apes generally chose rationally in testing trials. Taken together these results indicate that apes can choose rationally but this depends on the value of the food items.