AJP-June 2018

'''Freeman, S. M., Rebout, N., & Bales, K. L. (2018). Effect of reward type on object discrimination learning in socially monogamous coppery titi monkeys (Callicebus cupreus). American Journal of Primatology, e22868.'''

The impact of reward type on learning has been studied in many species, including rodents and nonhuman primates. Social rewards, such as access to or a photograph of a conspecific, have been found to be at least as effective as food rewards and in some cases even more so. Coppery Titi monkeys are a highly social, monogamous species forming strong social bonds with their pair-mates. Freeman, Rebout, and Bales (2018) hypothesized that access to a pair-bonded mate should be a more efficient reinforcer than food when coppery titi monkeys are learning a two-choice discrimination task. In order to test this hypothesis, 28 monkeys, housed in heterosexual pairs and offspring if applicable, participated in the study. Pair-mates and offspring were separated from the test subject prior to testing. Subjects were visually separated but auditory and olfactory contact was still possible. Training sessions occurred to acclimate test subjects to separation with 12 animals reaching criterion and moving on to testing. The stimuli used were three different shapes (triangle, circle, square) in three colors (black, white, gray). The design was within-subjects with reward type counterbalanced across subjects. Subjects made their decision regarding two stimuli, one of which was designated the correct choice (S+) while the other was the incorrect choice (S-). If S+ was chosen, the monkey would, depending on their pre-designated reward type for the first round, either get a food reward or social reward which entailed releasing the previously out of view pair-mate into the enclosure. If the monkey chose S-, no reward was given and the monkey could continue choosing until either correct or five minutes had elasped. After reaching criterion (10/12), the monkey moved onto the other type of reinforcer for the second round of testing. It was found that the food reward group took less time to engage in the task, had higher percentage correct, had fewer trials needed to reach criterion, and were less likely to balk (make no response during trial) than monkeys receiving a social reward. Further, 83% of the food reward group learned the task while only 45% of the social reward group learned the task. Explanations for the results included: high value of food reward (bananas), food reward usage during training, and the fact that monkeys may be overly familiar with short separations from their pair-mates during other activities. Future studies could be conducted to flesh out the relative value of social rewards compared to values of food rewards as well as the impact of food reward use during training on later testing sessions.