Fairly flexible: brown-tufted capuchins and a squirrel monkey adjust their motor responses in a foraging task

Prior research on non-human primates has produced contradictory results regarding behavioral flexibility and habit formation. Most observational studies of wild primates show flexibility in foraging behavior, whereas experimental data suggest captive primates tend to form habits, thus displaying conservative tendencies. Jacobson and Hopper (2019) proposed and supported the hypothesis that captive apes' conservatism resulted from causally-unclear experimental apparatuses rather than a lack of flexibility as previous studies concluded. We replicated the experiment conducted by Jacobson and Hopper (2019) on apes with 18 brown capuchin monkeys (Cebus [Sapajus] apella) and five squirrel monkeys (Saimiri sciureus). Our goal was to investigate if they showed a similar degree of flexibility to chimpanzees (Pan troglodytes) and western lowland gorillas (Gorilla gorilla gorilla) when presented with a causally-clear task. Thus, the primary aim of this study was to determine whether this task was causally clear to monkeys, and if so, to compare their performance to that of apes. Monkeys were presented with a baited, clear tube where the removal of rods would allow the reward to drop, thus enabling the subject to retrieve said reward. Phase 1 of the study allowed us to determine whether the monkeys had a causal understanding of the task and provided an opportunity for habits to develop. Phase 2 presented the monkeys with a new reward configuration, requiring the removal of fewer rods to retrieve the reward to test if their causal understanding of the task would result in a flexible, more efficient response. The capuchins demonstrated cognitive flexibility and possible causal understanding in a manner similar to that of the apes. However, only one of five squirrel monkeys was efficient, suggesting the majority may not have understood a causal relationship between removing the rods and receiving the reward. Our study supports Jacobson and Hopper's (2019) conclusion that causally-clear tasks reduce habit formation and conservatism in capuchins, but more evidence is needed with respect to squirrel monkeys.