Capuchin monkeys' ability to choose beneficial options is inhibited by added complexity.

How does ecological complexity influence decision making? To facilitate interpretation, laboratory studies often focus on decision tasks with limited options, but animals presumably face more variety in the wild. For example, sometimes species must choose between ephemeral and permanent options, as with choosing between mobile prey and stationary food. The optimal choice is to prioritize the ephemeral option, because it will disappear if not selected first, whereas the permanent option will always be available. In experimental tasks with just these two choices, capuchin monkeys (Sapajus (Cebus) apella) learn to maximize their rewards. However, in the wild, animals presumably face additional sets of choices, for instance two ephemeral or two permanent options, which may make it more difficult to learn the best way to maximize their payouts. Here we show that adding configurations during learning lowers the capuchins' preference for choosing the ephemeral option first. Because recent theoretical work suggests that this more complex version could be solved by grouping the elements through configural learning, half of our subjects underwent training proposed to aid in the configural learning process prior to experiencing the added complexity. This training did not improve the capuchins' ability to pick the ephemeral option first. We consider both what this means for capuchins' and other species' decision making in more complex environments and how we use experimental results to understand animals' cognition and behaviour. • Capuchins and cleaner fish can solve a dichotomous choice to maximize rewards. • Capuchins failed to do so when complexity was added, even after training. • Capuchins' decisions might differ in more complex natural environments. [ABSTRACT FROM AUTHOR]