Searching in the middle - Capuchins' and Bonobos' behavior during a spatial search task

In this study we show that bonobos and capuchin monkeys can learn to search in the middle of a landmark configuration in a small-scale space. Five bonobos (Pan paniscus) and two capuchin monkeys (Cebus apella) were tested in a series of experiments with the expansion test paradigm. The primates were trained to search in the middle of a 4- or 2-landmark configuration, and were then tested with the same configuration expanded. Neither species searched in the middle of the expanded four-landmark configuration. When presented with a two-landmark configuration and a constant or variable inter-landmark training distance, the subjects sometimes searched preferentially in the middle of the expanded configuration. We discuss two alternative explanations of the results: extracting a middle rule or averaging between different goal-landmark vectors. In any case, compared to adult humans, primates appear highly constrained in their abilities to search in the middle of a configuration of detached landmarks. We discuss some of the factors that may influence the primates' behavior in this task.