Working memory (WM) is a core executive function that allows individuals to hold, process and manipulate information. WM capacity has been repeatedly nominated as a key factor in human cognitive evolution; nevertheless, little is known about the WM abilities of our closest primate relatives. In this study, we examined signatures of WM ability in chimpanzees (Pan troglodytes). Standard WM tasks for humans (Homo sapiens) often require participants to continuously update their WM. In Experiment 1, we implemented this updating requirement in a foraging situation: zoo-housed chimpanzees (n = 13) searched for food in an array of containers. To avoid redundant searches, they needed to continuously update which containers they had already visited (similar to WM paradigms for human children) with 15 s retention intervals in between each choice. We examined chimpanzees' WM capacity and to what extent they used spatial cues and object features to memorize their previous choices. In Experiment 2, we investigated how susceptible their WM was to attentional interference, an important signature, setting WM in humans apart from long-term memory. We found large individual differences with some individuals remembering at least their last four choices. Chimpanzees used a combination of spatial cues and object features to remember which boxes they had chosen already. Moreover, their performance decreased specifically when competing memory information was introduced. Finally, we found that individual differences in task performance were highly reliable over time. Together, these findings show remarkable similarities between human and chimpanzee WM abilities despite evolutionary and life-history differences.
Keywords: chimpanzees; cognitive control; executive functions; primate cognition; short-term memory; working memory.