article application/pdf Nature Portfolio 2024 CC BY 4.0 International http://creativecommons.org/licenses/by/4.0/ Human cooperation can be facilitated by the ability to create a mental representation of one's own actions, as well as the actions of a partner, known as action co-representation. Even though other species also cooperate extensively, it is still unclear whether they have similar capacities. The Joint Simon task is a two-player task developed to investigate this action co-representation. We tested brown capuchin monkeys (Sapajus [Cebus] apella), a highly cooperative species, on a computerized Joint Simon task and found that, in line with previous research, the capuchins' performance was compatible with co-representation. However, a deeper exploration of the monkeys' responses showed that they, and potentially monkeys in previous studies, did not understand the control conditions, which precludes the interpretation of the results as a social phenomenon. Indeed, further testing to investigate alternative explanations demonstrated that our results were due to low-level cues, rather than action co-representation. This suggests that the Joint Simon task, at least in its current form, cannot determine whether non-human species co-represent their partner's role in joint tasks. The Joint Simon task is not joint for capuchin monkeys doi:10.1038/s41598-024-55885-x https://phaidra.vetmeduni.ac.at/o:2963 Martínez, Mayte (University of Veterinary Medicine Vienna / Georgia State University) Brosnan, Sarah F. (Georgia State University) Babb, Matthew H. (Georgia State University) Range, Friederike (University of Veterinary Medicine Vienna) Animals; Cebusphysiology; Cues eng Scientific Reports 14(1) (2024)