Comparing chimpanzees' preparatory responses to known and unknown future outcomes.

When humans plan for the future, we recognize not only that one particular event may occur, but that the future can have different, mutually exclusive possible outcomes. A recent study by Suddendorf et al (Suddendorf 2017 Biol. Lett. 13, 20170170 (doi:10.1098/rsbl.2017.0170)) suggests that young children (less than 3 years) and apes lack this capacity, as demonstrated by their failure to cover each of two tube openings to ensure catching an object that would drop randomly from one of the tubes. Before drawing conclusions based on these negative results, however, it is important to assess subjects' failures and test the reliability of the task itself. To explore whether the apes' performance resulted from an inability to represent mutually exclusive futures or from extraneous factors related to the task, we replicated the methods of Suddendorf et al (Suddendorf 2017 Biol. Lett. 13, 20170170 (doi:10.1098/rsbl.2017.0170)) with a group of six chimpanzees but included a key control condition in which subjects were expected to cover both tubes on every trial (i.e. the rewards would consistently emerge from both tubes). We show that even in this straightforward condition in which the outcome of the trial was known, only four of the six subjects ever covered both tubes, typically doing so after a minimum of 17 trials, and often reverting back to covering one tube on later trials. We conclude that this task is not valid for testing the ability to represent mutually exclusive futures. We discuss what potential factors may explain the results and outline a new suggested method to continue testing for this capacity in the future.