In both humans and non-human animals, collectives can sometimes overcome individual cognitive biases or shortcomings to execute more rational behaviour than individuals. To investigate differences in strategy and outcome between individuals and collectives in a logical reasoning task, we presented an inverted U-shaped tube to individuals and pairs of chimpanzees (Pan troglodytes) and examined their preparatory actions towards rewards that could fall from either end of the tube. Given that individual chimpanzees have typically produced a suboptimal one-handed strategy in past variants of this task, we predicted that pairs would outperform individuals primarily through subjects sharing the apparatus, each placing one hand under one tube end such that they collectively account for both possible outcomes. Unexpectedly, over half of our chimpanzees spontaneously produced the optimal two-handed behaviour (covering both ends) on their own, providing evidence that individuals may be able to reason about mutually exclusive future possibilities. This reduced the capacity for pairs to improve upon individual performance. Notably, however, we observed an increase in individual usage of the two-handed strategy in the collective setting. This individual improvement may have arisen from an effect of collective facilitation, such as competition, suggesting an alternative mechanism through which collectives may outperform individuals. This article is part of the theme issue 'The evolution of collective intelligence'.
Warren et al. (2026) studied this question.