Gibbons (nomascus Leucogenys), Siamangs (symphalangus Syndactylus), and Chimpanzees (pan Troglodytes) Account for Proportional Probabilities in a Two-Choice Task

Probabilistic reasoning-the ability to predict outcomes based on the likelihood of different possibilities-is a key component of numerical cognition and is critical for navigating uncertain environments. However, it is unclear whether this capacity is shared with our closest relatives, the great apes, or with even more distantly related species such as the small apes. Here, we presented two gibbons (Nomascus leucogenys), two siamangs (Symphalangus syndactylus), and three chimpanzees (Pan troglodytes) with a proportional probability task requiring them to reason from the distribution of items in a population to the likely contents of a sample. In contrast to previous research, we ensured that participants could not rely on two salient heuristics-the absolute quantity of the preferred item or majority sampling-to solve the task. We found that individuals of all species were able to distinguish between 0% versus 50% and 50% versus 100%, and that two individuals (one chimpanzee and one siamang) were able to convincingly distinguish between 33% versus 67%. These results suggest that an antecedent to probabilistic reasoning was shared by a common ancestor of humans, great apes, and small apes. (PsycInfo Database Record (c) 2026 APA, all rights reserved).