Chimpanzees Exhibit Rational Belief Revision in Ugandan Sanctuary Study

A study published in the journal Science in late 2025 provided substantial evidence that chimpanzees possess the capacity for rational belief revision when presented with new data. This research challenges established assumptions regarding the exclusivity of human-like reasoning processes and carries implications for understanding intelligence across various animal species, including cetaceans.

The core experiments were conducted at the Ngamba Island Chimpanzee Sanctuary in Uganda, utilizing a controlled two-box paradigm to assess the primates' decision-making under varying levels of evidentiary strength. Researchers presented the chimpanzees with either a 'weak' clue, such as an auditory signal from a shaken box, or a 'strong' clue, like visual confirmation of a treat's location. The study focused on how these primates evaluate and adapt their internal conclusions based on the quality of incoming evidence, a fundamental component of rationality.

In subsequent trials, the subjects successfully used conflicting or stronger evidence to override initial assumptions. For example, when a 'defeater' clue contradicted the initial strong evidence—such as seeing food in one box while hearing a sound suggesting food in another—the chimpanzees significantly adjusted their choice toward the more compelling information. The research team, which included Professor Jan Engelmann of UC Berkeley and postdoctoral researcher Emily Sanford, employed rigorous methodologies to confirm genuine reasoning.

Computational models were used to analyze the choices made by the twenty chimpanzees across five experiments. These models confirmed that the decision-making aligned with rational belief updating strategies, effectively ruling out recency bias. The chimpanzees followed the evidence above chance in approximately 80% of the tests. Professor Engelmann frames these findings as evidence that rationality exists on a continuum across species, rather than being a strictly binary human trait.

Sanford likened the flexible reasoning skills demonstrated by the chimpanzees to those observed in four-year-old human children, suggesting shared evolutionary roots for critical thinking. This work informs models of learning and reasoning, including those being developed for artificial intelligence systems. The findings contribute to a growing body of evidence suggesting that metacognition—the ability to evaluate one's own knowledge—may be more widespread in the animal kingdom than previously accepted.

This comparative context extends to highly intelligent marine mammals. Cetaceans, such as dolphins and whales, possess large brains with complex structures, including numerous neocortical neurons associated with advanced problem-solving and social complexity. For instance, the bottlenose dolphin's brain mass slightly exceeds that of humans, and the long-finned pilot whale has more neocortical neurons than any other mammal examined to date. Future research plans include extending these belief revision tasks to young human children to draw direct comparisons with the chimpanzee data.