Chimpanzees Revise Beliefs Like Scientists — They Weigh, Combine and Update Evidence, Study Finds

Chimpanzees revise beliefs when new evidence appears, study suggests

New research indicates that chimpanzees (Pan troglodytes) don’t just react to single cues — they evaluate multiple pieces of information, weigh their strength, and change plans when stronger or contradictory evidence appears. The findings, led in part by Jan Engelmann, a comparative psychologist at the University of California, Berkeley, provide behavioural evidence that chimps engage in metacognitive reasoning: thinking about their own thinking when making decisions.

How the experiments worked

Researchers trained chimps to search for food hidden inside boxes and presented them with different combinations of cues. The team contrasted strong evidence (for example, a window cut into a box that allowed the animal to see the food) with weak evidence (for example, rattling a box to suggest something inside). Across a sequence of experiments the apes were shown one cue, made an initial choice, and then were offered additional, sometimes conflicting, evidence before making a final choice.

Key results

Across the tests the chimps were more likely to reverse an initial choice when the second cue provided strong, direct confirmation (visible food) than when the second cue was merely weak (rattling). A follow-up test with three boxes — one showing strong evidence, one weak, and one with no evidence — found that when the strong-evidence box was removed the apes reliably preferred the weak-evidence box over the no-evidence option. This suggests they kept track of multiple cues rather than simply following the most salient one.

In later trials the researchers probed how apes combined pieces of weak evidence. The animals were more likely to change their choice when presented with two different weak cues (for example, rattling plus the sound of a researcher dropping food) than when the same weak cue was repeated. Finally, when new evidence directly undermined an earlier cue — for example, revealing a pebble that could have caused the rattling sound — the chimps frequently abandoned their original choice. That behaviour indicates they linked the original and new information and updated their beliefs accordingly.

Interpretation and implications

The authors interpret these results as evidence that chimpanzees perform several metacognitive operations: representing evidence explicitly, weighing cue strength, combining distinct cues, and revising decisions when warranted by new, contradictory information. Cathal O'Madagain, a cognitive scientist at Mohammed VI Polytechnic University in Morocco, who was not involved in the research, described the decisive experiments as demonstrating a higher bar of rationality because they show apes track and relate separate pieces of information over time.

These findings raise important methodological and conceptual points: failures to detect complex cognition in other species may reflect limitations of test design as much as animals’ capacities. Engelmann and colleagues say they plan to adapt the paradigm for other non-human primates to see how widespread this form of evidence-based decision-making is across species.

Bottom line: The study provides converging behavioural evidence that chimpanzees do more than follow single cues — they compare, combine and revise based on the quality and compatibility of information, a process broadly analogous to human metacognition and a rudimentary form of scientific reasoning.