When, in 1960, the British primatologist Jane Goodall first observed chimpanzees at Gombe National Park in Tanzania using twigs to fish termites out of anthills, she was making scientific history. Researchers had until then believed only humans could use tools. Goodall’s discovery erased that line as well as raised important questions about what it means to be human.
Now, a study published in Science on October 30 has suggested that another supposedly uniquely human trait — the ability to weigh evidence, revise beliefs, and change one’s mind — may not be ours alone. (This is of course setting aside machines.)
A team of experts from the Netherlands, Uganda, the UK, and the US observed 15-23 chimpanzees at the Ngamba Island Chimpanzee Sanctuary in Uganda through a series of carefully designed behavioural tests. The question they were trying to answer was simple: when faced with conflicting clues about where food is hidden, do chimpanzees follow the most recent signal or do they combine information, remember it, and update their beliefs, much like humans do when they think rationally?
Five tests
The researchers used five simple tests to examine how chimpanzees revised their beliefs.
“To run this study, we really had to break it down and ask: what is rationality at its roots?” Utrecht University assistant professor and the study’s lead author Hanna Schleihauf said. “What are its most fundamental components? And how do you test this in a non-verbal way?”
The team used a Bayesian model to generate predictions about the great apes’ decisions and compared them with their actual behaviour. A Bayesian model is a statistical way to understand or predict something by continuously updating one’s beliefs with new evidence.
The first test checked how chimpanzees responded to strong and weak clues. A strong clue could be in the form of seeing food dropped into a container; a weak clue could be hearing food rattling inside the container. The researchers also switched the order in which these clues appeared.
This way, the team observed that when the strong clue came second, the chimpanzees were more likely to change their original choice, suggesting they weren’t simply reacting to the most recent signal but were weighing how reliable each clue could be.
The second test examined whether this pattern held when the team swapped the clues. This time, the strong clue was auditory while the weak clue involved vague movements or traces of food. As before, the chimpanzees consistently favoured the stronger, more reliable clues regardless of when it appeared.
The third test explored how well the chimpanzees remembered the clues. The scientists introduced three containers, one with a strong clue, one with a weak clue, and a third with no clues. After the chimpanzees picked the container with the strongest clue, the researchers removed it and presented them with the remaining two (somewhat like the Monty Hall problem for humans). The chimpanzees more often than not picked the container with the weak clue rather than the one with no information, suggesting they kept multiple possibilities in mind rather than focus solely on the strongest clue.
Next, the scientists checked whether the chimpanzees could differentiate between information they already had and something new. The great apes were made to hear either the same sounds of food rattling in a can twice or a new sound the second time. The chimpanzees changed their choice only when the second signal added something new, indicating they could tell old information from genuinely new evidence.
The final test explored how the chimpanzees responded when their initial belief was undermined. When the scientists presented a “defeater” clue that directly contradicted a prior strong clue, for instance by revealing that a rattling sound had come from a pebble rather than food, the chimpanzees revised their choices. However, they ignored the same ‘defeater’ when it wasn’t linked to previous positive evidence.
Not uniquely human
Taken together, the tests show the chimpanzees didn’t simply respond to the last or loudest clues. Instead they evaluated how relevant and reliable each piece of information was, kept multiple possibilities open, and updated their choices in ways that resemble rational, evidence-based reasoning in humans.
“This shows [the study] that they are aware of the reasons for their choices, a skill that has been believed to be uniquely human,” Prof. Schleihauf said.
How would this sort of rational thinking turn up in the everyday lives of wild chimpanzees? Since this study was carried out in semi-captive conditions and with rescued chimpanzees, the researchers could offer only hypothetical examples.
As Prof. Schleihauf said: “Imagine a chimpanzee sees a red fruit on the ground and walks towards a tree because it assumes the fruit must have fallen from there. But when it reaches the spot, it realises the ‘fruit’ was only a cluster of red leaves. In that moment, the chimpanzee should revise its belief: it should no longer think that this tree is full of ripe fruit.”
‘Bigger questions’
According to Tetsuro Matsuzawa, a primatologist and former director of Kyoto University Primate Research Institute (and who wasn’t involved in the study), it would be extremely difficult to observe such traits in wild chimpanzees in their daily lives because researchers simply wouldn’t be able to make enough repeat observations to show reliably that chimpanzees can revise their beliefs in ways demonstrated in the study.
Nonetheless, he added, “This study also raises bigger questions about the evolutionary roots of human rationality. It shows that rational thinking is not limited to humans, and prompts us to ask how much of this ability is shared with chimpanzees and other great apes, a question now brought to the forefront.”
Ipsita Herlekar is an independent science writer.
Published – November 24, 2025 05:30 am IST
