Medina, a female chimpanzee in the study. Credit: Innocent Ampeire
In a paper published in Science, researchers show that humans are not the only ones making decisions based on evidence, and importantly, chimpanzees judge the quality of the evidence when making decisions.
This is one of the aspects of rational thought that was widely thought to be uniquely human.
The study was led by Utrecht University in the Netherlands, in collaboration with the University of St Andrews and the University of Portsmouth in England, and University of California Berkeley in the US.
The research team examined whether and how chimpanzees (Pan troglodytes) update their initial belief about the location of a reward in response to conflicting evidence.
Food was hidden in two boxes and presented to the chimpanzees. Researchers would then provide either a strong or weak piece of evidence about one of the boxes to suggest whether it contained food, such as shaking it to make a sound or directly showing them what was inside. The chimpanzee could make an initial choice of where they thought the food was hidden, before the researcher provided a new piece of evidence about the other box that was either stronger or weaker than the initial evidence.
When the first clue was stronger, the chimpanzees tended to stay with their initial choice, but when it was weaker they switched. This suggests that they were updating their beliefs in line with the evidence.
However, when researchers offered a clue about one of the boxes and revealed it to be misleading - for example demonstrating that the 'food' contained in the box was merely a picture - the chimpanzees understood that the initial evidence was not valid.
Senior author of the study, Dr Esther Herrmann from the University of Portsmouth, said: "Our findings reveal that humans aren't the only ones who make rational decisions based on evidence - chimpanzees, our closest living relatives, also weigh the strength and quality of the evidence before making decisions.
Professor Josep Call from the Global Research Centre for Diverse Intelligences at the University of St Andrews and co-author of the study, added: "This study opens a new avenue of research into the psychological mechanisms underlying decision-making in chimpanzees. Together with previous work on bargaining games, it contributes to portray chimpanzees as rational decision-makers."
This paper points to a conclusion that some elements of rational thought, did not begin with our species.
