OSAKA — According to an investigation by Nagoya University cognitive science professor Nobuyuki Kawai, it is the texture and appearance of snakes rather than their unsettling movement that instinctively causes primates, including humans, to feel a sense of fear.
It is thought that the small mammal ancestors of primates lived in trees approximately 65 million years ago, with snakes capable of climbing tall trees being their primary predators. A “snake detection theory” has been proposed, suggesting that primates developed advanced vision and large brains in order to quickly recognize their greatest threat, snakes. According to the World Health Organization, tens of thousands of individuals worldwide succumb to snake bites annually.
Researchers found that monkeys, who had not seen a snake before, could identify snake photos more quickly than images of other animals, and human infants displayed significant brainwave responses to snake photos. Nevertheless, the specific characteristics of snakes that triggered these reactions remained unclear.
In an investigation centered on snake-like scales, Kawai conducted experiments on three Japanese macaques that had never encountered real snakes. The monkeys were incentivized to identify the unique image among nine black and white photos of snakes by receiving a reward. Subsequently, they were tasked with identifying a snake among nine pictures, of which eight were of salamanders. All three macaques were able to locate the snake photograph much faster than they could spot a salamander picture when it was randomly inserted among eight snake photographs. These findings validated previous research showing that monkeys can rapidly detect images of snakes as opposed to images of other animals.
Similarly, experiments were conducted by digitally altering salamander photos to make them appear “clad in” snake scales, in order to compare the detection speed. It was observed that two out of three monkeys were able to spot the salamander at the same speed they would a snake, and the third monkey identified the salamander even faster than the snake. The results suggest that monkeys are sensitive to snake scales rather than their slithering movements or shape.
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