OSAKA -- Primates, including humans, instinctively fear snakes due to their scales, not their unsettling, slithering movements, Nagoya University cognitive science professor Nobuyuki Kawai has found.

It is believed that the small mammalian ancestors of primates began living in trees around 65 million years ago, with snakes capable of climbing tall trees being their main predators. A "snake detection theory" has even been proposed, suggesting that primates developed advanced vision and big brains to quickly spot their greatest enemy, snakes. According to the World Health Organization, tens of thousands of people worldwide die each year from snake bites.

Kawai said that previous experiments showed that monkeys who had never seen a snake could spot snake photos faster than pictures of other animals, and human infants exhibited strong brainwave reactions to snake photos. However, the specific features of snakes that triggered these reactions were not yet understood.

Focusing on the scales of snakes, Kawai conducted experiments with three Japanese macaques that had never seen real snakes. The monkeys were rewarded with food for touching the one among nine black and white photos that was unique. They were then tasked with selecting a snake from nine pictures, the other eight of which were of salamanders. All three of macaques found the snake photo faster than a salamander picture mixed in with eight photos of snakes. These results aligned with previous experiments showing that monkeys spot snakes quicker than other animals.

Next, similar experiments were conducted with salamander photos digitally altered to be "clad in" snake scales, comparing the detection speed. Two of the three monkeys spotted the salamander as quickly as they would a snake, and the third identified the salamander faster than the snake. These results indicate that monkeys are sensitive to snake scales rather than their slithering movements or shape.

Kawai stated, "It's likely that the ancestors of primates developed their visual systems to detect snake scales during the course of evolution." The research findings were published in the Nov. 10, 2024, edition of Scientific Reports.

(Japanese original by Yosuke Tsuyuki, Osaka Lifestyle, Science & Environment News Department)