How do monkeys recognize snakes so fast?

Dr. Nobuyuki Kawai from Nagoya University in Japan has found that the rapid detection of snakes by monkeys is because of the presence of snake scales as a visual cue. His findings highlight an evolutionary adaptation of primates to identify snakes based on specific visual characteristics. Understanding these mechanisms provides insight into the evolution of visual processing related to threat detection. The findings were published in Scientific Reports.

Rapid detection of dangers and threats is important for personal safety. Since our ancestors first appeared, snakes have posed a deadly danger to primates, including humans. Even monkeys and human infants who have never encountered a snake react to pictures of them, demonstrating our innate fear of these creatures.

Kawai's first experiment demonstrated that monkeys exhibited an immediate response to images of snakes but not to images of salamanders, implying a specific fear of snakes. On the basis of this observation, he wondered what would happen if he edited the images of the salamanders to have snakeskin without changing anything else. Would monkeys respond to the skin, or would they recognize the harmless salamander in snake clothing?

To answer this question, Kawai presented monkeys that had never seen a real snake with nine images on a board and trained them to find the one that was different to receive a reward. When presented with a single snake amidst a group of salamanders, monkeys exhibited a faster response time to locate the snake compared to identifying a salamander among snakes. This observation suggests that the monkeys had a strong response to the potentially dangerous reptile.

However, when Kawai showed the edited images of salamanders with snakeskin to monkeys, they reacted to the altered images of the harmless creature equally fast, or even faster, than to the snake.

“Previously we demonstrated that humans and primates can recognize snakes quickly; however, the critical visual feature was unknown,” Kawai said. “The monkeys did not react faster to salamanders, a species that shares a similar elongated body and tail with snakes, until the images were changed to cover them with snakeskin.” This suggests that the monkeys were most threatened by the scales.

“This may be because during evolution our primate ancestors evolved a visual system to identify scales, which are characteristic of snakes,” he continued. “These insights into primate evolution will likely improve our understanding of vision and brain evolution in animals, including ourselves.”