Beware the slithering scales: Monkeys fear snakeskin even when it’s not on a snake, study suggests

A new study suggests monkeys can identify snakes by their scales, and know to fear them, even when those scales aren’t on a snake.

Published in the journal Scientific Reports in November, the study pitted a trio of Japanese monkeys – Pero, Ume and Shiba – against a battery of tests designed to measure their reaction time.

In each experiment, study author Nobuyuki Kawai presented the monkeys with a grid of nine black-and-white images of animals. Initial tests challenged the monkeys to pick out a single snake from eight harmless salamanders, and vice versa — a task they performed both quickly and nearly without mistakes, the study found.

The monkeys showed an “immediate response to images of snakes but not to images of salamanders, implying a specific fear of snakes,” a release announcing the study’s results reads.

Kawai also wanted to determine what it was about snakes that caused that response. Accounting for things like colour and shape, the Nagoya University researcher tested whether it was the scaly pattern of the snake’s skin, which salamanders do not naturally possess, that made the difference.

Using image-editing software, Kawai copied the snakes’ scales and applied them to the bodies of salamanders, then re-ran the test to see how the monkeys reacted.

The results indicated that Pero, Ume and Shiba were just as fast, if not faster, at identifying the artificially scaly salamanders than they were snakes, the study reads, suggesting it could be the scales the monkeys were looking for, after all.

“Humans and primates can recognize snakes quickly; however, the critical visual feature was unknown,” Kawai said in the release. “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.”

While some aspects of the primates’ reactions remain up to interpretation, Kawai says the experiments may help understand how potential danger is identified and processed, and what specifically triggers the fear response.

“These insights into primate evolution will likely improve our understanding of vision and brain evolution in animals, including ourselves,” he said.