A new study suggests that monkeys have developed ophiophobia or fear of snakes as part of their natural evolution to avoid these predators. It may also explain why human experience fear when faced with these species.

A group of scientists from the University of California in the United States, the University of Toyama in Japan and the Primate Center of the University of Brasilia in Brazil collaborated to understand why ophiophobia is one of the top fears among the human race. Although this fear of snakes is not an innate characteristic of humans, it is known to be developed in the early growth stages.

Evolutionary biologist and lead author Lynne Isbell said that fear of snakes is not inborn for humans but detecting snakes is easier for us than other phenomena. This so-called amplified vigilance may cause humans to develop early a fear-motivated evasion of snakes.

The study supports the theory that monkeys did not develop the fear as a result of adaptation from hunting but out of response from being preyed upon by snakes. The snakes were “the first of the modern predators of primates, and the most persistent, that continued to this day -- and that look the same as they did 100 million years ago,” wrote Isbell.

To test the theory, the scientists selected two Japanese Macaques, also known as snow monkey, which never had any encounter with snakes. They inserted probes into their brains to measure their responses to stimuli and their reactions to images of snakes, geometric shapes, and other monkeys.

The scientists observed from their data that the neuron responses in the monkeys’ brains were remarkably more rapid and intense when presented with images of snakes, compared to other images.

The pulvinar neuron pulses recorded around 60 microseconds when the snakes were shown to the macacques, which may be a sign that the brain processing was done involuntarily or unconsciously. A second neuron pulse was detected and recorded around 250 microseconds, which may be linked as a response from the brain’s cortex, the part of the brain which has cognitive controls.

According to Isbell, we have distinct attributes to detect the snakes better than other mammals. Generally, mammals are able to pick up moving objects or things immediately.

“But snakes lie in wait. They don’t move very much, so it’s crucial to see them before they see us and to avoid them.”

The study was published in the Oct. 28 issue of the Proceedings of the National Academy of Sciences.