As Halloween continues to draw closer, it seems appropriate to address one of mankind’s most common fears: ophidiophobia, or the fear of snakes. Those with the phobia often try to explain it with a personal traumatic experience, cultural conditioning, or an aversion to the way the animals move, but some scientists believe that the fear has much deeper roots.
Large and venomous snakes are most prevalent in the tropics, as are primates, and the threat snakes posed to our ancestors may have been a selective pressure that left an evolutionary mark. After all, primates who were unable to identify and deal with snakes would have been less likely to reproduce and pass on the traits leading to that naive behavior. In a recent study, Quan van Le of Toyama University in Japan and his colleagues found evidence to bolster this hypothesis: particular neurons in the brains of several macaque monkeys that respond to images of snakes.
The researchers implanted electrodes on individual neurons in two regions of the brain, the medial and dorsolateral pulvinar, that are found only in primates. Previous research has suggested that these areas are important to the visual detection of threats, particularly the medial pulvinar, which receives direct input from the eyes in addition to processed input from the rest of the brain.
After placing the electrodes, the authors showed the macaques a series of images: geometric figures, monkey hands, monkey faces, and snakes. The scientists then measured how many neurons responded to each type of image, how powerful the response was, and how quickly the response occurred. When snakes were shown, the response was faster, stronger, and happened in more neurons than for any of the other stimuli.
Lynne Isbell, a coauthor of the study, considers its results to be exciting support for her “snake detection theory” of visual evolution. Eyes that face forward, combined with brainpower capable of sophisticated pattern recognition and the detection of elongated, moving forms, would have been an invaluable advantage for early primates at the risk of falling prey to serpents. She has argued that the evolutionary importance of stronger sight even led to the reduction of the sense of smell to free up the necessary resources in the brain.
Although the fear of snakes may be evolutionarily derived, it has taken on a life of its own in human myths and legends. From the deceiver of the Garden of Eden to the shape-shifting nemesis of Disney’s Aladdin, the reptiles have been cast as villains time and again. Yet snakes often provide ecological benefits, preying on pests such as rodents and slugs, and the vast majority are harmless to humans. Taking a more scientific approach to snakes may help ophidiophobes recognize the basis of, and perhaps finally overcome, their fears.