Monkeys may hold the key to understanding why some primates fear snakes.
According to a new study published in the journal Proceedings of the National Academy of Sciences, scientists found ophidiophobia develops early in primates and may be an evolutionary response to avoid being harmed by the dangerous reptiles.
“The characteristics we have help us to see them better than other mammals can see them,” Lynne Isbell, an evolutionary biologist from UC Davis and one of the authors of the paper, told the Los Angeles Times. “Mammals in general are really good at picking up movement. 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 involved analyzing the brains of Japanese macaques to see how they reacted to images of snakes. The results showed that there were many snake-sensitive neurons in parts of the monkeys’ brains that gave stronger and quicker responses to snake images compared to visuals of hands or geometric shapes.
"The results show that the brain has special neural circuits to detect snakes, and this suggests that the neural circuits to detect snakes have been genetically encoded," Hisao Nishijo, of Toyama University in Japan, said in a statement.
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Researchers came to the conclusion after electrical pulses from the monkeys’ pulvinar neurons took place about 60 microseconds after a snake was introduced to the monkeys, followed by another after 250 microseconds, suggesting they came from the cortex. The monkeys, which were bred in captivity, had never seen snakes before. Given their quick and strong responses, scientists concluded their response was hardwired by evolution.
“They were actually prey,” Isbell said, who developed the evolutionary hypothesis in 2006 that suggested primates developed good close-range vision to spot dangerous snakes millions of years ago. “And 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 -- are snakes.”
Some scientists argue more research needs to be done to draw hard conclusions about the fear of snakes.
“A Japanese macaque is not a good model for the ancestor of primates -- or the even ancestor of macaques, for that matter -- so it will be difficult to generalize these results to other primates . . . without falling prey to the myth of the ‘typical primate,’” Seth Dobson, a biological anthropologist from Dartmouth College who wasn't involved in the study, told the Scientist, suggesting that the pulvinar neurons in ancient primate ancestors may have been more focused on snake detection than modern primates, which now recognize facial expressions.
Isbell said she plans on testing other areas of the brain to see whether other neurons show selective responses to predators besides snakes. “Future collaborations of neuroscientists and evolutionary biologists will really advance our understanding of why animals do what they do,” she said.